Field of Science

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.

The Osmiin Mason Bees

As I'm sure I must have had cause to say before, the world of solitary bees is a spectacularly diverse. Literally tens of thousands of species have been described to date, and no doubt many more remain. The classification of bees was reviewed by in great detail by Charles Michener (2007) in his monumental Bees of the World, and it was there that I turned to learn about the subject of today's post, the osmiins.

Female Osmia ferruginea, copyright Gideon Pisanty.


The Osmiini are currently recognised as a tribe of the Megachilidae, one of the two families of long-tongued bees (the other is the Apidae, including, among others, the majority of social bees). Megachilids are most easily characterised by the position of the scopa, a dense array of hairs used by bees for carrying pollen. In most bees possessing a scopa (it tends to be reduced or lost in kleptoparasitic forms), it is located on the hind legs but in megachilids it covers the underside of the metasoma. Osmiins are distinguished from other megachilids by the combination of a well developed sting, elongate stigma on the fore wing, arolia between the claws, and the lack of a pygidial plate. They are often smaller bees, less than a centimetre in length, though the largest osmiins grow close to two centimetres. Some osmiins are also more or less metallic in coloration, an unusual condition for megachilids. No feature has been identified that is unique to osmiins as a whole and their monophyly relative to other megachilid tribes (particularly the Megachilini) has long been called into question. A number of authors have recognised a division of living osmiins between two subtribes, the Osmiina and Heriadina. Osmiina have generally been distinguished from Heriadina by features such as a smaller stigma in the fore wing, a mesopleuron (a plate forming much of the side of the mesosoma) that is shorter ventrally than dorsally, and a propodeum that generally slopes downward from the base (rather than being initially flat). Again, however, the validity of this division has been questioned as no one feature uniformly distinguishes the two groups. A phylogenetic analysis of the Megachilidae by Gonzalez et al. (2012) did not support monophyly for Osmiini or either of its subtribes, but a proper revision of the group's higher classification remains to be done.

Female Hoplitis parana, copyright Gideon Pisanty.


Like other solitary bees, osmiins nest in cavities (a handful are kleptoparasites that do not construct their own nests). They often do not construct these cavities themselves but occupy pre-existing ones such as abandoned beetle burrows and hollows in wood, or crevices between rocks. Some species of Osmia have a predilection for nesting in empty snail shells. Cells are most commonly demarcated in the nest by walls constructed of chewed leaves, often held together with a sticky substance such as mud, resin or (more rarely) nectar. In some cases, the amount of leaf material used is reduced or abandoned, so the cell walls are made entirely of mud or resin. In some European species of Hoplitis, the cells are lined with petals; the species H. papaveris, for instance, lines its cells with bright red poppy petals. Osmia brevicornis, a species found in southern Europe and central Asia, is unusual in that its nest is not divided into cells. Instead, the nest cavity (an abandoned beetle burrow) is uniformly packed with pollen, with eggs being progressively inserted into the pollen mass as it is laid down. The larvae feed on the pollen around them after they hatch, and cocoons end up randomly scattered through the remains of the mass as they mature.

REFERENCES

Gonzalez, V. H., T. Griswold, C. J. Praz & B. N. Danforth. 2012. Phylogeny of the bee family Megachilidae (Hymenoptera: Apoidea) based on adult morphology. Systematic Entomology 37: 261–286.

Michener, C. D. 2007. The Bees of the World 2nd ed. John Hopkins University Press: Baltimore.

Raffrayia

In previous posts, I have introduced you to various representatives of the Pselaphinae, bizarre-looking little gorgon-headed beetles dwelling in soil. But as with all elements of the world's biodiversity, I have not even begun to scratch the surface of what this group has to offer. So for today, a post on another pselaphine genus: the African Raffrayia.

Male Raffrayia dilatata, from Jeannel (1955).


The genus Raffrayia was first established by Reitter in 1881 for a species found in Ethiopia and since then over twenty species have been recognised. The great majority of these have been from southern Africa, in particular from various locations in the Cape Province. Only the meerest handful have been described from scattered localities in east Africa. Nevertheless, it would not be at all surprising if this disparity between regions turns out to be in part an artefact of study effort; there may be more species yet to be described.

The most distinctive feature of the genus is one or two rings each of small button-like nodules on the median segments of the antenna (I am unable to guess the functional significance of these, if any). Jeannel (1955) recognised a number of smaller genera closely related to Raffrayia that share these antennal nodules but differ in having a more elongate basal segment on the abdomen and/or being uniformly flightless forms with the humeri ('shoulders') of the elytra reduced (flightless Raffrayia [see below] retain more distinctly pronounced humeri). Both of these features are derived and these segregate genera may well be expected to be derivatives of Raffrayia. Jeannel also distinguished two subgenera Raffrayia sensu stricto and Raffrayola based on the structure of the first abdominal segment. Raffrayola is restricted to southern Africa whereas Raffrayia sensu stricto is found across the genus' range.

Sexual dimorphism within the genus is strong: males are winged but females are flightless. Elytra are somewhat reduced in females as a result (but again, not as much as in consistently flightless genera). Unfortunately, while we can make some obvious inferences from this about their relative life styles, there seems to be little in the way of direct observations on how Raffrayia spend their lives.

REFERENCE

Jeannel, R. 1955. Les psélaphides de l'Afrique australe. Mémoires du Muséum National d'Histoire Naturelle, nouvelle série, Série A, Zoologie 9: 1–196.

Mites of Marine Sands

Mites may be the most ecologically diverse group of animals on the planet. It is something of a challenge to think of a habitat supporting complex life in which mites may not be found. Nevertheless, it can fairly be said that the marine environment has provided them with a challenge. Though a wide variety of mites can be found in habitats along the shoreline, few lineages have learnt to make a life for themselves beyond the littoral fringe. The most diverse group of truly marine mites is the Halacaridae, of which the genus Simognathus is a representative.

Simognathus sp., from Banks (1915).


Halacarids are notably armoured mites, their bodies protected by an array of reticulate plates. They are found in a wide range of marine habitats and pursue the gamut of lifestyles: representatives of halacarids include algal grazers, micropredators, and parasites. Despite their aquatic lifestyle, they are not swimmers. Instead, they cling to their substrate and crawl slowly on legs bearing large claws. The diversity of halacarid morphologies is reflected in their classification with over a dozen subfamilies currently recognised.

Simognathus is a genus of halacarids found around the world though the greater diversity of species are known from the Southern Hemisphere. They are found at depths ranging from near the low tide mark to around 500 m, and from the full range of tropical, warm-temperate and cold-temperate waters. Bartsch (2005) speculated that the only reason they are not known from even colder waters may be a question of sampling effort rather than true absence. Most Simognathus species are known to live among coarse sand, or in other interstitial microhabitats such as among coral rubble, among colonies of sessile animals such as barnacles or tubeworms, or within algal holdfasts. I haven't come across any specific comments on their diet but their robust chelicerae and grasping fore legs leads me to suspect that Simognathus species are probably micropredators.

Simognathus and the closely related genus Acaromantis form the subfamily Simognathinae. Simognathines differ from other halacarids in their spindle-shaped body with short rostrum, reflecting their interstitial habitat. The first leg ends in a pincer arrangement formed from the terminal claw and a spine on the underside of the tibia. Acaromantis species have a two-segmented palp, no lateral claws at the end of the first leg, and a spinose seta on the genu (the segment between the femur and tibia) of the first leg. Simognathus species have a three-segmented palp, a pair of slender lateral claws on the first leg as well as the terminal claw, and no spinose seta on the first genu. The defining features of Simognathus are all likely to be primitive relative to those of Acaromantis and it has been suggested for some time that Acaromantis may be a derived subgroup of Simognathus. This suggestion is bolstered by a recent molecular analysis of halacarids by Pepato et al. (2018) which found the two Simognathus representatives included to be paraphyletic to the included species of Acaromantis.

REFERENCES

Bartsch, I. 2005. Lohmannella and Simognathus (Halacaridae: Acari) from Western Australia: description of two new species and reflections on the distribution of these genera. Records of the Western Australian Museum 22: 293–307.

Pepato, A. R., T. H. D. A. Vidigal & P. B. Klimov. 2018. Molecular phylogeny of marine mites (Acariformes: Halacaridae), the oldest radiation of extant secondarily marine animals. Molecular Phylogenetics and Evolution 129: 182–188.

The Mouse Shrews of Africa

Shrews are one of the less appreciated groups of mammals. Small (some are among the smallest mammals on earth), skulking, they are often overlooked but are nevertheless represented by a diversity of species in many parts of the world. Among this diversity are the mouse shrews of the genus Myosorex.

Forest shrew Myosorex varius, from Roberts (1951).


Myosorex is a genus of nearly twenty known species of shrew, some of which have only been identified very recently, with more probably yet to be described. They are known in the modern fauna only from sub-Saharan Africa, though the fossil record indicates they once extended as far north as Spain (Furió et al. 2007). Mouse shrews differ from most other living shrew genera (except the closely related Congosorex) in the presence of a tiny vestigial tooth in the lower jaw behind the first antemolar (the tooth behind the incisors, so called in shrews because it is unclear whether it corresponds to a canine or premolar in relation to the teeth of other mammals). Because their teeth lack the red pigment found in shrews of the subfamily Soricinae, Myosorex have historically been classified with the white-tooth shrews of the Crocidurinae. However, the presence of white teeth is, of course, a primitive feature of questionable significance phylogenetically. Instead, more recent authors have pointed to the retention of the second antemolar and other features to support recognising Myosorex and related African shrew genera as relictual members of a third subfamily, the Myosoricinae, that may also include a number of earlier fossil shrews (this group has also been known as the 'Crocidosoricinae' but 'Myosoricinae' is the name with priority; the argument by Furió et al., 2007, that the latter name cannot be used for the broader subfamily because it was originally used only for the African genera has no standing under current nomenclatorial rules).

Foraging forest shrew, copyright Johnny Wilson.


In the modern fauna, Myosorex species have a very scattered distribution. Species found in central and eastern Africa are restricted to high mountains, among moist, densely vegetated environments. Species found in southern Africa are often found in similar habitats. However, the species M. varius is also found in drier locations at lower altitudes, closer to the South African coast. Nevertheless, it is still restricted to areas with high seasonal rainfall, or (in part of Western Cape Province) zones dominated by succulent vegetation where low levels of actual rainfall may be compensated for by precipitation from mist (Meester 1958). The distribution of the genus as a whole is marked by a broad gap of over 1600 km separating the northern limit of species in South Africa and Zimbabwe from their nearest neighbours to the north in the DRC and Kenya, a gap that they were presumably only able to cross in the past when climate conditions were more amenable.

This sensitivity to environment means that Myosorex species may be very vulnerable to changes in habitat. Several species are restricted to limited ranges and several are recognised as potentially endangered. The prospect of climate change makes this vulnerability even worse: as levels of rainfall decrease, mouse shrews will be forced to retreat to ever higher altitudes, and there's only so high they can go before running out of mountain.

REFERENCES

Furió, M., A. Santos-Cubedo, R. Minwer-Barakat & J. Agustí. 2007. Evolutionary history of the African soricid Myosorex (Insectivora, Mammalia) out of Africa. Journal of Vertebrate Paleontology 27 (4): 1018–1032.

Meester, J. 1958. Variation in the shrew genus Myosorex in southern Africa. Journal of Mammalogy 39 (3): 325–339.

Five-fingers and Lancewoods

Longtime readers of this blog will know that my knowledge of plants has always been fairly rudimentary. As a young'un, I only ever learnt to distinguish some of the more common and visible varieties. As a student, I did take a few botany classes, but only really enough to learn that plant biology is complicated and terrifying. Since then, I've continued in much the same vein. But for today's post, I'm looking at something I do recall being aware of in my youth: the lancewoods and five-fingers of the genus Pseudopanax.

Horticultural variant of coastal five-finger Pseudopanax lessonii, copyright Leonora Enking.


Pseudopanax is a genus of a dozen species of small tree (mostly growing about five to seven metres in height) found only in New Zealand (Perrie & Shepherd 2009). Various species have also been assigned to the genus from locations around the Pacific (China, Tasmania, New Caledonia and Chile) but recent studies have lead to their exclusion. A handful of New Zealand species previously included in Pseudopanax have also been separated as the genus Raukaua (Mitchell et al. 1997). The historical taxonomy of the group is confusing, with species being variously attributed to genera Panax, Nothopanax, Neopanax and Polyscias. Things seem to have settled down a bit in recent years but there is still the possibility we may one day see Neopanax rise again (Perrie & Shepherd 2009).

Chatham Islands lancewood Pseudopanax chathamicus, copyright Krzysztof Ziarnek, Kenraiz.


Pseudopanax belongs to the family Araliaceae, a group that is primarily composed of tropical and subtropical shrubs and trees. Araliaceae are commonly referred to as "the ivy family", after one of their best-known members, the common ivy Hedera helix, but, as is not uncommon when a tropical family gets named after one of their European outliers, ivy is pretty weird by Araliaceae standards. Pseudopanax species are perhaps a bit more typical. They have large leaves, often more or less toothed or lobed along the margins. In a number of species, the leaves are palmately divided into three or five separate leaflets, hence the aforementioned vernacular name of 'five-finger'. In one group of species, the lancewoods, the lateral leaflets have been lost and the now undivided leaf is more or less long and narrow. Hybrids between five-fingers and lancewoods may have multiple leaflets like a five-finger but the leaflets shaped like those of a lancewood; New Zealand botanist Leon Perrie has written a post about hybridisation in this genus that you can read here. The trees are usually dioecious (male and female flowers are borne on separate trees) and the individually small flowers are borne aggregated in compound umbels. Fruits are fleshy berries.

Collection of lancewoods P. crassifolius showing the variation in leaf form, copyright Petra Gloyn. Two individuals on the left are young tress with hanging leaves; to the right is a more mature individual with spreading leaves.


Within Pseudopanax, the lancewoods are particularly renowned for their exhibition of heteroblasty, a phenomenon where the appearance of the leaves changes significantly as the tree matures. Juvenile leaves of the common lancewood P. crassifolius and toothed lancewood P. ferox are remarkably long, slender, strongly toothed along the margin, stiff and leathery, and hang downwards around the young tree like a skirt. As the tree approaches its mature height, it starts producing shorter, softer, less serrate leaves that are held in a more or less horizontal position.

Changes in growth habit with maturity seem to be surprisingly common among New Zealand plants and there has been a lot of discussion about why this might be. One suggestion that has certainly received a lot of public attention is that it is a relic of browsing by the large herbivorous birds such as moa that dominated the New Zealand environment prior to human settlement. Juvenile plants developed a habit that was energetically expensive but discouraged browsing by birds; as they grew high enough to escape the reach of such browsers, they changed to a less demanding form. I personally tend to be skeptical of these kinds of claims of historical baggage, not least because the extinction of one-half of the equation makes them very hard to test in any way, but I will admit that this case does perhaps have a bit more credibility than, for instance, claims elsewhere of giant fruits being dependent on long-extinct megafauna. Alternatively, it has been suggested that changes in growth habit may be related to climatic conditions; the juvenile leaves of P. crassifolius dissipate heat more effectively than those of mature trees (Gould 1993). Heteroblasty is less pronounced in the montane lancewood P. linearis of the South Island and almost absent in the Chatham Islands lancewood P. chathamicus, an insular derivative of P. crassifolius. Were these species insulated from the selective pressures affecting the other two? It should also be pointed out that the two proposals mentioned here are not mutually exclusive; the consideration of one as a factor does not automatically rule out the other.

REFERENCES

Gould, K. S. 1993. Leaf heteroblasty in Pseudopanax crassifolius: functional significance of leaf morphology and anatomy. Annals of Botany 71: 61–70.

Mitchell, A. D., D. G. Frodin & M. J. Heads. 1997. Reinstatement of Raukaua, a genus of the Araliaceae centred in New Zealand. New Zealand Journal of Botany 35 (3): 309–315.

Perrie, L. R., & L. D. Shepherd. 2009. Reconstructing the species phylogeny of Pseudopanax (Araliaceae), a genus of hybridising trees. Molecular Phylogenetics and Evolution 52: 774–783.