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.
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.
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:
– Spines similar to those of the modern spiny mice.
– The venom spurs.
What can you even say to that?
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.
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.
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
Updated 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.
Taeniolabis 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:
Artist uncredited but hosted here.
Two depictions (alongside other Paleocene mammals) credited as “De Agostini Picture Library”
Another uncredited picture, hosted here.
Picture by Stanton F. Fink (also depicting Psittacotherium, meaning that the animal is also undersized)
Depiction (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:
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
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.
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
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.
Mystriosuchus planirostris by @paleoart, the inspiration for this post.
2016 was many things, but one of the best was definitely being the call out year for many archaic paleoartist mistakes. One of these was the absence of lips in many reconstructions, from the skin-wrapped maws of theropod dinosaurs to the bare-toothed saber-toothed cats to the rather ridiculous depictions of entelodonts and other prehistoric mammals as fanged demons. This year saw the publication of various papers showing that teeth do generally in fact need lips to be protected from damage and moistened, meaning that many animals traditionally reconstructed as bared-toothed monsters need a healthy amount of oral tissue.
That said, things aren’t black and white. Crocodilians, after all, still have bare teeth. In one of these papers, Larson et al 2016, it’s been suggested that their aquatic habits compensate for their lack of lips, as humidity certainly isn’t a problem. However, as the Mark Witton link above informs you, many crocodiles go through prolonged periods of life on land without tooth degradation. It also doesn’t cover how terrestrial crocodylomorphs would have coped with the absence of lips, or why many aquatic vertebrates like dolphins (Platanista aside) still kept their lips.
It seems, therefore, that crocodiles are simply off in this regard. Their liplessness actually appears to derived from a highly unusual facial development process, which essentially renders their entire face a single “scale”. This seems to have evolved in order to develop the extensive Integrumentary Sense Organs (ISOs), thinning the facial skin in order to increase sensivity, and it carried over into their terrestrial descendants.
This obviously raises the question of whereas groups similar ecologically and morphologically to aquatic crocodilians underwent a similar process. Where they also lipless, or did they in fact retain their lips, making comparisons to crocodiles all the more questionable?
Phytosaur head diversity by Darren Naish. Taxa included: Smilosuchus gregorii, Pravusuchus hortus, Mystriosuchus westphali, Paleorhinus bransoni and Pseudopalatus pristinus.
Phytosaurs were, in some respects, the “original crocodiles”, having evolved and prospered long before crocodylomorphs ever touched the water. Although they weren’t particularly closely related (birds are closer to crocodiles than phytosaurs are), these archosauriform reptiles did hit most of the same notes as crocodiles: barrel-shaped bodies, extensive osteoderm armours (in some cases even better protected, due to the bell-shaped cap on the throat and various scutes on the forelimbs and belly), generally short limbs and large, paddle-like tails.
While some phytosaurs explored odd ecological niches – Nicrosaurus and similar taxa are adapted to a primarily terrestrial lifestyle, while Mystriosuchus was inversely so specialised to life in the water that it was practically the Triassic Metriorhynchus -, a generally semi-aquatic lifestyle for most phytosaurs can be inferred due to due sheer prevalence in freshwater and shallow marine deposits, limb proportions and shape, laterally flattened and powerful tails and retracted nostrils (though keep reading).
Various tracts attributed to these animals similarly imply a close functional match between phytosaurs and crocodiles. Various swimming tracts have been attributed to phytosaurs, while the Apatopus footprints show an interesting insight on these animals’ terrestrial locomotion capacities, being capable of an erect gait like archosaurs and mammals, including modern crocodiles and alligators. Paleopathology studies indicate similar behaviours such as interspecific biting (hence the need for strong armour), and perhaps more damningly endocast studies show that the general phytosaur brain shape was rather similar to that of modern crocodilians (albeit with a few differences, like the size of the brain and the presence of multiple sinuses; see below).
For all intents and purposes, phytosaurs were functionally crocodilian, offering one of the most extreme cases of convergent evolution ever recorded. But no matter how close, phytosaurs were still off the mark in various ways.
Phytosaur facial anatomy and morphology
Pseudopalatus buceros skulls, exemplifying the general morphology of phytosaur skulls as well as interspecific variation. Notice massive premaxila.
The most classical thing you’ve ever heard about phytosaurs was how they differ from crocodiles in having the nostrils be close to the eyes/on top of the head rather than at the tip of the snout. This is true; as you can see, the nostrils are located in front or above the eyes in a “volcano-like” elevation; combined with the nostril-less and often conical snouts, this gives them a distinctive dolphin-like profile.
Like in cetaceans, this nostril placement would come in handy on a mostly aquatic lifestyle, avoiding drag and allowing the animal to surface only a small part of the head and remain concealed underwater. However, unlike cetaceans – and marine reptiles such as plesiosaurs -, this nostril position is not derived from nasal retraction. In fact, phytosaur nostrils are sometimes noted as being rather protracted, sometimes as a result of the general elevation of the nasal region.
Instead, what happened is that phytosaurs elongated the premaxila at the expense of the other skull bones. Unlike crocodiles – and whales and plesiosaurs and many other aquatic tetrapods -, half or more of the phytosaur upper jaw is composed of a single bone, normally a vestige at the end of the jaw in most amniotes, that expanded radically. This hints at a pretty rapid elongation of the snout, explaining maybe why long-snouted phytosaurs appear “out of nowhere” in the fossil reccord.
Predictably, this could also hint at rather atypical development, which is etremely important in dictating the presence or absence of lips.
Another frequently cited difference is the presence of antorbital fenestrae. These are the famous “holes” in front of the eyes present in most dinosaurs and other archosauriform reptiles. Crocodiles have lost them, but they are present in phytosaurs, though they can be reduced in some species. Perhaps associated with this, phytosaurs also have extensive antorbital sinuses, while crocodilians lack them altogether. Phytosaurs also have an extensive premaxillary sinus, though as crocodilians have most of their snout taken by the nasal airways this may not make a lot of difference.
With a few exceptions, most aquatic crocodilians have conical teeth; they compensate for the lack of meat-cutting speciations with the infamous “death-rolls”. Phytosaurs, by contrast, generally have serrated teeth, and combined with the presence of crests on many specimens it seems unlikely that these animals engaged in “death-rolls”, instead opting for more typical meat-eating behaviours. To date longirostrine phytosaurs are the only “gharial-like” vertebrates with serrated teeth, and it might explain why they were frequently associated with the carcasses of terrestrial vertebrates like rhynchocephalians and dinosaurs.
Unlike the teeth of crocodiles, phytosaur teeth seem to be rarely interlocked. Even without lips, it seems likely that the upper jaw teeth covered the lower jaw ones.
What about the lips?
Leptosuchus skull, illustrating the basic points for and against phytosaur lips. For are in red: anteorbital fenestra and serrated teeth. Against are in green: long prexmaxila, POSSIBLE ISOs, front teeth POSSIBLY too large to fit within lips. The latter two are of course ambiguous.
With the above in mind, the absence for or against phytosaurian lips is…mixed.
The rapid premaxilary development in phytosaurs is the key to understanding how the jaw integument of these animals worked. It is possible that the premaxila’s growth prevented the formation of conventional lips, either due to physical and metabolic constraints or because the same genes triggering it could have prevented the development of lips. Perhaps the same pressures causing the crocodilian “single scale” would have been forced on phytosaurs by this developmental quirk.
On the other hand, other parts of the phytosaur skull anatomy seem to suggest the presence of lips:
- The aforementioned antorbital fenestrae suggests that the phytosaur skull was less “skin-tight” than that of crocodilians. In modern birds, the only living reptiles with antorbital fenestrae, that area of the skull is covered by various soft tissues, and indeed areas of the avian beak devoid of a rhamphotheca tend to be covered by fleshy lips.
- Serrated teeth tend to be more vulnerable than conical teeth to degradation, so most predatory animals that possess them have them covered by lips. The only crocodilians with clearly serrated teeth are terrestrial species and the fairly basal thalattosuchians, which are still on the limbo on whereas they had lips or not.
It is possible that phytosaurs found themselves in an unique integumental arrangement. Perhaps they did become lipless, with a “single scale” covering the jaws, while the rest of the head had a more normal integument.
A deciding factor in this argument would be the discovery of ISOs on phytosaur jaws. However, structures associated with these organs, such as pits, are rarely discussed outside of the context of pathology when it comes to these animals. There is plenty of literature on pits and holes in phytosaur skulls being caused by fights and bites, but few on any possible natural ones.
Modern Ganges River Dolphin. Although it has exposed teeth, it’s also not the norm among cetaceans.
Just because something resembles another doesn’t mean that there is an exact equivalency. Case in point: no matter how close phytosaurs got to crocodilians, they still differed in many aspects, and could not be mistaken for them in life.
It’s clear that skin-wrapping is a tremendous lack of apreciation for the organic nature of extinct animals. The lack of lips in crocodilians has been taken far too long to be the “norm”; but, as it turns out, it is an anomaly among the usual amniote tendencies.
We may never know for sure whereas phytosaurs had lips or not. Hell, it’s even possible that some had while others went full Platanista. However, far too often are they taken to be crocodile-like for granted, without other possibilities, equally as valid as they are, taking into consideration.
Hopefully, further research will grant us insights on how these already spectacular animals looked in life.
Reisz, R. R. & Larson, D. (2016) Dental anatomy and skull length to tooth size rations support the hypothesis that theropod dinosaurs had lips. 2016 Canadian Society of Vertebrate Paleontology Conference Abstracts, 64-65.
Grigg, G., & Kirshner, D. (2015). Biology and evolution of crocodylians. Csiro Publishing.
Soares, D. (2002). Neurology: an ancient sensory organ in crocodilians. Nature, 417(6886), 241-242.
Stocker, M. R. & Butler, R. J. 2013. Phytosauria. Geological Society, London, Special Publications 379, 91-117.
Kimmig, J. 2013. Possible secondarily terrestrial lifestyle in the European phytosaur Nicrosaurus kapfii (Late Triassic, Norian): a preliminary study. Bulletin of the New Mexico Museum of Natural History and Science 61, 306-312.
Gozzi, E. & Renesto, S.A. 2003. Complete specimen of Mystriosuchus (Reptilia, Phytosauria) from the Norian (Late Triassic) of Lombardy (Northern Italy). Rivista Italiana Di Paleontologia e Stratigrafia 109(3): 475-498.
Michelle R. Stoker; Sterling J. Nesbitt; Li-Jun Zhao; Xiao-Chun Wu; Chun Li (2016). “Mosaic evolution in Phytosauria: the origin of long-snouted morphologies based on a complete skeleton of a phytosaur from the Middle Triassic of China”. Society of Vertebrate Paleontology 76th Annual Meeting Program & Abstracts: 232.