Basic rundown of Choristodera, the “freshwater marine reptiles”

choristodera
a) Simoedosaurus lemoinei (modified from Sigogneau-Russell and Russell, 1978) –b) Ikechosaurus pijiagouensis (IVPP V13283) –c) Tchoiria namsarai (redrawn from Efimov, 1975) –d) Champsosaurus gigas (SMM P77.33.24): non-neochoristoderan –e) Cteniogenys sp. (Evans, 1990)–f) Lazarussuchus inexpectatus (Hecht, 1992) –g) Hyphalosaurus baitaigouensis (redrawn from Gao and Ksepka, 2008) –h) Monjurosuchus splendens (Matsumoto et al., 2007) –i) Philydrosaurus proseilus (Gao and Fox, 2005).

Choristoderes are a group of extinct reptiles I’ve talked about before. Now that several years have passed, I’m going to discuss them once more.

What are choristoderes?

hyphalosaurus_by_kahless28-d34r8g9
Hyphalosaurid choristoderes (and Manchurochelys) by kahless28

Choristoderes are a lineage of extinct, generally fully-aquatic reptiles. Beyond that, however, it’s very hard to tell.

See, it’s been a long standing debate whereas choristoderes are either archosauromorphs (the lineage of reptiles leading to archosaurs, including crocodylomorphs, pterosaurs and dinosaurs), lepidosauromorphs (the lineage leading to squamates and sphenodontians), or neither (more basal than either group). Similar debates have also plagued classical marine reptile groups like plesiosaurs and ichthyosaurs.

Part of the reason why figuring out where choristoderes fit in the phylogenetic tree of reptiles is because they lack definitive features affiliating them to either major reptile group. Their skulls vaguely resemble those of lepidosauromorphs, but lack the same quadrate configuration.

A particularly radical proposition was suggested by Miller 2004, in which choristoderes are linked to classical marine reptiles (ichthyosaurs and sauropterygians) under Euryapsida.

Personally, I lean slightly towards a lepidosaur identity, but further results may prove this hunch wrong.

 

When did they live?

hyphalosaurus_web

Hyphalosaurus by Matt Martuniuk.

Pachystropheus and Actiosaurus might represent examples of Rhaetian Triassic choristoderes. There is still some ambiguity; the former has also been regarded as a sauropterygian (Matsumoto 2009) or as a thalattosaur (Renesto 2005), while the latter was initially interpreted as a theropod, then an ichthyosaur before its proposed choristodere identity more recently (Mortimer 2010).

If both or at least one are choristoderes, they’re both fairly specialized marine reptiles and, in Pachystropheus‘ case, fairly derived, implying an even earlier origin. This is consistent with the long temporal gaps in choristodere fossil record history, the Lazarus taxon effect.

Otherwise, choristoderes are first known from the mid/late Jurassic. These Jurassic species are less specialized, being lizard-like freshwater reptiles. This would set a norm for the rest of the group’s existence: highly conservative “living fossils” whose lineages bear a spotty fossil record, attesting to both their resilience as well as avoidance of preservation. These include the complicated “Cteniogenys” assemblage, which first arose in the mid-Jurassic of Europe and North America, including the famous Morrison Formation.

By the Early Cretaceous, choristodere diversity in Asia undergoes an explosive radiation. Besides the more classical, salamander-like monjurosuchids, we also see the arrival of the long-necked hyphalosaurids and the long-jawed neochoristoderes, expanding the ecological niches available to the group.

Its unclear why choristoderes underwent such a sudden diversification. The local extinction of aquatic crocodilians due to colder temperatures might have triggered the evolution of neochoristoderes (Matsumoto 2010), but this doesn’t explain the evolution of hyphalosaurids.

From the mid-Cretaceous onward, there are no Asian choristodere remains, other than the largely understudied Eotomistoma (Carroll 1988). Hyphalosaurids disappear, but North America and Europe see the presence of both salamander-like species (“Cteniogenys”) as well as the diverse neochoristodere Champsosaurus. Both groups co-exist with a variety of aquatic crocodilians, though Champsosaurus apparently prevents long-snouted crocodilians from occurring in freshwater habitats (Matsumoto 2010).

Choristoderes survived the KT event, with Champsosaurus displaying dietary changes (Matsumoto 2015). It undergoes an adaptive radiation, producing several species including C. gigas, the largest known choristodere at around 3.5 meters long. It is joined by another genus, Simoedosaurus, which given its sudden appearance in the fossil record probably evolved from a Cretaceous Asian species.

Cretaceous “Cteniogenys” species are not known to have made it past the KT event. However, the genus Lazarussuchus debuts in the Paleocene of France, a much more basal animal with a ghost lineage quite possibly extending to the Jurassic or Triassic (Hecht 1992), unless they are related to the Cretaceous “Cteniogenys” species (Matsumoto 2013).

Neochoristoderes suddenly disappear in the medial Eocene, for no currently explainable reasons (see below). However, Lazarussuchus would continue to endure until the mid-Miocene, if not possibly until the Pliocene, when Europe grew too cold for these reptiles.

Unlike many clades, in which the most specialized members were the last representatives, choristoderes seem to have ended more or less as they started.

Fully Aquatic

hyphalosaurus_by_hyrotrioskjan-d460fje
Hyphalosaurus giving birth by Joschua Knüppe.

Most choristoderes were fully aquatic animals.

While some basal taxa like Lazarussuchus could be interpreted as amphibious, both monjurosuchids and hyphalosaurids have evidence of vivipary (Wu 2010) while in Champsosaurus only adult females have limbs robust enough to carry them ashore, while males and juveniles could not support their weight on land (Katsura 2007). As such, choristoderes as a whole appear to not have left the water much, if at all.

All known choristoderes possess laterally flatted tails, which were probably their main propulsion mechanism, with neochoristoderes also having paddle-like limbs. At least hyphalosaurids, monjurosuchids and neochoristoderes all have smooth skins with small, non-overlapping scales, though monjurosuchids do bear rows of scutes similar to those of modern alligator-lizards (Gao 2000). In neochoristoderes, the torso is dorsoventrally flattened, the gastralia are large and the ribs are short and massive, further adaptations for diving. There was a source mentioning pachyostic bones, but I can’t seem to find it.

Probably the most remarkable adaptation for life in the water are the nostrils. Aside from basal choristoderes like Lazarussuchus, which simply have receded nostrils, in more derived taxa the nostrils are fused and oriented towards the tip of the snout, allowing the animals to effectively use them as a snorkel, surfacing only the very tip of the snout while the rest of the body remained underwater (Acorn 2007).

Perhaps tellingly, the eyes are forwardly oriented. This is in contrast to amphibious animals like crocodilians and hippos, which have dorsally-oriented nostrils and eyes.

Cold Waters

Fig-2-Paleogeographical-distribution-of-Choristodera-during-the-Mesozoic-plotted-on

Palaeogeographical distribution of Choristodera in the Jurassic and Cretaceous periods.

Choristoderes as a whole have a pan-Laurasian distribution, ocurring in fossil sites in North America, Europe and Asia. There are two possible exceptions: basal choristoderes from the Jurassic/Early Cretaceous of North Africa (Haddoumi 2014) and possible neochoristodere teeth from Timor (Umbgrove 1949).

Other than these examples, however, nearly all choristodere remains occur in high latitudes. Although some do occur in sites that had a paratropical climate, most occur in temperate regions. Some Champsosaurus fossils even ocur in the high Arctic, in the Cretaceous of Greenland and the Eocene and Cretaceous of Axel Heiberg (Matsumoto 2010).

Thus, it can be seen that choristoderes tolerated colder environments, if not outright preferred them (Matsumoto 2014). Their ability to survive in colder climates in fact probably allowed them to diversify in the absence of crocodilians during the Early Cretaceous, when colder temperatures in Asia caused the local extinction of aquatic crocodilians (Matsumoto 2014). Both groups ultimately co-existed in warmer areas, however.

I previously speculated that neochoristoderes might have been endothermic, which fits well with their extreme speciation to an aquatic lifestyle. However, given that cold-tolerance was also present in more basal taxa, monjurosuchids and hyphalosaurids, I wonder if it endothermy was more widespread across the clade.

Ironically, cooling temperatures in Europe probably lead to the extinction of the group, as Lazarusussuchus probably survived until the glaciations.

Feeding

 

Figure-8-Palatal-dentition-of-Simoedosaurus-sp-SMNS-59026-a-e-and-Slemoinei-f-i.png

Palatine teeth in Simoedosaurus.

Basal choristoderes were probably generalistic feeders, ambushing small fish, crustaceans and other prey. Monjurosuchus has evidence of arthropod cuticle in its intestines, suggesting that it fed on aquatic arthropods (Gao 2000).

Hyphalosaurids were certainly specialized animals, bearing long necks. It’s likely that they were doing whatever the convergently similar plesiosaurs were doing, whatever that was. In particular, they appear to have preferred softer prey, which they probably hunted actively on the deeper areas of their lake environments (Gao 2008).

By contrast, the long-snouted neochoristoderes are much easier to figure out. These predators were doing what crocodilians, whales, gars and countless other large, predatory aquatic vertebrates do best: snatch up larger prey. Long, thinner jaws allowed them to accomplish this task with less drag.

While they are often compared to gharials, neochoristoderes may not be exactly analogous. Unlike other crocodilians, gharials have weak bite forces, rarely exceeding 497 N; they can tackle animals as large as goats, but they prefer small fish. By contrast, Champsosaurus had a bite force of 1194 to 1910 N, more comparable to that of crocodiles (James 2010). This may not have necessarily translated to a preference for larger prey, but it does convey a much larger degree of strength and speed when catching small fish.

Simoedosaurus dakotensis and Champsosaurus gigas, by contrast, have semi-brevirostrine snouts, and might have a diet more similar to that of crocodiles, feeding on not just fish but aquatic tetrapods like turtles and waterfowl and maybe ambushing terrestrial animals near the water. Notably, they co-existed with crocodile-like crocodilians like Borealosuchus (Matsumoto 2013 and 2014).

A neat feature choristoderes have is the retention of palatine teeth. Lost in various tetrapod groups like mammals, crocodylomorphs and dinosaurs, these teeth basically serve the role palatal grooves have in these groups, helping the animal to hold and manipulate food items in the mouth. In basal choristoderes these teeth are fairly generic, while in neochoristoderes they are more specialized, indicating a higher degree of food manipulation in the mouth. Such speciation might have evolved multiple times, as the simoedosaurid Ikechosaurus has palatine teeth more similar to those of non-neochoristodere choristoderes (Matsumoto 2015).

Through subtle differences in palatine teeth, we know that Champsosaurus changed its diet across the KT event, and that Simoedosaurus lindoei prefered softer prey than S. dakotensis (Matsumoto 2015).

Extinction

Lazarussuchus_NT_small

Lazarrusuchus, the last choristodere, by Nobu Tamura.

As mentioned previously, the extinction of choristoderes isn’t clear.

Competition with crocodilians is sometimes attributed to the demise of neochoristoderes, but both groups co-existed across the Late Cretaceous, Paleocene and Eocene of Laurasia and possibly Timor, and if anything neochoristoderes seem to have been dominant over long-snouted crocodilians in freshwater habitats (Matsumoto 2010 & 2014).

This also doesn’t explain the demise of hyphalosaurids, or of basal choristoderes in North America, North Africa and Asia.

Ultimately, European choristoderes such as Lazarussuchus appear to have died out due to glaciations in Europe.

Conclusion

There is still much I haven’t covered. Hopefully this will introduce you to these amazing extinct reptiles.

References

Ryoko Matsumoto; Shigeru Suzuki; Khisigjav Tsogtbaatar; Susan E. Evans (2009). “New material of the enigmatic reptile Khurendukhosaurus (Diapsida: Choristodera) from Mongolia”. Naturwissenschaften. 96 (2): 233–242. doi:10.1007/s00114-008-0469-6. PMID 19034405.

Silvio Renesto (2005). “A possible find of Endennasaurus (Reptilia, Thalattosauria) with a comparison between Endennasaurus and Pachystropheus“. Neues Jahrbuch für Geologie und Paläontologie – Monatshefte. Jg. 2005 (2): 118–128.

http://theropoddatabase.blogspot.pt/2010/06/actiosaurus-is-choristodere-not.html

R. Matsumoto and S. E. Evans. 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberian Geology 36(2):253-274

R. L. Carroll. 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Company, New York 1-698

R. Matsumoto and S. E. Evans. 2015. Morphology and function of the palatal dentition in Choristodera Article in Journal of Anatomy 228(3):n/a-n/a · November 2015 DOI: 10.1111/joa.12414

Hecht, M.K. (1992). “A new choristodere (Reptilia, Diapsida) from the Oligocene of France: an example of the Lazarus effect”. Geobios. 25: 115–131. doi:10.1016/S0016-6995(09)90041-9.

Matsumoto, R.; Buffetaut, E.; Escuillie, F.; Hervet, S.; Evans, S. E. (2013). “New material of the choristodere Lazarussuchus(Diapsida, Choristodera) from the Paleocene of France”. Journal of Vertebrate Paleontology. 33 (2): 319. doi:10.1080/02724634.2012.716274.

Ji Q., Wu, X.-C. and Cheng, Y.-N. (2010). “Cretaceous choristoderan reptiles gave birth to live young.” Naturwissenschaften, 97(4): 423-428. doi:10.1007/s00114-010-0654-2

Yoshihiro Katsura. 2007. Fusion of sacrals and anatomy in Champsosaurus (Diapsida, Choristodera), doi:10.1080/08912960701374659

Gao, K.; Evans, S.; Ji, Q.; Norell, M.; Ji, S. (2000). “Exceptional fossil material of a semi-aquatic reptile from China: the resolution of an enigma”. Journal of Vertebrate Paleontology. 20 (3): 417–421. doi:10.1671/0272-4634(2000)020[0417:efmoas]2.0.co;2.

John Acorn, Deep Alberta: Fossil Facts and Dinosaur Digs, University of Alberta, 07/02/2007

Hamid Haddoumi, Ronan Allain, Said Meslouh, Grégoire Metais, Michel Monbaron, Denise Pons, Jean-Claude Rage, Romain Vullo, Samir Zouhri, Emmanuel Gheerbrant, Guelb el Ahmar (Bathonian, Anoual Syncline, eastern Morocco): First continental flora and fauna including mammals from the Middle Jurassic of Africa, doi:10.1016/j.gr.2014.12.004

J. H. F. Umbgrove, Structural History Of The East Indies

Gao, K.-Q. and Ksepka, D.T. (2008). “Osteology and taxonomic revision of Hyphalosaurus (Diapsida: Choristodera) from the Lower Cretaceous of Liaoning, China.” Journal of Anatomy, 212(6): 747–768. doi:10.1111/j.1469-7580.2008.00907.x

James, Michael, The jaw adductor muscles in Champsosaurus and their implications for feeding mechanics, 2010-08-30T19:01:39Z

Six Reasons Why Eutriconodonts Are Awesome

People who know will definitely tell you how much I’ve come to appreciate Mesozoic mammals. Usually dismissed as small rat things, mammals in the Mesozoic were a highly diverse bunch of animals, including swimmers, diggers, anteater like forms, large terrestrial predators, hoppers and many, many more.

Of these, eutriconodonts are by far among the more spectacular. I’ve already talked at length about the possible flight capacities of volaticotheres, but really the whole clade is pretty neat, and here’s why:

1- The first mammalian carnivores

Jugulator amplissimus by @paleoart

Eutriconodonts are notable for being among the first mammals specialized to dedicated carnivory. Zofia Kielan-Jaworowska identified numerous features associated with obligate carnivory: long, sharp canines (or canine-like incisors in the case of gobiconodontids), premolars with trenchant main cusps that were well suited to grasp and pierce prey, strong development of the mandibular abductor musculature, bone crushing ability in at least some species and several other features.

Their iconic triconodont dentition, usually taken as “primitive”, might actually be specialized for shearing (Zofia Kielan-Jaworowska 2004, Sigogneau-Russell 2016), making it vaguely analogous to the carnassials of placentals and marsupial predators. Their exact shearing mechanism has no real analogue among mammalian carnivores, but the function is considered very similar at least (Rougier 2015)

Equally important is eutriconodont size. Eutriconodonts are among the largest mammals in Mesozoic faunal communities, which has been inferred as standing the highest among mammals in contemporary trophic webs ( Zofia Kielan-Jaworowska 2004). At their size, they were perfectly capable of taking down vertebrate prey, and the largest gobiconodontids like the infamous Repenomamus might have been apex predators in their environment.

Repenomamus itself has been found with dinosaur remains in its belly, and scavenging marks associated with Gobiconodon have also been found. These mammals could, in fact, tackle dinosaurs, and if modern analogues like wolverines, tasmanian devils and ratels are of any indication then the largest eutriconodonts could in fact be “top guns” in their environments.

Other Mesozoic synapsids have also been inferred to be specialized carnivores, like Sinoconodon and deltatheroideans. But they lived either before eutriconodonts spread, or after they became extinct, and as such their range was much more limited.

2- Their diversity

Speculative depictions of

Ichthyoconodon by @alphynix. While I argue for a slightly different lifestyle, they help summarize the range of known eutriconodont bauplans, such as the otter like Yanoconodon and Liaoconodon and the aerial Volaticotherium and Argentoconodon

Better only than carnivorous Mesozoic mammals are carnivorous Mesozoic mammals that come in all shapes and sizes. In spite of being pretty much highly specialized carnivores and certainly more restricted in terms of diet than, say, symmetrodonts or early therians, eutriconodonts were much more diverse than these groups were (until therians got them beat after eutriconodonts went extinct, that is).

The group ranged from shrew analogues (amphilestids, amphiodontids, basal gobiconodonts and some triconodontids), arboreal, tree-shrew like forms (Jeholodens), large, robust carnivores (gobiconodontids, Jugulator, triconodontids), a quilled species with an immensely thick spine (Spinolestes), at least two lineages of swimmers (Liaoconodon and Yanoconodon) and of course the aforementioned volaticotheres, conservatively gliders if not outright flyers.

The exact smallest triconodonts probably weighted around 50 grams. The largest, Repenomamus giganticus, as much as 14 kg.

This level of ecological diversity is so far unmatched by any Mesozoic mammal group save for multituberculates and perhaps Late Cretaceous metatherians. It is even larger than the diversity of most therian carnivore groups, save for carnivorans.

Every possible niche taken by carnivorous mammals under 14 kg was taken, and it’s amazing.

3- Brains!

Yes, we know about eutriconodont brains. In fact, Triconodon mordax is one of the first extinct animals to have its endocast studied (Simpson 1928).

From what we can tell, at least from this one specimen, eutriconodonts had fairly “primitive” brains for mammal standards. The cerebral hemisphere is long, oval and flat, lacking the inflated appearance present in modern mammals (including monotremes, which are generally held to be more basal than eutriconodonts!) as well as the also extinct multituberculates. The cerebrum is similarly not expanded as much as in those groups. Like multies, Triconodon has a large, semi-triangular bulge, thought to be a large cistern.

What this means about eutriconodont intelligence is unclear. It might seem like they were fairly stupid mammals, but mammals with fairly simplistic brains are known to be fairly intelligent (Weisbecker 2010). They probably weren’t as cunning as modern cats and dogs, but probably capable of complex behaviors nonetheless.

4- Everywhere For A Long Time

Different mammaliaform tooth types across the Mesozoic. Eutriconodonts, alongside the unrelated morganuconodonts, were the only mammals to bear a “triconodont” tooth type.

While eutriconodonts fall short of multituberculates as the longest living mammal lineage, they were still very successful. The first eutriconodont fossils – Argentoconodon, Victoriaconodon and Huasteconodon – all date to the Toarcian and represent a large variety of lineages, indicating an even earlier origin.

Eutriconodonts would then keep on going in full force for another 111 million years. Even when other mammal groups display gaps in their fossil record, eutriconodonts continue across fossil sites in Europe, Asia, North America, Africa and South America, rendering them a truly global presence in Mesozoic faunas as much as dinosaurs and pterosaurs.

Alas, they faced a final challenge with the spread of angiosperm plants, which drastically altered faunal components across the globe and was particular harsh on carnivorous mammals. Only one lineage survived the Turonian, Alticonodon, to still endured all the way to the Campanian.

5- Poison-Heels

Gobiconodon

by Dmitry Bogdanov. Notice the spurs on the heels.

Okay, not something exclusive to eutriconodonts among mammals, but it bears repeating.

Venomosity is inferred to be an ancestral trait for mammals (Hurum 2006). Various Mesozoic mammal groups possess heel spurs similar to those of the modern platypus, which delivers a powerful neurotoxin infamous for how painful it is. This includes similar canals, which implies an identical function.

This spurs have been found in nearly all non-therian mammal groups, suggesting that either venom evolved multiple times among Mesozoic mammals, or, most likely, that it was an ancestral feature later lost in therians.

Eutriconodonts, of course, preserve such spurs. They are best known in gobiconodontids, which combined with other features would make these some of the most ridiculously over-engineered killing machines of the time.

6- Tough As Nails

Fantastic Mr. Spinolestes.

Gobiconodontids were probably the honeybadgers of the Jurassic (and early Cretaceous). The largest of all eutriconodonts, they included not only the infamous Repenomamus, but the also fairly sizeable Gobiconodon. These animals are racoon-to-wolverine sized beasts, bearing thick skeletons, robust jaws and sharp fang-like incisors.

As such, not only were they large carnivorous mammals for the time, but also specifically designed to fight violently. Combined for evidence for scavenging for Gobiconodon and outright dinosaur-consumption for Repenomamus, it’s hard to not see these as competitors for small to mid-sized theropod dinosaurs in their local environments. “Small”, but incredibly brute fighters, fighting their way into carcasses and perhaps even harassing fellow predators.

That said, even the smaller gobiconodontids were nothing to laugh at.

Spinolestes, a more conventionally shrew-sized animal, bears:

– A massively thick, xenarthrous spine similar to that of xenarthrans and the hero shrew. This probably allowed it to survive being smashed by animals up to 75 kg.

– Spines similar to those of the modern spiny mice.

– The venom spurs.

What can you even say to that?

Conclusion

I think Mesozoic mammals are underrated in general, but eutriconodonts in particular are a very fascinating group. Besides these undeniably awesome facts, there’s also the fact that they bear some of the most exquisitely preserved Mesozoic mammal fossils, something even the more well known multituberculates currently lack.

Dismissed as just archaic “missing links”, they were a dynamic, fascinating group of animals, which I believe deserve some recognition.

References

Zofia Kielan-Jaworowska, Richard L. Cifelli, Zhe-Xi Luo (2004). “Chapter 7: Eutriconodontans”. Mammals from the Age of Dinosaurs: origins, evolution, and structure. New York: Columbia University Press. pp. 216–248. ISBN 0-231-11918-6.

Percy M. Butler; Denise Sigogneau-Russell (2016). “Diversity of triconodonts in the Middle Jurassic of Great Britain” (PDF). Palaeontologia Polonica 67: 35–65. doi:10.4202/pp.2016.67_035.

Chen, Meng; Wilson, Gregory P. (2015). “A multivariate approach to infer locomotor modes in Mesozoic mammals”. Paleobiology. 41 (02): 280–312. doi:10.1017/pab.2014.14. ISSN 0094-8373.

Vera Weisbecker and  Anjali Goswami, Brain size, life history, and metabolism at the marsupial/placental dichotomy, Proc Natl Acad Sci U S A. 2010 Sep 14; 107(37): 16216–16221. Published online 2010 Sep 7.   doi:  10.1073/pnas.0906486107

Flying Volaticothere Developments

argentoconodon-color

triconolestes-polished

vh8msc8

volbatUpdated depictions of Argentoconodon, Triconolestes and Ichthyoconodon (Ceri Thomas and Dylan Bajda, respectively), and a fictional species for the Speculative Dinosaur Project by Tim Morris.

So a while ago I made some  posts on whereas a group of Mesozoic mammals, the volaticotheres, were capable of true powered flight. There have been relatively few responses since then, other than the expected “take this with a grain of salt” (which is fair; it’s an outrageous hypothesis), and even fewer addressals to the points I provided. I had a conversation with paleofail-explained on Julio Lacerda’s commission, but it was abandoned quickly. A more successful counter-argument was made by fezraptor, which I will address below.

In the past few months there have been relatively few papers involving eutriconodont mammals, and none of them involving volaticothere taxa. Thus, for now, things remain as they remain.

However, I previously did miss an important study, which deals with the dietary ranges of Mesozoic mammals and their relation to angiosperm diversity. Here, both Argentoconodon and Volaticotherium are present, firmly within the animalivorous range, with the former ranking more with carnivorous taxa and the latter with insectivorous taxa. Animalivorous mammals are shown to have declined substantially during the mid-Cretaceous; both points strengthen the arguments for volaticotheres being specialised animalivores, and thus unlikely to have been gliders.

Now, unto for the addressal:

The manus of Volaticotherium is actually sufficiently preserved to show that it is smaller than the pes (barring the presence of unprecedentedly weird distal phalanges), precluding it looking bat-like.

The appendix for Meng 2003 describes the manus as “poorly preserved”, and doesn’t mention size. Granted, the fact that the metacarpals don’t seem to be particularly specialised in relation to those in the foot makes a bat-like wing less likely, but the aforementioned elongated phalanges might suggest an atypical wing structure.

Ichthyoconodon is actually outside of the clade that preserves any evidence of adaptations for gliding, so kenbrasai’s reasons for why Ichthyoconodon would be atypical for a glider actually strike me as evidence it wasn’t volant at all.

True, though it’s worth to note that Ichthyoconodon‘s exclusive status from this clade is based on one character less (Gaetano et al 2011). In any case Ichthyconodon‘s molars are less recurved than those in Volaticotherium and Argentoconodon, and as these taxa are already disparate in their jaw morphology it could mean that Ichthyoconodon was functionally very different. We do have evidence of truly aquatic eutriconodonts, after all.

I do stand by my hypothesis, however, as it is still a rare taxon, so an aquatic lifestyle seems less likely.

The digitigrade Taeniolabis meme

taeniolabis_nt_smallTaeniolabis taoensis by Nobu Tamura.

Taeniolabis, like most multituberculates, has relatively few artistic depictions. What few depictions there are, however, seem to all be plagued by a common feature: digitigrady.

Above is the relatively recent work of Nobu Tamura, uploaded to Wikimedia, which only the latest in a long line:

taeniolabisArtist uncredited but hosted here.

492758687

89164541Two depictions (alongside other Paleocene mammals) credited as “De Agostini Picture Library”

s-l500Another uncredited picture, hosted here.

taeniolabis_psitaccotheriumv1_by_avancna-d1gjs3aPicture by Stanton F. Fink (also depicting Psittacotherium, meaning that the animal is also undersized)

fossil_mammal_parade_by_paleoaeolos-d64bdloDepiction (alongside many other mammals) by Martin Chavez; also severely undersized.

This leads me to believe that this is yet another paleomeme, one that seems unjustifiable.

There is some evidence that derived multituberculates displayed facultative digitigrady, and indeed I wouldn’t be surprised that some multies were fully digitigrade. However, from what little I can gather from Taeniolabis‘ tarsal anatomy, it seems to have fit the plantigrade model offered above.

The depiction for it’s closest relative, Kimbetopsalis, by  one of its describers Sarah Shelley, seems to agree on a plantigrade model for taeniolabidids:

kimbetopsalis-simmonsae

References

http://www.palaeontologia.pan.pl/PP67/Luo.pdf

Willamson, T.E.,; Brusatte, S.L.,; Secord, R.,; Shelley, S (2015), “A new taeniolabidoid multituberculate (Mammalia) from the middle Puercan of the Nacimiento Formation, New Mexico, and a revision of taeniolabidoid systematics and phylogeny”, Zoological Journal of the Linnean Society, doi:10.1111/zoj.12336

The evolution of herbivory in mammals

f3-large

f2-large

So there’s this 2013 study that examines mammalian diversity in relation to the spread of angiosperms, and there are several interesting results. For instance, carnivorous/insectivorous species underwent a decline with the spread of angiosperms, while Argentoconodon is apparently a carnivore while Volaticotherium is an insectivore (both relevant to my flying volaticotheres post). Most interestingly, it paints a rather interesting picture on the development of mammalian herbivory.

As you can see above, through most of the Mesozoic mammals were predominantly animalivorous. By the Early Jurassic there was already a vast diversity of mammalian and quasi-mammalian species; insectivores such as Megazostrodon and Kuehneotherium branched into hard-shelled and soft-prey specialists respectively, while we see the appearance of relatively large sized carnivores like Sinoconodon and the aerial volaticotheres. This trend continues into the Late Jurassic and Early Cretaceous periods, which see a further diversification of mammals into aquatic, fossorial, arboreal and even larger sized carnivorous species. Essentially, we see a guild of insectivores and carnivores in just about any niche available.

However, through most of the Jurassic and Early Cretaceous only one lineage of mammals, the multituberculates, appear to have ventured into herbivorous niches. Even haramiyidans, traditionally considered herbivores or omnivores, range within the insectivore space (though in fairness only one genus is taken into account). And through most of the Mesozoic, multituberculates only do so tentatively, mostly staying within granivore or animalivorous niches. Only in the Late Cretaceous do they venture into fully herbivorous niches, alongside at least two other mammalian clades, the eutherian zhelestids and dryolestoid mesungulatoids (both unaccounted for in the graph but the latter alluded to in the paper).

So, in essence, through most of the age of the dinosaurs mammals were predominantly carnivorous, only occasionally touching granivorous niches until the very end, when fully herbivorous mammals explode in diversity. In spite of their diversity in locomotion methods and size, mammals remained more or less barred from predominantly plant-eating habits until late in the game.

This is in contrast with other groups, such as lepidosaurs and theropods, which did experiment with herbivory early on. Crocodylomorphs appear to follow a similar pattern, with most of the Mesozoic seeing a variety of carnivorous species but only witnessing the rise of herbivorous taxa in the Late Cretaceous; the same might also apply to pterosaurs, if the edentulous tapejarids were in fact significantly herbivorous. The consistent dietary range through time seems to imply that preservation bias is not influencing the results (beyond showing exactly when the “herbivorous turnover” occurred).

This is quite interesting for a variety of reasons. While the paper does warn against equating the evolution of mammalian herbivory with the spread of angiosperms, the fact that the first mammalian herbivores were seed-eaters might imply that mammals were unable to convert into conventional herbivory directly, having to go through a granivore stage first. This clearly applies to multituberculates, though it remains to be seen if it also applies to zhelestids and mesungulatoids.

This might give an insight to how herbivory developed in tetrapods. Tetrapods as a whole are ancestrally animalivorous, but explored herbivorous niches multiple times. It is possible that granivory could have bridged between insectivorous or carnivorous habits and full-fledged herbivory it at least some groups, drawing in through their metabolic rewards but offering a degree of structural complexity that needs to be dealt with. This is particularly interesting in groups such as dinosaurs and anomodonts, in which herbivorous representatives are often beaked or have “buck-teeth” and, like mammals, are endothermic, higher energetic needs that could imply a need for such a transition.

Another important insight is how Mesozoic trophic dynamics changed through time. Mammals were for the longest time barred from an important part of terrestrial ecologies and fulfilled mostly secondary and above consumer roles. This might explain the decline of carnivorous and insectivorous species in the medial Cretaceous, as the higher trophic levels would render them more vulnerable to sudden ecological turnovers. As pointed out in the paper, the more omnivorous therians and meridiolestidans managed to thrive and expanded into the niches left by non-multituberculate mammal groups.

More importantly, this might be part of a much larger turnover. Amidst Jurassic tetrapods only dinosaurs and tritylodontid synapsids appear to have specialised significantly towards herbivory, with a few crocolymorphs and sphenodonts probably veering towards omnivorous habits. Given that the Late Cretaceous sees a much higher diversity of herbivorous tetrapods, including notosuchians, sphenodonts, squamates, turtles and of course mammals, it might suggest that Mesozoic ecosystems couldn’t support many herbivorous guilds we now take for granted, and that floral turnovers such as the spread of angiosperms created new ecological niches that didn’t exist before.

References

http://rspb.royalsocietypublishing.org/content/280/1771/20132110

http://www.nature.com/nature/journal/v512/n7514/full/nature13622.html

https://www.researchgate.net/publication/272791355_A_multivariate_approach_to_infer_locomotor_modes_in_Mesozoic_mammals

Michael J. Benton,Mikhail A. Shishkin,David M. Unwin, The Age of Dinosaurs in Russia and Mongolia

JENNIFER BOTHA-BRINK and KENNETH D. ANGIELCZYK, Do extraordinarily high growth rates in Permo-Triassic dicynodonts (Therapsida, Anomodontia) explain their success before and after the end-Permian extinction?, Version of Record online: 26 JUL 2010 DOI: 10.1111/j.1096-3642.2009.00601.x

 

Largest Mesozoic Mammals

Schowalteria clemensi: Known only from one skull, but seems comparable to latter taeniodonts in size, ranging somewhere between 10 to 50 kg. A specialised herbivore.

2. Bubodens magnus: Represented by a single tooth. It is enormous by multituberculate standards, and probably indicative of an animal above 16 kg (it is described as “beaver sized”). Presumably a specialised herbivore.

3. Repenomamus giganticus: Only “giant” Mesozoic mammal known from fairly good material. Measuring about a meter long and weighting at least up to 14 kg. A specialised carnivore.

4. Kollikodon ritchiei: Conflicting sources on this one. It may have been up to a meter long, certainly putting it above R. giganticus (monotremes are proportionally much more robust), but some sources also list it as “platypus size”. A molluscivore or piscivore.

5. Oxlestes grandis: Possibly slightly smaller than R. giganticus. There is some debate about how large its skull was (10 vs 7.5 centimeters), though the former seems to be the most convincing measurements for now. A carnivore.

6. Khuduklestes bohlini: “Subequal” in size to O. grandis. Possibly carnivorous.

7. Mesungulatids: Most sources are rather vague on estimated sizes (in part due to the lack of modern analogues, in part due to how rare postcranial material is), but the larger forms likeColoniatherium seem to be around 6-13 kg. Specialised herbivores.

8. Vintana sertichi: Known from only one skull. Estimated to be around 9 kilos. Specialised herbivore.

9. Altacreodus magnus: Known from various specimens. At around 9 kilos, it is the largest of the Hell Creek mammals. Specialised carnivore.

10. Didelphodon vorax: Known from several remains. The largest of the Hell Creek metatherians at 6-9 kilos. Molluscivore or carnivore.