Field of Science

Fusulinellidae, -inae, summat like that...

In an earlier post, I introduced you all to the fusulinids, a group of complex foraminiferans that were abundant during the later Palaeozoic. In that post, I alluded to the complex array of terminology that can be used when describing fusulinids but said that I would rather not cover it at that time. Well, this time I'm going to be dredging some of it up because I've drawn the Fusulinellidae as the topic for today's post.

Sectioned reconstruction of Fusulinella, from here. Labels: нк = primary chamber, са = septal folds, с = septa, сб = septal furrows, х = chomata, у = septal aperture, т = tunnel.


The Fusulinellidae as recognised by Vachard et al. (2013) are a family of fusulinids with fusiform or oblong tests known from the Middle to Late Pennsylvanian (during the later part of the Carboniferous). One genus, Pseudofusulinella, persists into the early Permian (Ross 1999). They are a part of the larger superfamily Fusulinoidea, a group of fusulinids characterised by what is known as a diaphanotheca. This is a thick, more or less translucent layer in the test wall. As noted in my earlier post, such a test structure may have functioned to allow light through to symbiotic microalgae (or possibly captured chloroplasts from algal prey) sheltered within. Fusulinellids are distinguished from other fusulinoids by the structure of the septa dividing chambers within the test, which are mostly flat except for some folding near the poles of the test (in the Fusulinidae, in contrast, the septal walls were folded throughout). As the test developed, sections of the septa were resorbed to form tunnels connecting adjacent chabers (and presumably allowing the transmission of materials between chambers in life). The course of the tunnels is commonly delimited within the chambers by chomata, discrete ridges of shell material. In other species, the chomata are absent but axial fillings of calcite were formed in the chambers instead.

How fusulinids are more commonly seen: sections of fusulinellid Dagmarella iowensis from Vachard et al. (2013). Image on left = subaxial section (scale bar = 0.1 mm); image on right, larger individual = tangential section (scale = 0.5 mm). The smaller individual on the right is a juvenile Profusulinella cf. fittsi, which depending on the author may or may not be considered a fusulinellid.


Being so widespread and abundant when they lived, fusulinellids are commonly used as index fossils for identifying when a deposit was formed. However, this process is complicated somewhat by ongoing debates about fusulinid systematics. Rauzer-Chernousova et al. (1996) proposed a classification of fusulinids that represented an extensive modification from previous systems. Part of this was simply a question of ranking, with Rauzer-Chernousova et al. recognising many groups at higher ranks than previously (so, for instance, recognising the separate family Fusulinellidae as opposed to its previous recognition as a subfamily of Fusulinidae). Nevertheless, some subsequent authors have felt that Rauzer-Chernousova et al. and their followers attribute too much significance to relatively minor variations. For instance, Kobayashi (2011) synonymised several genera under Profusulinella that Rauzer-Chernousova et al. regarded as belonging to distinct families (and Vachard et al. 2013 even placed in separate superfamilies). Some of the features regarded by Rauzer-Chernousova et al. as indicating separate genera were regarded by Kobayashi as representing variation within a single species. Indeed, there have even been arguments that some 'significant' features may represent post-mortem preservation artefacts (I've come across the term 'taphotaxa' used to refer to taxa based on such features). At present, my impression is that there is something of a geographical divide in preferred systems with eastern European authors following the lead of Rauzer-Chernousova et al. whereas authors from elsewhere may keep to a more conservative arrangement. The Berlin Wall may be down but the Fusulinid Cold War continues.

REFERENCES

Kobayashi, F. 2011. Two species of Profusulinella (P. aljutovica and P. ovata), early Moscovian (Pennsylvanian) fusulines from southern Turkey and subdivision of primitive groups of the family Fusulinidae. Rivista Italiana di Paleontologia e Stratigrafia 117 (1): 29–37.

Rauzer-Chernousova, D. M., F. R. Bensh, M. V. Vdovenko, N. B. Gibshman, E. Y. Leven, O. A. Lipina, E. A. Reitlinger, M. N. Solovieva & I. O. Chedija. 1996. Spravočnik po Sistematike Foraminifer Paleozoâ (Èndotiroidy, Fuzulinoidy). Rossijskaâ Akademiâ Nauk, Geologičeskij Institut, Moskva "Nauka".

Ross, C. A. 1999. Classification of the Upper Paleozoic superorders Endothyroida and Fusulinoida as part of the class Foraminifera. Journal of Foraminiferal Research 29 (3): 291–305.

Vachard, D., K. Krainer & S. G. Lucas. 2013. Pennsylvanian (Late Carboniferous) calcareous microfossils from Cedro Peak (New Mexico, USA). Part 2: smaller foraminifers and fusulinids. Annales de Paléontologie 99: 1–42.

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.

Morion

Morion monilicornis, copyright Charles Schurch Lewallen.


Just a quick one today. This is a typical member of Morion, a genus currently recognised as including over forty species of carabid beetles though there may be many more yet to be described. Characteristic features of this genus include a somewhat flattened body form, moniliform antennae (that is, the antennal segments are all short and similar in form, like beads on a string), a more or less cordiform (heart-shaped) pronotum, and a bilobed median tooth on the mentum (a sclerite on the underside of the head that might be thought of as the 'lower lip' of the mouth) (Will 2003). Though currently recognised as pantropical, Will (2003) suggested that its defining features were potentially plesiomorphic relative to some closely related genera. Further studies may identify Morion in its current sense as a paraphyletic grade to those genera, possibly leading to a reclassification.

The flattened body form of Morion and its relatives (the Morionini) reflects their preferred habitat. Like other carabids, Morion species are voracious predators (both as adults and larvae). Morionins are specialised for hunting in dead wood and under back, forcing themselves through enclosed gaps in search of other insects that might have thought themselves secure in their lignified fortresses.

REFERENCE

Will, K. W. 2003. Review and cladistic analysis of the generic-level taxa of Morionini Brullé (Coleoptera: Carabidae). Pan-Pacific Entomologist 79 (3–4): 212–229.

Libellulidae: On the Wing

Dragonflies of the order Odonata are unquestionably one of the more familiar groups of insects to the general public. They are large, visible and eye-catching, and may be quite colourful. Some have even taken to 'twitching' dragonflies in the same manner as bird species, identifying species observed on the wing and keeping a tally of how many they have seen.

And at the top of many people's list: the wandering glider Pantala flavescens, copyright Jeevan Jose, the world's most widespread dragonfly species.


Ecologically, in contrast, dragonflies may be called a relatively conservative group. All begin their lives as aquatic predators before emerging with adulthood as fast-moving aerial predators. All are generalists, feeding on whatever other insects may be unfortunate enough to fall into their grasp. All dragonflies conform to a fairly similar overall bauplan when compared to the diversity of forms that may be found in many other insect orders (for instance, there are no flightless dragonflies). Classification of dragonflies has often focused heavily on features of the wing venation, tracing its lines in their criss-crossing network.

Hind wing of a libellulid with the anal loop highlighted, from here.


The largest of the generally recognised families of dragonflies is the Libellulidae, containing over 1000 of the approximately 6000 known species of Odonata (Pilgrim & von Dohlen 2008). Characteristic features of the Libellulidae include the presence of the 'anal loop', an arrangement of veins in the hind wing forming what has been described as a boot shape. In the case of the genus Libellula, at least, the shape of the anal loop rather reminds me of one of the legs on the Manx flag. Members of the Libellulidae are commonly known as perchers or skimmers in reference to their hunting behaviours; others have similarly composed names such as darters or pondhawks. A number of members of the family have strikingly banded or coloured wings, leading to vernacular labels such as amberwings or pennants. Members of the genus Tramea are commonly known as saddlebags in reference to the dark patches at the base of their hind wings.

Common picturewing Rhyothemis variegata, copyright Tarique Sani.


Members of the Libellulidae have been divided between about a dozen subfamilies, again primarily defined on the basis of wing venation. However, distinctions between the subfamilies have always been vague with many subfamilies recognised by particular combinations of characters rather than characters unique to each subfamily alone. This vagueness has been underlined by recent molecular studies which have identified most subfamilies as polyphyletic. It seems likely that the defining features of these subfamilies are convergences related to similar ecologies. The 'Sympetrinae' include species with a preference for open watery habitats such as ponds and marshes where they spend a lot of time perched on exposed vegetation (Pilgrim & von Dohlen 2008). The 'Tetrathemistinae', with narrow wings with somewhat reduced venation, are found along forest streams (Fleck et al. 2008). The genera Tramea and Pantala, falsely united in the subfamily Trameinae by broadened bases on the hind wings, are specialised for long-distance flights spending extended periods on the wing (Pilgrim & von Dohlen 2008). Indeed, the wandering glider Pantala flavescens is the world's most widespread dragonfly species, being found in warmer regions of the entire globe and seemingly capable of migrations between separate continents.

The slightly freakish-looking larva of Orionothemis felixorioni, from Fleck et al. (2009).


So if we're going to have a stable classification for libellulids, we need to look past their wings. Intriguingly, larval features may prove more useful in this regard than adult characters. Fleck et al. (2008) examined a group of genera previously classified in the Tetrathemistinae but whose larvae were more similar to those found among members of the Libellulinae. A molecular phylogeny showed that, whereas the Tetrathemistinae as a whole were polyphyletic, these genera were indeed associated with the Libellulinae as their larvae indicated. With further research, we find that libellulid classification need not be all in vein.

REFERENCES

Fleck, G., M. Brenk & B. Misof. 2008. Larval and molecular characters help to solve phylogenetic puzzles in the highly diverse dragonfly family Libellulidae (Insecta: Odonata: Anisoptera): the Tetrathemistinae are a polyphyletic group. Organisms, Diversity & Evolution 8: 1–16.

Pilgrim, E. M., & C. D. von Dohlen. 2008. Phylogeny of the Sympetrinae (Odonata: Libellulidae): further evidence of the homoplasious nature of wing venation. Systematic Entomology 33: 159–174.

The Life and Times of Dissodinium

I've referred before to the position of the minute crustaceans known as copepods as one of the major groups of animals making up the marine zooplankton. Copepods form a significant part of the diet for a wide range of other marine animals: fish, molluscs, jellyfish, you name it. They are also targeted by other organisms coming in at a different scale.

Dissodinium pseudolunula: dinospores waiting to be released from the shell of a secondary cyst, copyright Gabriela Hannach.


Dissodinium is a genus of dinoflagellates, another group of organisms that has appeared on this site in the past. Most dinoflagellates are primarily photosynthetic but not Dissodinium: it's a parasite. Specifically, it's a parasite of copepod eggs. Copepods produce relatively large eggs compared to their body size that are full of tasty lipids and other nutrients so it's hardly surprising that they would attract attention. The free-swimming dinospore of Dissodinium initially looks much like a typical dinoflagellate but once they attach to a copepod egg they produce a sucker-like organelle through which they slurp up the egg's contents, swelling to a globular blob. When feeding is finished, this blob detaches from the remains of the egg to begin the process of reproduction.

There are two species of Dissodinium whose asexual life cycles were described by Elbrächter & Drebes (1978). I haven't found any reference to a known sexual reproduction cycle for Dissodinium. In both species, the replete individual forms a spherical primary cyst that floats free within the plankton. The contents of the primary cyst divide within the cyst wall to form the next stage, the secondary cysts. In the most commonly seen species, Dissodinium pseudolunula*, these secondary cysts are distinctively crescent-shaped. Following their release from the original primary cyst wall, the cytoplasm within the secondary cysts further subdivides to form the actively swimming dinospores. These dinospores presumably function as the infective stage for another round of the cycle but it should be noted that Gómez et al. (2009) were unable to induce infection when they incubated newly released dinospores together with copepod eggs. Instead, the dinospores encysted themselves in a hyaline membrane and Gómez et al. suggested that some sort of maturation period may be necessary before infection can take place. The second species of Dissodinium, D. pseudocalani, differs in that the secondary cysts are not crescent-shaped, and divide to release the dinospores while still themselves contained within the original primary cyst wall so the breakdown of the latter releases dinospores directly into the environment. This compression of the life cycle has also sometimes been observed with D. pseudolunula.

*This species has often masqueraded in the past under the name of Dissodinium lunula. The name 'Gymnodinium lunula' was originally used for crescent-shaped cysts by Schütt in 1895. Unfortunately, Schütt's figured examples of this 'species' included representatives of two quite different dinoflagellates, now classified as Dissodinium and another genus Pyrocystis that is not parasitic. The name lunula has become properly attached to the latter species, requiring a different name for the Dissodinium.

Stages in the life cycle of Dissodinium pseudolunula, from Elbrächter & Drebes (1978), running from a freshly released primary cyst at top left to a newly attached parasitic dinospore at bottom right.


Elbrächter & Drebes (1978) included Dissodinium in the Blastodiniales, a morphologically diverse group of parasitic dinoflagellates. The advent of molecular analyses would later demonstrate this grouping to be polyphyletic with parasitic dinoflagellates evolving on numerous occasions from free-living ancestors. Instead, Dissodinium and another parasite of copepod eggs, Chytriodinium, form a clade that is closely related to the major free-living genus Gymnodinium (Gómez et al. 2009). Gómez et al. also found that D. pseudolunula retains some elements of its free-living ancestry: it still retains chlorophyll (chlorophyll is absent in D. pseudocalani and Chytriodinium). Just how functional this chlorophyll remains is an open question: it appears less concentrated within the cell than in a typical photosynthetic dinoflagellate, and Gómez et al. were unable to maintain a culture of D. pseudolunula under conditions that would support a free-living species. Nevertheless, they suggested that a low level of photosynthesis might supplement the dinoflagellate's nutrient requirements while it waited out the aforementioned incubation period before finding itself a host.

REFERENCES

Elbrächter, M., & G. Drebes. 1978. Life cycles, phylogeny and taxonomy of Dissodinium and Pyrocystis (Dinophyta). Helgoländer wiss. Meeresunters. 31: 347–366.

Gómez, F., D. Moreira & P. López-García. 2009. Life cycle and molecular phylogeny of the dinoflagellates Chytriodinium and Dissodinium, ectoparasites of copepod eggs. European Journal of Protistology 45: 260–270.

Protacanthopterygii: A Brief History of a Vague Idea

There are some taxon names whose concepts are rock-solid, that have been universally recognised since their inception almost without variation. There are some taxon names that are coined, potentially linger through one or two subsequent uses, then disappear into the mists of history never to be used again. And then there are some taxon names that are used regularly but whose actual concept shifts wildly over time: names that seem to be used not so much for their own sake as because authors seem to think they need to be in there somewhere. Witness today's subject, the Protacanthopterygii.

Brown salmon Salmo trutta, photographed by Eric Engbretson, about as close to a definitive 'protacanthopterygian' as you're going to get.


The Protacanthopterygii has widely been recognised as a major group of ray-finned fishes since the name was established by Greenwood et al. (1966). Using the modern parlance, Greenwood et al.'s Protacanthopterygii was an explicitly paraphyletic group of euteleost fishes that could be recognised as branching off the lineage leading to the Acanthopterygii and Paracanthopterygii but lacked the full suite of characteristics of the latter group. As such, many of the characters listed by Greenwood et al. as diagnostic of the Protacanthopterygii were expressed in the form of trends: "widespread trend toward the development of premaxillary processes", for instance, or "hyoid and branchiostegal skeleton approaching paracanthopterygian and acanthopterygian form". We also get a number of references to majority rather than universal features: "glossohyal teeth usually prominent", or "few species with opercular spines or serrations". Greenwood et al. included the bulk of their Protacanthopterygii in the order Salmoniformes, but recognised this order in a much broader sense than modern authors. As well as the Salmonidae itself, their Salmoniformes included taxa that would now be placed in the orders Galaxiiformes, Esociformes, Myctophiformes, Aulopiformes and Stomiiformes, among others. Greenwood et al.'s Protacanthopterygii was also supposed to include the orders Cetomimiformes, Gonorynchiformes and Ctenothrissiformes. Their concept of Cetomimiformes is now recognised as polyphyletic and neither Cetomimiformes and Gonorynchiformes include any taxa closely related to Salmonidae; the case of Ctenothrissiformes has been discussed on this site previously.

Northern pike Esox lucius, copyright Jik jik.


In the intervening years, of course, the philosophy of systematics has shifted to prioritising the recognition of monophyletic taxa, requiring the dissolution of the original Protacanthopterygii. Unfortunately, calculating basal euteleost relationships has not proven an easy task. As a result, authors have differed considerably on exactly which fishes should be regarded as 'protacanthopterygians'. About the only constant factor in all circumscriptions of the taxon has been the inclusion of the Salmonidae, the salmons, trouts and the like. Indeed, the most extreme restriction of the Protacanthopterygii would treat it as including this family alone.

Recent molecular studies have agreed on the recognition of a clade uniting the Salmonidae with the Esociformes. The Esociformes is a small order of a bit over a dozen species of freshwater fish found in the Holarctic region, uniting the pikes of the genus Esox with the mudminnows of the Umbridae. Betancur-R et al. (2017) recognised Protacanthopterygii as the name for a clade uniting the Salmonidae, Esociformes, Argentiniformes (a marine order including herring smelts, barreleyes and the like) and Galaxiidae (whitebaits). However, other studies have not supported this clade.

Spotted galaxias Galaxias truttaceus, copyright Nathan Litjens, an Australian member of the whitebait family. Though galaxiids are rather salmon-like in overall appearance, it remains an open question whether this resemblance indicates any sort of direct relationship or just a shared hold-over from some ancestral neoteleost.


Considering the difficulty in defining it, one might question why the concept of a 'Protacanthopterygii' persists at all. Really, there doesn't seem to be much reason for it other than that the Greenwood et al. (1966) classification was long the base standard for teleost classifications, leaving subsequent authors loathe to discard any taxon recognised therein lightly. It might, in theory, be possible to rescue the Protacanthopterygii concept by phylogenetic definition: for instance, as those species more closely related to Salmo than Perca (indeed, I would not be surprised to learn this has already been done). But considering that the uncertain composition of the resulting clade would reduce the practicality of its recognition, I don't think I would be weeping too much if someone would just take the Protacanthopterygii concept out the back and shoot it.

REFERENCES

Betancur-R., R., E. O. Wiley, G. Arratia, A. Acero, N. Bailly, M. Miya, G. Lecointre & G. Ortí. 2017. Phylogenetic classification of bony fishes. BMC Evolutionary Biology 17: 162.

Greenwood, P. H., D. E. Rosen, S. H. Weitzman & G. S. Myers. 1966. Phyletic studies of teleostean fishes, with a provisional classification of living forms. Bulletin of the American Museum of Natural History 131 (4): 339–456.

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.