Key Limpets

On two occasions before, I've presented you with members of the Fissurellidae, the keyhole and slit limpets. It's time for a return visit to the fissurellids, in the form of the diverse keyhole limpet genus Diodora.

Various views of shell of Diodora italica, copyright H. Zell.

Species have been assigned to Diodora from coastal waters pretty much around the world except for the coolest regions. They are small to moderate-size limpets, the largest species growing about three centimetres in length and two centimetres in height. The shell opens through a 'keyhole' at the apex through which the animal ejects waste matter and water that has been passed over the gills. The internal margin of this keyhole is surrounded by a callus on the underside of the shell; a distinguishing feature of Diodora is that this callus is posteriorly truncate. The external ornament of the shell is cancellate (arranged in a criss-cross pattern) and the margin of the shell is internally crenulated (Moore 1960). Moore (1960) listed three subgenera of Diodora distinguished by features of the keyhole shape and position but Herbert (1989) notes that these subgenera are not clearly distinct. A phylogenetic analysis of the fissurellids by Cunha et al. (2019) did recognise a clade including the majority of Diodora species analysed. However, species from the eastern Pacific formed a disjunct clade that may prove to warrant recognition as a separate genus.

As far as is known, Diodora species have a long lifespan, surviving for some ten to twenty years. They do not have a planktonic larva; young Diodora hatch directly from the egg as benthic crawlers. For the most part, they are presumed to graze on algae in the manner of other fissurellids and limpets. However, the northeastern Australian species D. galeata has been found feeding on the soft tissues of coral (Stella 2012), a habit that went unrecognised until fairly recently owing to the animal's cryptic nature, hiding deep among the branches of the host. Whether other Diodora species might exhibit similar lifestyles would require further investigation.

REFERENCES

Cunha, T. J., S. Lemer, P. Bouchet, Y. Kano & G. Giribet. 2019. Putting keyhole limpets on the map: phylogeny and biogeography of the globally distributed marine family Fissurellidae (Vetigastropoda, Mollusca). Molecular Phylogenetics and Evolution 135: 249–269.

Herbert, D. G. 1989. A remarkable new species of Diodora/i> Gray, 1821 from south-east Africa (Mollusca: Gastropoda: Fissurellidae). Annals of the Natal Museum 30: 173–176.

Moore, R. C. (ed.) 1960. Treatise on Invertebrate Paleontology pt I. Mollusca 1. Mollusca—general features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—general features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia. Geological Society of America, Inc. and University of Kansas Press.

Stella, J. S. 2012. Evidence of corallivory by the keyhole limpet Diodora galeata. Coral Reefs 31: 579.

Donaldina: Palaeozoic Turrets

Within the last few decades, we've developed a reasonably good idea of what are the primary subdivisions of gastropods alive today. One such generally accepted lineage is the Heterobranchia (or, depending on the author, the Heterostropha), a group that includes (among others) the air-breathing pulmonates as well as the marine sea slugs and bubble shells. In the fossil record, the roots of this lineage extend well back into the Palaeozoic with early members recognisable by their distinctive mode of shell development. The larval shell, the protoconch, of these forms spirals in the opposite direction from the mature teleoconch, so the animal will start its life sinistral (spiralling left) and end it dextral (spiralling right; if you're having difficulty imagining how this works, the protoconch often ends up sitting upside down relative to the teleoconch). Among the earliest heterobranchs in the fossil record is the genus Donaldina.

Specimen of Donaldina (1.2 mm in height), with close-up of protoconch, from Bandel et al. (2002).

Fossils of Donaldina have been found around the world and the genus persisted for a long time. The earliest potential Donaldina have been described from the Early Devonian but their inclusion in the genus is uncertain (Bandel et al. 2002). The protoconch on these early forms is poorly preserved and it is uncertain whether they truly showed a heterobranch development. The genus was definitely present by the early Carboniferous and persisted into the lower Permian. This is an impressive length of time: the Carboniferous alone last for around sixty million years.

Donaldina was a genus of small, high-spired gastropods, less than a centimetre in height. Many early members of the Caenogastropoda, the likely sister group of the Heterobranchia, also had shells of this kind and it may have represented the ancestral form for the two lineages. The sinistral protoconch of Donaldina was almost planispiral (spiralling in a flat plane) and completed between one and two whorls. The multi-whorled, dextral teleoconch was characterised by an ornament of spiral cords, usually only on the lower half of the whorl.

So what were Donaldina doing with their time when alive? Modern high-spired gastropods occupy a range of lifestyles, including free-living grazers, burrowers, or sedentary forms that live as filter feeders or parasites of other animals (Signor 1982). The morphology of Donaldina suggests that it is unlikely to be a burrower. The whorls are individually rounded whereas those of habitual burrowers tend to be flattened so the shell moves more smoothly through the sediment. The ornamentation on the underside of the whorl would presumably also have presented resistance to burrowing. The shape of the aperture in Donaldina is more suggestive of a free roamer, as a sinus in the upper part of the outer margin would have allowed the animal to pull back into its shell while the plane of the aperture was held as flat as possible against the substrate to protect against predators. Overall, the lifestyle of Donaldina may not have been dissimilar to that of the modern mudsnails of Cerithium and similar genera, crawling about in search of algae and other tasty morsels.

REFERENCES

Bandel, K., A. Nützel & T. E. Yancey. 2002. Larval shells and shell microstructures of exceptionally well-preserved Late Carboniferous gastropods from the Buckhorn Asphalt Deposit (Oklahoma, USA). Senckenbergiana Lethaea 82 (2): 639–689.

Signor, P. W., III. 1982. Resolution of life habits using multiple morphologic criteria: shell form and life-mode in turritelliform gastropods. Paleobiology 8 (4): 378–388.

Air-breathing Limpets

For many people, the most familiar members of the gastropods are the terrestrial snails. Gastropods started their evolution as marine animals, breathing through gills, but members of one lineage would instead evolve their own version of a lung, a large hollow in the mantle cavity opening through a hole alongside the head called the pneumostome. Possession of this lung cavity would enable slugs and snails to thrive in the terrestrial environment but the structure had originally evolved in a marine context, and even today one may find marine lung-bearers occupying habitats along the coast. One such group is the siphon limpets or 'false limpets'* of the Siphonarioidea.

*You know, normally I don't overly concern myself with vernacular names. They are not regulated and not obliged to follow reason. But even so, the name 'false limpet' makes me grit my teeth. The name is presumably inspired by the fact that siphonariids are not direct relatives of the 'true' limpets of the Patellogastropoda. But the limpet morphotype, where the typical spiral gastropod shell is reduced to a simple cap, has evolved on multiple occasions. As well as the siphonariids and patellogastropods, there are the keyhole limpets of the Fissurellidae, the freshwater limpets of the Ancylini, and many others, all consistently referred to as 'limpets'. The name refers to a morphology, not to a clade, and by that measure the siphonariids are no more 'false' than any other limpets.

Flat siphon limpets Siphonaria atra, copyright Ria Tan.

Living siphonarioids are placed within a single family, the Siphonariidae, whose members with their cap-shaped, often radially ribbed shells are found in littoral environments in temperate and tropical regions of the world. A second family, the Acroreiidae, is recognised from the Cretaceous and early Tertiary; the inclusion of these smooth, thin shells in the Siphonarioidea is somewhat tentative (classification of limpets in the fossil record is always a challenge because their simple shell form renders them light on distinguishing characters). Siphonariids are readily distinguished from other living limpets by the presence of a groove on the underside of the right side of the shell marking the position of the pneumostome. In dorsal view, this groove is often indicated by an asymmetry in the outline of the shell with one side produced. The pneumostome is also associated with a broad gap in the ring of muscle holding the shell in place; the ring is more complete in other limpets. Seemingly as a result of this lower extent of muscle, siphonariids cling to their home rocks with less tenacity than other limpets and are mostly restricted to more sheltered locations (Simone & Seabra 2017). On the other hand, they do have a more flexible foot than their competitors, allowing them to potentially move more quickly. Like other limpets, siphonariids are grazers, scraping microalgae as they crawl about. Siphonariids have a weaker radula than patellogastropods and so scrape somewhat less forcefully; when members of the two clades occupy the same habitats, patellogastropods are generally the more abundant. The majority of siphonariids (where known) have planktonic larvae but some species are known to be direct developers.

Siphonaria lessonii, copyright Mikelzubi.

Obviously, the marine but lung-possessing siphonariids are potentially of great interest in understanding how the gastropod lung evolved. Many earlier researchers thought that the siphonariids may have evolved from terrestrial ancestors who had returned to the seashore but this is no longer thought likely to be the case. In most lunged gastropods, gas exchange is effected in the mantle cavity via dense blood vessels in the cavity wall but in siphonariids a gill structure is present within the lung (this lung-gill combination makes the siphonariids particularly well suited for moving freely both above and below the water surface). The gill of siphonariids is quite similar to that of the sacoglossans, a group of herbivorous sea-slugs. Though it was long presumed that the lung-bearing gastropods belonged to a single clade, more recent molecular phylogenies have confused the issue (Kocot et al. 2013). The sacoglossans are likely to be close to the ancestry of lunged gastropods as a whole, but it is possible that siphonariids are more closely related to the sacoglossans than the other lung-bearers. It remains an open question whether the siphonariid combination of lung and gill represents an intermediate stage towards the vascular lung of the terrestrial forms, or whether siphonariids and other lung-bearers each evolved their pneumostome from close but distinct ancestors.

REFERENCES

Bouchet, P., J.-P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl & E. E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia 61 (1–2): 1–526.

Simone, L. R. L., & M. I. G. L. Seabra. 2017. Shell and body structure of the plesiomorphic pulmonate marine limpet Siphonaria pectinata (Linnaeus, 1758) from Portugal (Gastropoda: Heterobranchia: Siphonariidae). Folia Malacologia 25 (3): 147–164.

Crystal Butterflies of the Sea

All chains of life in the open ocean are ultimately dependent on plankton. Photosynthetic micro-plankton convert the energy of sunlight into their own stores that are in turn commandered by animal plankton through consumption and digestion. Both animal and photosynthetic plankton provide food sources for larger animals, both plankton and nekton, and even deeper dwelling organisms may take their sustenance from the rain of planktonic corpses settling from above. A wide range of animal lineages may be identified among oceanic plankton, one of the most prominent being the gastropod group known as the Thecosomata.

An orthoconch thecosomatan, Clio pyramidata, copyright Russ Hopcroft.

The Thecosomata are small gastropods, rarely exceeding a couple of centimetres in size at their largest. Many marine molluscs will spend at least part of their lives as planktonic larvae but relatively few mollusc groups have taken the route that the Thecosomata have, remaining part of the plankton through their entire life cycle. They maintain their position in the plankton by means of broad expansions of the foot on either side of the mouth, known as parapodia. The appearance and movement of these parapodia have given the Thecosomata the vernacular name of 'sea butterflies'. They also inspired the name Pteropoda ('wing-foot'), used for a clade that unites the Thecosomata with another group of planktonic gastropods, the Gymnosomata. Historically, many authors have questioned the association of the pteropods, in part because of the differing dispositions of the parapodia in the two component groups (Gymnosomata, commonly known as 'sea angels', have the wings of the parapodia located further back on the body rather than around the mouth). Nevertheless, more recent studies have corroborated pteropod monophyly (Klussmann-Kolb & Dinapoli 2006). The Thecosomata themselves fall between two major sublineages, known as the Euthecosomata and Cymbulioidea (or Pseudothecosomata). Members of the Euthecosomata have well-divided parapodia and the viscera are contained within a delicate, translucent, calcareous shell. In some euthecosomes such as the genus Limacina, this shell is coiled like that of other gastropods, but in others the shell has become straight and bilaterally symmetrical, being conical or globular with lateral projections. Recent phylogenetic analysis suggests that the straight-shelled sea butterflies may form a single monophyletic lineage known as the Orthoconcha* (Corse et al. 2013). In the Cymbulioidea, the parapodia are fused around the front of the animal to form a single swimming plate. Of the three families of cymbulioids, the Peraclidae have a calcareous shell as in the euthecosomes. The Cymbuliidae shed the larval calcareous shell over the course of their development and replace it with a pseudoconch, a hardened gelatinous, slipper-shaped structure that is still secreted by the mantle. In the third family, the Desmopteridae, the shell has been lost entirely.

*In the early days of invertebrate palaeontology, pteropod affinities were suggested for a number of groups of early Palaeozoic conical shells of uncertain affinities, such as the hyoliths and tentaculitoids. It is worth noting that, at the time, the pteropods themselves were often thought to represent a distinct molluscan class independent of the gastropods. Such proposals have long since fallen by the wayside. Not only is there nothing to connect such Palaeozoic forms with modern pteropods but the most superficial of resemblances in overall shape to certain Orthoconcha, but all indications now are that the Orthoconcha themselves did not evolve until some time in the Cenozoic (Corse et al. 2013), leaving a gap of some hundreds of millions of years between them and their erstwhile forebears.

Cymbulia peronii, copyright Vincent Maran.

Live sea butterflies feed on a wide range of organisms, including both micro-algae and other planktonic animals. The ancestral radula is reduced or lost and prey is captured by means of a mucous web, globular in euthecosomes and a funnel-shaped sheet in cymbulioids (Gilmer & Harbison 1986). This web may be absolutely gigantic relative to the animal itself, reaching up to two meters in diameter. While the web is extended, the sea butterfly does not swim actively but hangs suspended in the water column below, drawing food into the mouth by means of tracts of cilia on lobes of the foot. A further mucous array may trail away from the animal containing faecal particles and/or particles rejected as food; this may keep such particles from re-entering the feeding web. Should the animal be disturbed, the mucous web is rapidly ingested or abandoned before swimming away. Sea butterflies have commonly been referred to as suspension feeders but Gilmer & Harbison (1986) noted that a case could potentially be made for considering them as predators. Though micro-algae make up a large proportion of thecosome gut contents, the mucous web allows them to also capture active prey such as copepods that might have otherwise eluded them. It is possible that such prey is in fact more important overall for satisfying the sea butterfly's nutritional requirements. A number of sea butterflies possess brightly coloured mantle appendages that may lure active prey; the faecal trails may also assist in this way.

Limacina helicina, copyright Russ Hopcroft.

Sadly, any discussion of Thecosomata is forced to end on a tragic note. Recent increases in atmospheric carbon dioxide have lead to oceanic waters becoming more acidic than previously, which in turn reduces the concentration of dissolved carbonate. Because carbonate is a vital component of molluscan shells, ocean acidification compromises shell production. Studies of recent thecosome samples show that their shells have become thinner and more porous as acidification increases (Roger et al. 2012). If this trend continues, we may reach a point where shell secretion becomes impossible for these animals, leading to tragic consequences both for the thecosomes themselves and for the countless other organisms ecologically dependent on them. In recent years, concern has been expressed that ecological degredation may mean that we can no longer see butterflies flying in our gardens; their marine analogues are no less vulnerable.

REFERENCES

Corse, E., J. Rampal, C. Cuoc, N. Pech, Y. Perez & A. Gilles. 2013. Phylogenetic analysis of Thecosomata Blainville, 1824 (holoplanktonic Opisthobranchia) using morphological and molecular data. PLoS One 8 (4): e59439.

Gilmer, R. W., & G. R. Harbison. 1986. Morphology and field behavior of pteropod molluscs: feeding methods in the families Cavoliniidae, Limacinidae and Peraclididae (Gastropoda: Thecosomata). Marine Biology 91: 47–57.

Klussmann-Kolb, A., & A. Dinapoli. 2006. Systematic position of the pelagic Thecosomata and Gymnosomata within Opisthobranchia (Mollusca, Gastropoda)—revival of the Pteropoda. Journal of Zoological Systematics and Evolutionary Research 44 (2): 118–129.

Roger, L. M., A. J. Richardson, A. D. McKinnon, B. Knott, R. Matear & C. Scadding. 2012. Comparison of the shell structure of two tropical Thecosomata (Creseis acicula and Diacavolinia longirostris) from 1963 to 2009: potential implications of declining aragonite saturation. ICES Journal of Marine Science 69 (3): 465–474.

The Crossostomatinae of the Mesozoic

The hard shell of many molluscs has left them with an excellent fossil record, one with few rivals among other groups of organisms. As a result, we are aware of a great many molluscan lineages that have inhabited this planet in the past, only to fade away long before the present day. One such group is the gastropod subfamily Crossostomatinae.

Crossostoma specimen, from Szabó et al. (1993).

The Crossostomatinae were Mesozoic representatives of the vetigastropods, one of the major subdivisions of gastropods corresponding to what used to be referred to as the archaeogastropods. Vetigastropods are primarily marine (off the top of my head, I can't think of any that are found in freshwater or terrestrial habitats, though I'm happy to be corrected) and crossostomatines were no exception. The classification of vetigastropods has tended to be rather unsettled but crossostomatines were definitely part of the lineage that includes the modern top shells (Trochidae) and cat's-eyes (Turbinidae), recognised as the superfamily Trochoidea in the recent synoptic classification of Bouchet et al. (2017). Within this lineage, the crossostomatines belong to the group of families possessing a calcareous operculum (sometimes treated as a separate superfamily Turbinoidea, but the significance of the calcareous vs horny operculum division in the trochoids seems to be the subject of debate). In recent treatments, the Crossostomatinae have been included within the family Colloniidae, characterised by the lack of a nacreous layer on the inside of the shell (Monari et al. 1996).

In general, crossostomatines were small shells with a smooth outer surface and broadly rounded whorls. They varied in shape from forms resembling modern cat's-eyes to lower-coiling, almost planispiral forms. A notable feature of the group is a tendency for the top of the aperture to be filled by a callus so the aperture appears almost perfectly circular. Other modifications of the mature shell opening are also common: Crossostoma, for instance, has the outer lip strongly thickened (Knight et al. 1960) whereas the final whorl of Adeorbisina turns away slightly from the regular coiling axis so that in top-down view the shell appears to bulge outwards before the terminus (Szabó et al. 1993).

Though they persisted through most of the Mesozoic, the number of known crossostomatine genera does not appear to be large. They seem to be associated with hard-ground deposits (Conti & Szabó 1987) so it is possible the group was more diverse in high-energy environments (organisms living in such environments, for instance along rocky shores, tend not to get preserved in the fossil record because their remains are broken up by wave action). It is possible that their lineage did not truly go extinct in the Mesozoic: Szabó et al. (1993) allude to the possibility of crossostomatines being ancestral to the subfamily Colloniinae, members of which may have survived to the Pliocene. Nevertheless, the Colloniidae as a whole did not survive to the present day, and it seems the line of the crossostomatines may have entirely passed from this Earth.

REFERENCES

Bouchet, P., J.-P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl & E. E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia 61 (1–2): 1–526.

Conti, M. A., & J. Szabó. 1987. Comparison of Bajocian gastropod faunas from the Bakony Mts. (Hungary) and Umbria (Italy). Annales Historico-Naturales Musei Nationalis Hungarici 79: 43–59.

Knight, J. B., L. R. Cox, A. M. Keen, R. L. Batten, E. L. Yochelson & R. Robertson. 1960. Gastropoda: systematic descriptions. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1: Mollusca—General Features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—General Features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I169–I331. Geological Society of America, and University of Kansas Press.

Monari, S., M. A. Conti & J. Szabó. 1996. Evolutionary systematics of Jurassic Trochoidea: the family Colloniidae and the subfamily Proconulidae. In: Taylor, J. D. (ed.) Origin and Evolutionary Radiation of the Mollusca pp. 199–204. Oxford University Press: Oxford.

Szabó, J., M. A. Conti & S. Monari. 1993. Jurassic gastropods from Sicily; new data to the classification of Ataphridae (Trochoidea). Scripta Geologica, Special Issue 2: 407–416.

Digging for Tellina

When I was a child, a large part of my extended family would gather over the Christmas period to park their tents and caravans alongside the estuary downhill from my great-grandparents' house (in the usual way of these things, my memory has these summer camping periods lasting for ages, but I don't think they could have been longer than a week or so). While we were there, I would spend a fair chunk of the day looking for the wildlife that inhabited the slightly muddy estuary beach. Among these were various bivalves whose shells could be found littering the shoreline, or which might be found by digging in the sand at low tide. Close to the surface were New Zealand cockles Austrovenus stutchburyi (not actually a direct relative of the English cockle but a member of the Veneridae family that has adopted a similar body form). A little deeper were pipis and tuatuas. And a little deeper again were the flat, slender shells of Tellina.

Thin tellin Tellina tenuis, copyright S. Rae.

I should note that Tellina species are not really deep burrowers in the grand scheme of things, generally only embedding themselves about one to three centimetres below the surface, but again I must ask that you make allowances for childhood memories. Their low profile and weakly inflated shells also make them fast diggers so they were probably able to elude most casual explorations. Like most subsurface bivalves, Tellina species are sediment feeders. Their usual aspect is lying horizontally beneath the sediment, extending their long, unfused siphons to the surface to gather detritus (Ujino & Matuskuma 2010; the shells of Tellina are usually twisted slightly to one side at the end to facilitate the siphons' passage). Even if you've not seen the Tellina animals themselves, you may have seen the radiating trails made by the siphons as they extend along the top of the sediment.

Sunrise tellin Tellina radiata, copyright James St. John.

Tellina is an extremely diverse genus, with species found worldwide and recognised through the entirety of the Mesozoic (Moore 1969). These species vary greatly in appearance, with shells varying from almost completely smooth to strongly ornamented, and from subcircular to quite elongate. It should therefore come as little surprise that numerous attempts have been made to divide Tellina between various subgenera and genera but issues such as homoeomorphy in Tellina's evolution (where distinct lineages have converged on similar body forms) have lead to disagreement over the best system to adopt. In 1934, the malacologist A. E. Salisbury complained that, "The number of genera, subgenera, and sections into which the Tellinidae has been cut up is getting somewhat appalling; the list of names is still increasing every year, and, if every variation of form is magnified, it is quite possible to go on until at last each species becomes the representative of a different genus and each variety that of a subgenus" (of course, as seems almost inevitable when one encounters complaints of this kind, Salisbury himself then proceeds to add to the tally of generic names in that same paper). Though I suspect most modern malacologists would probably disagree with the extremely broad concept of Tellina advocated by Salisbury, the question of how best to handle the genus taxonomically remains an open one.

REFERENCES

Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology pt N. Mollusca 6. Bivalvia vol. 2. The Geological Society of America, Inc., and The University of Kansas.

Salisbury, A. E. 1934. On the nomenclature of Tellinidae, with descriptions of new species and some remarks on distribution. Proceedings of the Malacological Society of London 21: 74–91.

Ujino, S., & A. Matsukuma. 2010. Inverse life positions of three species in the genus Cadella (Bivalvia: Tellinidae). Molluscan Research 30 (1): 25–28.

The Age of the Ceratites

The ammonites are unquestionably one of the most famous groups of fossil mollusks, indeed of fossil invertebrates in general. Even those who have little consciousness of the fossil world might be expected to have a vague mental picture of a coiled shell housing a squid-like beast. But ammonites are far from being the only group of shelled cephalopod known from the fossil record. And though ammonites may have dominated the marine environment during the Jurassic and Cretaceous periods, during the preceding Triassic period they were overshadowed by another such group, the ceratites.

Reconstruction of Ceratites spinosus, from Klug et al. (2007).

The ceratites of the order Ceratitida (or suborder Ceratitina, depending on how you've tuned your rank-o-meter today) were close relatives of the ammonites, each deriving separately from an earlier cephalopod group known as the prolecanitids. The earliest forms regarded as ceratites appeared during the mid-Permian, though the exact dividing line between prolecanitid and ceratite seems to be somewhat arbitrary (as, indeed, is only to be expected with a well-known historical lineage). During the remainder of the Permian their diversity remained fairly subdued. When marine life was hit with the cataclysmic upheaval that was the end-Permian extinction, two lineages of ceratites managed to squeak through, together with a single other prolecanitid lineage that would give rise to the ammonites during the ensuing Triassic. With most of their competitors thus eliminated, ceratite diversity expanded rapidly.

Externally, the shells of ceratites and ammonites were very similar, and without knowing their evolutionary context one would be hard-pressed to tell one from the other. Most ceratite shells formed the typical flat spiral one associates with ammonoids, with different species being variously evolute (with successive coils lying alongside the previous one) to involute (outer coils overlapping and concealing the inner ones), and cross-sections varying from narrow and lenticular to broad and low (Arkell et al. 1957). One later Triassic family, the Choristoceratidae, had shells that began as an evolute coil but became uncoiled or straightened in later stages. Another Upper Triassic group, the Cochloceratidae, had turreted shells that might externally be mistaken for those of a gastropod.

Ceratites dorsoplanus, showing ceratitic sutures, copyright Hectonichus.

Internally, ceratites and ammonites often differed in the structure sutures, the lines formed by the join between the outer shell and the septa dividing the internal chambers. In ammonoids as a whole, the sutures are variously curved back and forth on the inside of the shell, with those parts of the suture going forwards (towards the shell opening) forming what are called saddles and those going backwards (away from the opening) forming lobes. In most ceratites, the sutures more or less form a pattern that is known (appropriately enough) as ceratitic: the saddles are simple and not future divided, but the lobes have multiple smaller digitations. In some later taxa, the sutures became goniatitic (with both saddles and lobes simple, secondarily similar to those found in earlier ammonoids) or ammonitic (with both saddles and lobes subdivided, the pattern more commonly associated with ammonites).

Our knowledge of the soft anatomy of ceratites remains limited. We know that they possessed an anaptychus (a leathery plate at the front of the body that may have functioned as an operculum, as I described in an earlier post). Known radulae have fairly simple, slender, undifferentiated teeth (Kruta et al. 2015) so they were probably micro-predators or planktivores in the manner of most ammonites. A black, bituminous layer sometimes preserved against the inside of the shell in the body cavity may represent the remains of the dorsal mantle. Similarity between this layer and the dorsal mantle of nautilids lead Klug et al. (2007) to infer the presence of a non-mineralised hood in ceratites, though I wonder how the presence of a hood would relate to an anaptychus. Conversely, Doguzhaeva et al. (2007) interpreted the black layer as the remains of ink from a ruptured ink sac.

Assemblage of Arcestes leiostracus, copyright Lubomír Klátil.

Ceratites were to remain the ecological upper hand throughout the course of the Triassic. Though ammonites (represented by the phylloceratidans) were not uncommon during this period, their diversity remained consistently lower. However, the end of the Triassic was marked by a spike in global temperatures and ocean acidification, generally regarded as connected to the volcanic rifting activity that marked the beginning of formation of the Atlantic Ocean (Arkhipkin & Laptikhovsky 2012). Of the two ammonoid lineages, only the ammonites survived into the Jurassic; the ceratites were wiped out. Whether some aspect of ammonite biology made them better suited to survive the stresses of global climate change, or whether their survival was a question of simple dumb luck, seems to be an open question. Nevertheless, with the ceratites out of the picture, the way was open for the ammonites to become the lords of the Mesozoic ocean.

REFERENCES

Arkell, W. J., B. Kummel & C. W. Wright. 1957. Mesozoic Ammonoidea. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt L. Mollusca 4. Cephalopoda: Ammonoidea pp. L80–L465. Geological Society of America, and University of Kansas Press.

Arkhipkin, A. I., & V. V. Laptikhovsky. 2012. Impact of ocean acidification on plankton larvae as a cause of mass extinctions in ammonites and belemnites. Neues Jahrbuch für Geologie und Paläontologie—Abhandlungen 266 (1): 39–50.

Doguzhaeva, L. A., R. H. Mapes, H. Summesberger & H. Mutvei. 2007. The preservation of body tissues, shell, and mandibles in the ceratitid ammonoid Austrotrachyceras (Late Triassic), Austria. In: Landman, N. H., R. A. Davis & R. H. Mapes (eds) Cephalopods Past and Present: New Insights and Fresh Perspectives pp. 221–238. Springer.

Klug, C., M. Montenari, H. Schulz & M. Urlichs. 2007. Soft-tissue attachment of Middle Triassic Ceratitida from Germany. In: Landman, N. H., R. A. Davis & R. H. Mapes (eds) Cephalopods Past and Present: New Insights and Fresh Perspectives pp. 205–220. Springer.

Kruta, I., N. H. Landman & K. Tanabe. 2015. Ammonoid radula. In: Klug, C., et al. (eds) Ammonoid Paleobiology: From Anatomy to Ecology pp. 485–505. Springer: Dordrecht.

Moles, Tortoises, Calves and Cowries

The cowries of the family Cypraeidae are one of the most readily recognisable groups of tropical and subtropical shells. Their distinctive shape (with no spire and a long narrow aperture running the length of the shell) and highly polished appearance are guaranteed to catch the eye (to the extent that one species, the money cowry Monetaria moneta, famously has a history of being used as a form of currency in many regions around the Indian Ocean). Though there are a large number of cowry species found around the world, they tend to be similar enough to each other that, until relatively recently, many authors would place all within a single genus Cypraea. This approach has fallen out of fashion in more recent years and, indeed, the current favoured approach divides the family between several subfamilies. One such subgroup is the subfamily Luriinae.

Live mole cowry Talparia talpa, copyright Juuyoh Tanaka.

In a phylogenetic analysis of the cowries, Meyer (2003) recognised the Luriinae as including two tribes, the Luriini and Austrocypraeini. This concept of Luriinae was essentially based on molecular phylogenetic analysis though it was also corroborated by radular morphology (with a reduced shaft on all teeth). The underside of the shell in luriines is mostly smooth with the 'teeth' being restricted to alongside the aperture. As in other cowries, the mantle is widely extended and mostly covers the shell in life (this is how cowry shells stay so shiny). In most luriines, the mantle is covered by warty papillae. In species of the genus Luria these warts are obsolete (Schilder 1939) but they are particularly prominent in the Indo-west Pacific mole cowry Talparia talpa. Members of the Luriinae vary greatly in size: the Pacific Annepona mariae is only a centimetre or two in length but the tortoise cowry Chelycypraea testudinaria of the Indian and western Pacific Oceans grows to ten centimetres or more. Species of Luriini have shells that are banded in coloration, with three or four broad dark bands divided by narrower light bands. The Austrocypraeini are most commonly marked with brown speckles or blotches on a pale background; these blotches may be irregular as in Chelycypraea testudinaria or more regularly rounded as in Annepona mariae. The calf cowry Lyncina vitellus of the Indo-Pacific is marked with white spots on a brown background, and some species or forms of Austrocypraeini may have coloration patterns more like the banded arrangement of Luriini.

Lynx cowry Lyncina lynx, copyright Patrick Randall.

My impression is that species of Luriinae tend to be mostly nocturnal, sheltering in crevices in coral reefs during the day before emerging to feed at dusk (the name of the aforementioned mole cowry is, I suspect, more likely to refer to its appearance in some way than to any actual burrowing habit). Though I haven't (though a cursory search, at least) found any reference to species of Luriinae in particular being endangered, a number of cowries in general have been threatened by overcollecting for their shells. Certainly, luriines would be subject to the broad range of threats that currently hang over coral reefs and their inhabitants anywhere in the world.

REFERENCES

Meyer, C. P. 2003. Molecular systematics of cowries (Gastropoda: Cypraeidae) and diversification patterns in the tropics. Biological Journal of the Linnean Society 79: 401-459.

Schilder, F. A. 1939. Die Genera der Cypraeacea. Archiv für Molluskenkunde 71 (5–6): 165–201.

Slippers on the Coast

The 'limpet' form is something that has evolved numerous times among gastropods, as various lineages of marine snail converted to a more or less unwhorled shell and low profile. In many cases, the evolution of the limpet form is also associated with high energy environments, the ability to nestle against rocks helping the gastropod maintain its grip against the surge of the waves. In the modern world, the most diverse and familiar lineage of limpets is that including the common limpets of the genus Patella and their relatives, but there also many independent lineages to be found. One of these is the slipper limpets of the genus Crepidula.

Various views of shell of Crepidula onyx, copyright H. Zell.

Slipper limpets get their vernacular name from the shape of their shell, whose more or less oval shape together with a jutting internal horizontal shelf (the septum) at one end gives the overall impression of a carpet slipper. About forty species (including fossils) of Crepidula are currently recognised worldwide. Species recognition has historically been difficult owing to their simple form and tendency to vary according to the environment in which they mature, but Hoagland (1977) identified a number of key distinguishing features such as disposition and shape of the muscle scars, features of the septum, and conformation of the apical beak of the shell. In contrast to the grazing common limpets, slipper limpets are filter feeders using their gill to capture micro-algae from the water column. They are protandric hermaphrodites, beginning their life as males but maturing into females as they grow. Eggs are brooded under the shell when first produced; in some species, the eggs are subsequently released to hatch into planktonic larvae whereas other species produce fewer eggs but retain them until the young have developed to the crawling stage. For instance, two species found on the east coast of North America that are very similar in adult appearance and have been confused historically differ in that Crepidula ustulatulina, found around Florida and the Gulf of Mexico, produces free-living larvae whereas the more northerly C. convexa does not.

Mating stack of Crepidula fornicata, copyright Dendroica cerulea.

The most renowned species of slipper limpet is the northern Atlantic Crepidula fornicata. This species was originally native to the eastern coast of North America but was accidentally imported to Europe in the late 1800s in association with oysters being transported as stock for farming (Blanchard 1997). In the subsequent years, C. fornicata has become increasingly widespread on the shores of Europe, and is often a significant fouling pest for oyster farms. It has also been introduced to even further flung locations such as Japan and Washington State. Crepidula fornicata is famed for its habit of forming high mating stacks with several smaller males living permanently on the dorsal surface of larger females. If the female of a stack dies, the largest male may develop into a female. Not all Crepidula species form such stacks: in some, just two or three individuals may form a temporary cluster when mating.

Historically, Crepidula has been distinguished from other genera in the limpet family Calyptraeidae by their posterior shell apex and flat septum (other calyptraeid genera may have a cone-shaped shell and/or cup-shaped septum). However, a molecular analysis of the family by Collin (2003) found that species of Crepidula sensu Hoagland (1977) did not form a single clade within Calyptraeidae, and the genus' prior members are now divided between at least four genera. While these genera may be distinguishable using features of the soft anatomy, they are almost indistinguishable from the shells alone.

REFERENCES

Blanchard, M. 1997. Spread of the slipper limpet Crepidula fornicata (L. 1758) in Europe. Current state and consequences. Scientia Marina 61 (Suppl. 2): 109–118.

Collin, R. 2003. Phylogenetic relationship among calyptraeid gastropods and their implications for the biogeography of marine speciation. Systematic Biology 52 (5): 618–640.

Hoagland, K. E. 1977. Systematic review of fossil and recent Crepidula and discussion of evolution of the Calyptraeidae. Malacologia 16 (2): 353–420.

The Solemyoida: A Taste for Sulphur

Atlantic awning clam Solemya velum, copyright Guus Roeselers.

The small bivalves that make up the Solemyoida were long a mystery, ecology-wise. Though they have a long history, potentially going back as far as the Ordovician (Cope 2000), they are not known to have ever been diverse, and only just over fifty species are known from the modern fauna. Living solemyoids are divided between two very distinct families that probably diverged near the origin of the group. The Solemyidae, awning clams, have relatively long shells that gape at each end, no teeth in the dorsal hinge, and tend to have an unusually thick periostracum (the overlying layer of horny proteinaceous matter that covers the outside of the mineral shell). They generally live in burrows buried deep in sediment. The Nucinellidae are a group of minute clams with an average length of about half a centimetre that are mostly found in deep waters, generally not buried quite so deep in the mud as the awning clams. They have a less elongate shell than the Solemyidae that does not gape and simple peg-like teeth in the hinge. What the two families do share is a markedly reduced gut and feeding appendages that initially caused much speculation about what exactly they were feeding on.

Nucinella sp. with foot extended, from Taylor & Glover (2010). Scale bar equals 1 mm.

The answer, as it turns out, was that they were not exactly 'feeding' on much, if anything. Solemyoids have relatively large gills that provide a comfortable living place for sulphur-oxidising bacteria, sheltered from the outside world while the host clam keeps up a continuous flow of water through its burrow from above the sediment surface. In return, the bacteria fix hydrogen sulphide rising from the underlying mud to provide both themselves and their host with nutrients. In this way, solemyoids have largely been able to get by without actively eating for close to 450 million years, achieving something the likes of Jasmuheen can only dream of.

REFERENCE

Cope, J. C. W. 2000. A new look at early bivalve phylogeny. In: Harper, E. M., J. D. Taylor & J. A. Crame (eds) The Evolutionary Biology of the Bivalvia pp. 81–95. The Geological Society: London.

Hypsogastropods: Gastropods on High

Historically, the classification of molluscs has been a challenging prospect. Early researchers focused almost entirely on the shell which provided a somewhat limited range of characters with a definite possibility for convergence. Over time, more attention came to be paid to features of the soft anatomy but that required access to freshly collected material that might be difficult or impossible to obtain. As such, it has only been in the last few decades that a well-structured classification for many molluscan groups has begun to develop, and even now many significant uncertainties remain.

Common periwinkles Littorina littorea, a pretty typical hypsogastropod, copyright Fritz Geller-Grimm.

Until maybe the late 1990s, gastropods were primarily classified using a heavily grade-based system that was established in the 1930s. Gastropods were divided between three subclasses: the torted, gill-breathing prosobranchs, the untorted opisthobranchs, and the lung-breathing pulmonates. Prosobranchs were in turn divided into three main groups whose names directly reflected the 'level' of evolution at which they were supposed to sit: the archaeogastropods, the mesogastropods and the neogastropods. Many of these subdivisions were implicitly assumed to be ancestral to others. As the philosophical underpinnings of biological classification came to favour recognition of monophyletic taxa, it was obvious that such a system had to change. The prosobranchs and archaeogastropods both faded away as formal taxa. A major clade uniting the neogastropods and most of the mesogastropods came to be recognised as the caenogastropods. And while many questions still remain about relationships within the caenogastropods, most recent analyses have agreed in supporting a clade that was dubbed the Hypsogastropoda by Ponder & Lindberg (1997).

False cowrie Dentiovula dosruosa, copyright Nick Hobgood.

The prefix 'hypso-' means 'high' and was chosen because this clade corresponded to a group that had previously been known as the 'higher' caenogastropods (including the neogastropods and a fair chunk of the 'mesogastropods'). Hypsogastropods include many of the best known marine gastropods, such as whelks, periwinkles, moon snails, cones, cowries, conches and doubtless a ton of other things beginning with C (they also include freshwater and terrestrial forms but these are mostly minute and lack the public image of their marine relatives). They are ecologically diverse, including grazers, detritivores, filter feeders, predators and even parasites. The violet snails of the genus Janthina are planktonic, using a raft of bubbles to float on the water's surface so they can feed on Portuguese men-of-war. The similarly pelagic heteropods of the superfamily Pterotracheoidea have the foot extended and flattened to form a fin for active swimming.

Paraspermatozoon of violet snail Janthina, from Buckland-Nicks (1998). The arrow indicates the much smaller euspermatozoa attached to the tail.

Among the characters originally cited by Ponder & Lindberg (1997) as uniting the hypsogastropods were features of the spermatozoa. Most hypsogastropods have vermiform paraspermatozoa, sterile sperm cells that are released by the male together with the functioning euspermatozoa. The function of the paraspermatozoa seems to warrant further study. In some cases they may actively assist in the transport of the euspermatozoa; for instance, in violet snails a large number of euspermatozoa will be attached to a single super-sized paraspermatozoon able to swim harder and faster than any of the smaller cells could do on their own. In others, however, the two sperm cell types are not directly associated. It is possible that the paraspermatozoa act as a nuptial gift, providing nutrients to the female as a reward for mating, or that they somehow function to suppress sperm cells from any other males the female might made with (Buckland-Nicks 1998). Other synapomorphies of the clade include an external penis located behind the right cephalic tentacle, and statocysts (balance organs) each containing a single large statolith (Simone 2011).

Relationships within the Hypsogastropoda remain more poorly supported. Most researchers have agreed that the traditionally recognised neogastropods represent a clade united by numerous features, many of them related to the digestive system. The 'mesogastropods' included in the Hypsogastropoda mostly possess a taenioglossan radula with seven teeth in each row. In neogastropods, the number of teeth becomes more varied and the teeth themselves become modified so that the lateral teeth are strongly distinct in form from the central tooth. Some of these neogastropod modifications have been discussed in earlier posts on this site. A number of recent analyses have further associated the neogastropods with 'mesogastropod' taxa such as cowries and tun shells that they resemble in possessing an inhalent siphon forming a groove at the front of the shell (Simone 2011). A number of the remaining 'mesogastropods', such as the periwinkles of the Littorinidae and the Rissoidae, have been united by molecular analyses into a group that has been labelled the 'asiphonate clade' or the 'GC group' (the latter name chosen by Colgan et al., 2007, in reference to a particular genetic sequence motif). This clade is less universally recovered, however, and the scope for further investigation certainly remains.

REFERENCES

Buckland-Nicks, J. 1998. Prosobranch parasperm: sterile germ cells that promote paternity? Micron 29 (4): 267–280.

Colgan, D. J., W. F. Ponder, E. Beacham & J. Macaranas. 2007. Molecular phylogenetics of Caenogastropoda (Gastropoda: Mollusca). Molecular Phylogenetics and Evolution 42: 717–737.

Ponder, W. F., & D. R. Lindberg. 1997. Towards a phylogeny of gastropod molluscs: an analysis using morphological characters. Zoological Journal of the Linnean Society 119: 83–265.

Simone, L. R. L. 2011. Phylogeny of the Caenogastropoda (Mollusca), based on comparative morphology. Arquivos de Zoologia 42 (4): 161–323.

The Australian Panda

The world is home to an incredible diversity of snails: there are literally thousands of species, some widespread, some restricted to very small areas. Most, as is the usual way of things, are tiny, barely discernible without very close examination. But then there are some that are very much not—such as the giant panda snail Hedleyella falconeri.

Giant panda snail Hedleyella falconeri, from the Queensland Museum.

Giant pandas are found on rainforest floors in northern New South Wales and southern Queensland, in a range spanning from the Barrington Tops to the D'Aguilar Range. They are Australia's largest land snail, reaching nine centimetres in diameter, about the size of a tennis ball. They have globose, reddish brown shells with a spiral pattern of darker broken bands. Their name bears no relation to any Asian mammals; instead, they were gifted the genus name Panda as a derivation from the Latin word pandere, meaning to stretch out or extend, presumably in reference to their size. The genus name later had to be changed but it survives in the vernacular (as well as in the name of a closely related genus of slightly smaller snails, Pygmipanda).

Panda snails are nocturnal, spending the days in moist spots such as buried in leaf litter or hidden under logs. At night, they roam in search of fallen leaves and fungal fruiting bodies. A study of giant pandas that tracked individual snails found that they wandered more or less randomly, up to about 20 metres over the course of a night, without returning to any particular 'home' site.

A demonstration of the size of H. falconeri, from Pollinator Link.

Like many other land snails, giant pandas are hermaphrodites, able to both fertilise and be fertilised during mating. They may have the largest sperm cells of any mollusc, each over a millimetre in length. Mating usually happens on a February night though observations in captivity suggest it may happen whenever consitions are suitable. The snail lays its hard-shelled eggs in batches of fifteen to twenty in a burrow in the leaf litter*. To continue with the theme, these are also realtively gigantic: close to two centimetres in diameter, comparable in size to those of a small bird. The young snails hatch at about 15 mm in size (I haven't found any reference to the eggs being tended by the parent in any way) and grow slowly. By the time they reach a year in age, they may not have even doubled in size, and it presumably takes several years for them to reach their full extent.

*So it turns out Paazan was right after all: pandas do hatch from eggs.

Pandas are not uncommon within their range and are not generally regarded as a conservation concern. Indeed, their nomadic habits have led to the suggestion that they may be well disposed to re-colonising regenerating forest (Parkyn & Newell 2013). Nevertheless, recent years have seen increasing fragmentation of suitable habitat within their range and this, together with their slow growth rate, means that I can easily imagine them becoming vulnerable if conditions deteriorate. I would hope that appropriate action is taken to ensure that there should always be giant pandas in eastern Australia.

REFERENCE

Parkyn, J., & D. A. Newell. 2013. Australian land snails: a review of ecological research and conservation approaches. Molluscan Research 33 (2): 116–129.

Alvania

It's a general rule with organisms that species diversity increases as size decreases (at least down to about the millimetre range, below which things get a bit more complicated). That's certainly the case with molluscs, whose range clearly favours the tiny.

Alvania cimex, copyright Alboran Shells.

Alvania is a cosmopolitan genus of marine gastropods, found in most parts of the world except the Antarctic and sub-Antarctic (Ponder 1984). The average Alvania species is less than five millimetres in total length, and other members of the family they belong to, the Rissoidae, are similarly wee. The shell of Alvania species varies from elongate-conical to more squatly conical in shape, and generally has a sculpture of both axial and spiral ridges. In some species the axial and spiral ribs are both similarly prominent; in others, the spiral ridges are more strongly developed.

Rissoids may be found crawling on seaweed or sheltered amongst stones or other rubble. Alvania species seem to be more likely to be found in the latter habitat than the former. Alvania have a smaller mucous gland on the rear of the foot than species of Rissoa, a related genus that is more likely to be found on the weeds. The mucus produced by this gland assists rissoids in clinging to their substrate or the surface film, and its reduction in Alvania is presumably connected to their preference for the low life. Rissoids are grazers on microalgae or deposit feeders; those species found on seaweeds will feed on diatoms and the like growing over the seaweed rather than on the seaweed itself. Among European species, A. punctura is known to selectively pick out diatoms and dinoflagellates from among detritus when feeding whereas A. jeffreysi may be less discriminating in what it swallows.

Alvania subcalathus, copyright H. Zell.

The greater number of Alvania species are planktotrophic as larvae, and as described in some of my previous posts on turrids, their shells have protoconches to match. Nevertheless, the genus also includes some direct-developing species with fewer protoconch spirals. The Mediterranean species A. cimex and A. mammillata are almost indistinguishable when mature except by features of the shell apex, which is broader with fewer spirals to the protoconch in the latter (Verduin 1986). If A. mammillata is a direct developer while A. cimex has a planktotrophic larva, it would tally up with the situation elsewhere seen among turrids.

REFERENCES

Ponder, W. F. 1984. A review of the genera of the Rissoidae (Mollusca: Mesogastropoda: Rissoacea). Records of the Australian Museum Supplement 4: 1–221.

Verduin, A. 1986. Alvania cimex (L.) s.l. (Gastropoda, Prosobranchia), an aggregate species. Basteria 50: 25–32.

Belemnitellidae: Reaching the End of an Era

Fossil cephalopods have featured on this site numerous times in the past. I've talked about nautiloids, I've talked about ammonoids. But one group of cephalopods that I haven't given that much time to to date is the group including the majority of living species: the coleoids. In coleoids, the ancestral cephalopod shell has become reduced and internalised (one group, the octopods, has lost the shell entirely) so it should not come as much of a surprise that their fossil record is more limited than that of other cephalopod groups. Nevertheless, the coleoid lineage does include at least one group known from an abundant fossil record: the Mesozoic belemnites.

Fossil guard of Belemnitella americana, from here, in ventral view with the ventral opening of the alveolus visible as a longitudinal fissure.

Belemnites were a significant part of the marine fauna during the Jurassic and Cretaceous. Externally, they were similar in overall appearance to modern squid, as demonstrated by rare finds of specimens with preserved soft body parts. However, whereas squid have the internal shell reduced to the thin, non-calcified pen, belemnites possessed a well-developed internal shell. The posterior end of the shell was a solid, bullet-shaped rostrum or guard, in front of which was a chambered section known as the phragmocone. Being completely calcified, the rostrum of a belemnite was readily preserved and isolated rostra make up the greater part of the belemnite fossil record (the more delicate phragmocone was less likely to survive the fossilisation process). Different belemnite taxa may be recognised by variations in rostral shape and structure and several families are recognised from various parts of the Mesozoic. The latest surviving belemnite family was the Belemnitellidae.

Reconstruction of a typical belemnite showing the life position of the shell (not actually visible externally), copyright Charlotte Miller.

Belemnitellids are characterised by rostra with an alveolus or pseudoalveolus (an anterior conical depression into which the phragmocone would have originally fit) that opens through a ventral fissure, and longitudinal dorsolateral impressions (Christensen 1997, 2002). The earliest belemnitellids appeared during the early part of the Cenomanian epoch of the Cretaceous period, about 98 million years ago (Christensen 1997). They reached their peak of diversity during the lower Santonian, about 86 million years ago, but they persisted in one form or another right up to the end of the Cretaceous, eventually disappearing in the giant colossal environmental clusterbump that brought that period to a close. Throughout their history, belemnitellids were restricted to the Northern Hemisphere, being known from what is now Europe and North America. By the late Cretaceous, of course, the modern continents were definitely approaching their modern forms and positions but were not quite there yet. For a large chunk of this period, sea levels were higher than they are now so much of modern Europe and the central part of North America were covered by shallow seas. The North Atlantic was still a developing prospect; it looks like there still would have been something of a continental shelf connection between what is now its two sides during the Santonian. This continental shelf and shallow seas was the habitat of the belemnitellids; it appears that they never made the shift to deeper waters. Hence their geographical restriction as the deeper Tethys Ocean still separated Eurasia from Africa and India. When the belemnitellids first appeared, these deeper Tethys waters were home to another belemnite family, the Belemnopseidae (the belemnitellids would make some inroads to the northern coast of the Tethys after the belemnopseids became extinct during the Cenomanian but never anything extensive). A third family, the Dimitobelidae, occupied the position of the belemnitellids in the Southern Hemisphere.

The earliest belemnitellids are known from northern Europe where they presumably evolved from belemnopseid ancestors (Christensen 1997). There do appear to be some questions about whether the belemnitellids as currently recognised represent a monophyletic group or whether the belemnopseid invasion happened more than once. However it be, northern Europe would remain the centre of diversity for the group. They reached North America during the Turonian, about ninety million years ago, but for whatever reason never quite diversified there as much as they did in their homeland. During the Campanian, from about 83 million years ago, there is a period of close to ten million years where belemnitellids disappeared from the North American fossil record entirely. Presumably this represents a local extinction followed by a later recolonisation from Europe.

North American belemnitellids also failed to quite make it to the end of the Cretaceous, dropping out about one or two million years earlier. In Europe, however, three species are known from the period's closing hours. Though not at their earlier levels of success, belemnitellids were diversifying right to the end: the distinctive Fusiteuthis polonica appears well within the last couple of million years. Nevertheless, there was precious little from that part of the world at that time in history that did not have the word DOOM stamped firmly on its forehead and belemnitellids were no exception. Their passing marked the final end of the belemnite hegemony and the stage was now completely clear for the more modern coleoids to rise.

REFERENCES

Christensen, W. K. 1997. The Late Cretaceous belemnite family Belemnitellidae: taxonomy and evolutionary history. Bulletin of the Geological Society of Denmark 44: 59–88.

Christensen, W. K. 2002. Fusiteuthis polonica, a rare and unusual belemnite from the Maastrichtian. Acta Palaeontologica Polonica 47 (4): 679-683.

A Neogene Moon

Back when I was a young lad, some time not so long after the end-Cretaceous extinction, we often spent part of the Christmas holidays camped at the estuary beach-front below my great-grandparents' house. Among the things I recall doing there was going out at low tide with my great-grandmother to dig up cockles for lunch. The New Zealand cockle Austrovenus stutchburyi is not an immediate relative of the bivalves of the family Cardiidae known as cockles in Europe but a member of a different bivalve family, the Veneridae. Venerids are shallowly burrowing bivalves that generally live buried below the sand or mud just shallowly enough to extend their short siphons to the surface for filter-feeding.

Dorsal (left) and lateral views of Marama hurupiensis, from Beu & Maxwell (1990).

Because they live pre-buried in this manner in fairly low-energy habitats, venerids have an excellent fossil record. Marama is a fossil genus of a dozen species of venerids known only from New Zealand and Tasmania (Beu & Maxwell 1990; Beu 2012). The genus was first recognised by Marwick (1927) who divided it between two subgenera, Marama sensu stricto and Hina. Both names derive from Maori names for the moon, presumably in reference to the clams' appearance. Marama species are similar in overall appearance to the modern New Zealand cockle, the primary defining characters of the genus reflecting features of the shell hinge. These include the presence of a moderate anterior lateral tooth or tubercle in the left valve. The size of the species varies from the small M. tumida, a bit less than two centimetres in length, to the relatively large M. hurupiensis which reaches six centimetres in length. The shells are sculpted with concentric lamellae, varying from fine and very dense in M. tumida to strong and widely spaced in M. pristina to weak and sparse in M. ovata.

Marama species are known from the Kaiatan to the Nukumaruan stages in the New Zealand stratigraphic system, corresponding to the ealy Late Eocene to the late Pliocene/earliest Pleistocene in the international stratigraphic divisions. Many regions of the world have their own local stratigraphic divisions that may be used in preference to the glocal system for various reasons. In some cases, this may be because of difficulties in correlating the local geological record to global events. There may not be suitable resources preserved for calculating a deposit's absolute age, or a geographically isolated region may lack fossils of cosmopolitan index species. As a result, it may be possible to recognise temporally successive biotas in a region's palaeontological record without being able to tell for sure whether a given biota is (for instance) Eocene or Oligocene. Alternatively, because stratigraphic divisions are commonly based on biotic turnovers such as mass extinctions, the major local biotic events may not exactly line up with the global average (for instance, the characteristic biota of a given geological period may have persisted longer in one region than it did in another). In the case of the New Zealand palaeontological record, Marama was one of a number of molluscan genera that became extinct towards the end of the Nukumaruan in relation to cooling temperatures representing the onset of the Pleistocene ice ages.

REFERENCES

Beu, A. G. 2012. Marine Mollusca of the last 2 million years in New Zealand. Part 5. Summary. Journal of the Royal Society of New Zealand 42 (1): 1–47.

Beu, A. G., & P. A. Maxwell. 1990. Cenozoic Mollusca of New Zealand. New Zealand Geological Survey Paleontological Bulletin 58: 1–518.

Marwick, J. 1927. The Veneridae of New Zealand. Transactions and Proceedings of the New Zealand Institute 57: 567-636.