Monday, 12 October 2026

Proto-mammals (probably) didn't look too mammal-like

I would not really consider myself a paleoartist, at least not in a professional sense. I have drawn extinct animals for various projects of mine but I see myself more as a writer. This might make it unusual that now I do not write merely about the history of paleoart but directly comment on paleoartistic tropes and trends and criticize those. But I hope you will see how it came to this point and why I want to talk about it.

My attempt at reconstructing the Early Triassic cynodont Cynognathus crateronotus, which used to be a popular staple in older books but is nowadays only rarely featured for some reason. Is this attempt too conservative? Shouldn't this animal have fur, a distinctive leathery nose, maybe pointy ears, looking basically like a dog with a lizard tail, as many like to reconstruct it? Well, as you will read, maybe not.

At my current job for the Natural History Museum Zürich (NMZ) I help with the research on quite a project, which we call the Great Journey (no relation to the Halo games, sadly), which will be a 60-metre long diorama depicting the last 600 million years of life on Earth. Currently we are working on the prototype, which will be “just” 10 metres and depict “just” a timespan of 100 million years, beginning with the start of the Triassic and ending in the Late Jurassic. This will be populated by around 300 miniature models and my task is to write the research-dossiers for these, which our 3D-artists can then use as reference material to make and 3D-print our models. I can not show those models off just yet, but trust me, you will love them.

Now, as you may guess, the first half of the Mesozoic holds a lot of extremely important events in the history of life. A marine ecosystem revolution, the rise of the archosaurs, including the dinosaurs and pterosaurs, the origin of various marine reptile clades, turtles, lepidosaurs, all our modern conifer families, and, of course, the evolution of mammals as we know them from the more archaic therapsids. Regarding the latter, we of course will have a couple of taxa which show these transitions. More basal therapsids, like the dicynodont Dinodontosaurus, early cynognaths like Cynognathus and Exaeretodon, mammaliamorph tritylodonts like Kayentatherium, famous mammaliaforms like Adelobasileus, Morganucodon and Castorocauda and eventually of course true crown-group Mammalia like Pseudotribos and Juramaia.

Fuzzy, snow-bound dinocephalians and therocephalians of the Middle Permian, as illustrated by Robert Bakker in the famous 1975 Dinosaur Renaissance issue of Scientific American. Are such speculations, which continue being made by paleoartists to this day, reasonable for early therapsids or do we now have sufficient data to doubt them? Source: Bakker 1975.

As you can imagine, my main goal has been figuring out for our artists what any of these animals would have looked like in life. Many artists in recent years (not naming names) have, likely influenced by the All Yesterdays movement, tended to give almost any of these animals features we associate with modern mammals but would rarely, if ever, fossilize: External ear pinnae, a soft, mobile rhinarium, long whiskers, a full fur coat and sometimes even the ability to lactate. But in my research, as well as after talking to experts, I have realized there is a noted discrepancy between how artists traditionally depict these animals and what current data, both directly from fossils as well as palaeoneurological work, says about the soft tissue anatomy of synapsids before the evolution of crown-group mammals. While these synapsids were closely related to or ancestral to mammals, this does not mean that they already possessed every trait we associate with mammals. In many ways these were very unique animals in their own right, not readily comparable with anything that survives today and drawing them in such an altogether too “modern” way may be just as erroneous and overenthusiastic as giving full feather-coats to sauropod dinosaurs. In some ways we might even have to go back and re-evaluate some of the supposedly outdated art that was made back when non-mammal synapsids were still thought of as “mammal-like reptiles” (just in case it needs to be said, synapsids do not descend from reptiles, they just share a common ancestor and are an amniote group of their own).

Let’s just go over a few traits:

The Myth of Thrinaxodon Whiskers

Early cynodonts such as Thrinaxodon and Cynognathus, as well as therocephalians and even gorgonopsians, are popularly depicted as having had whiskers (vibrissae) sprouting from their snouts. The origins of this go all the way back to D.M.S. Watson in 1931, who claimed that the densely arranged pits in the skulls of these animals were in fact anchoring points for vibrissae. This claim has often been repeated, rather uncritically, for the better part of a century. That is despite there being two rather major problems with the whisker-pit-hypothesis: 1) These supposed whisker-pits are in density and form nearly identical to the foramina also seen in various sauropsid reptiles, which obviously do not have whiskers and in which these pits instead house blood vessels that support lips and other facial tissues (Benoit et al. 2019), and, perhaps much more importantly: 2) No mammal today actually has such whisker pits in its skull (Benoit et al. 2019). Therefore, these pits cannot be used as an indicator for the presence of vibrissae. If anything, the foramina are more direct evidence for these animals having had thin, immobile, lizard-like lips, in contrast to modern mammals whose thick, muscly facial tissues are reflected in very smooth skull bone textures (Witton 2018).

Can we use other potential markers to determine when exactly vibrissae would have appeared in synapsids? Indeed, we can. In a series of papers starting in 2016, Julien Benoit et al. have looked at the evolution of the infraorbital nerve in synapsids and came to some rather insightful conclusions on the issue. In simple terms, the infraorbital nerve is what in modern mammals controls the movement and feel of the whiskers, as well as the rhinarium (the fleshy, mobile, wet nose typical of many mammals, more on which later). In mammals, the nerve is short and enters the infraorbital canal under the eye, where it lies unenclosed. This gives it enough space in order to be able to move the whiskers and rhinarium as effective tactile sense organs (Benoit et al. 2016). Reptiles also have this nerve, but it is much longer and passes together as a bundle with other nerves through a fully enclosing maxillary canal, giving it no room for mobility, thereby preventing the complex movement of facial tissues. (Note that the language used in the relevant papers is highly technical and I am not an expert in palaeoneurology, so I am sorry if the details here are a bit too oversimplified).

The evolution of the maxillary canal as well as the Msx2 gene coupled with the pineal eye both suggest that the genetic and neurological prerequisites for whiskers and more mammal-like snouts in general do not appear until the emergence of the derived cynodont clade Probainognathia. Source: Benoit et al. 2016.

How does the situation look alongside the therapsid-mammal spectrum? Here the data shows rather unequivocally that the typical mammalian condition of the infraorbital nerve does not evolve in cynodonts until the appearance of the clade Probainognathia (Benoit et al. 2016 & 2019). This is a rather advanced clade of cynodonts, consisting mainly of mammaliamorphs and their closest relatives, like the puma-like Trucidocynodon. It excludes its sister-group, Cynognathia (which Cynognathus and the large, herbivorous traversodonts are part of) and any earlier cynodonts, like Thrinaxodon. These animals, as well as more basal therapsids like the therocephalians and gorgonopsians, are instead found to have had the reptile-like (or rather ancestral amniote) condition, with the trigeminal nerve fully enclosed by the maxillary canal (Benoit et al. 2016 & 2019). This is not just a strong indicator but actually a direct falsification of the idea that these animals would have had mobile whiskers, as the ossification meant the nerve was immobile and therefore could not have controlled any tactile organs on the face in an effective manner (Benoit et al. 2019). Therefore, it is pretty safe to say, whiskers first appear in Probainognathia, not earlier.

What further supports this is that Probainognathia is the first synapsid clade in which the pineal/parietal eye (a primitive third eye on the forehead ancestral to all vertebrates) is permanently lost (Benoit et al. 2016). Studies on lizards, mice and humans have shown that the evolution and loss of this feature is controlled by the homeogene Msx2, which in mammals also affects the expression of certain skin and brain traits. Mice with a knocked-out Msx2 develop a rudimentary pineal eye, but at the cost of a deficient development of hair follicles and milk glands (Benoit et al. 2016), traits that are elemental to mammals as we know them, alongside other interesting losses, like a diminished cerebellum. The latter is further supported by the enlarged brain endocasts on probainognaths compared to earlier synapsids (Benoit et al. 2016). So, in short, if a synapsid still had a pineal eye, it's very unlikely it had whiskers, full-body fur or mammal-like mental faculties. The fact that we can use simple osteological characters like that to make assessments about the genetics and soft tissue traits of long extinct organisms is quite amazing, if I dare say so myself.

Splitting Hairs

So, where does this leave us in regards to one of the most characteristic mammal traits: Hair? Well, that depends on what hypothesis you subscribe to as to the original function of hair. Today, fur’s primary function is thermal insulation, allowing homeothermic endotherms to retain and keep steady the costly body warmth that has been generated by the animal’s elevated metabolism. Therefore, the appearance of fur has often been linked to the emergence of endothermy, which, according to bone histology, may potentially stretch back all the way to Permian pelycosaurs like Ophiacodon (Shelton & Sander 2015). Some recent work suggests that some form of endothermy may even have been ancestral to amniotes as a whole (Grigg et al. 2022). However, evolution does not work with foresight. You cannot go from naked skin to a fully insulating fur-coat overnight. There must have been some intermediate stage in which only part of the body was covered in hair, at which point it could not have had functioned sufficiently as an insulator. Going out at night with nothing on but socks may be kinky, but not good for survival. The original function of hairs must have therefore been something else, it being co-opted for insulation only later. As hair follicles are closely connected to nerves, the most commonly cited hypothesis has therefore been that hairs originally evolved purely as tactile sense organs, in most cases meaning whiskers. So, if you subscribe to the whiskers-first approach, then the paleoneurological works discussed to far seem to indicate that fur coats do not predate the emergence of Probainognathia, something also suggested by the authors. The soft-tissue fossil record seems to corroborate this. Even if endothermic in some form, all known skin imprints and trace fossils from pelycosaurs, including famous taxa like Dimetrodon, seem to show only reptile-like scales or scutes (Niedźwiedzki & Bojanowski 2012, Marchetti et al. 2025). Early therapsids, like the dinocephalian Estemmenosuchus, show only smooth, glandular skin reminiscent of that of big, hairless mammals today (Chudinov 1968), as does the recently described fossil mummy of the famous dicynodont Lystrosaurus from the Karoo Basin (Smith et al. 2022), a more derived neotherapsid. In other words, classic artists like Charles R. Knight and Zdeněk Burian seem to have portrayed these animals more accurately than some have tended to give them credit for. 

A fossilized mummy of a Lystrosaurus, described in 2022 from the Karoo Basin. It shows mainly a leathery skin with some pits and dimples, which may be skin glands. This largely affirms past reconstructions showing these animals as being naked instead of furry. Though taking taphonomy into account, could this animal still have had faint beginnings of hair somewhere on the body that did not preserve? Source: Smith et al. 2022. 

So, if you want to err on the safe side, you should probably not depict synapsids more basal than Probainognathia with fur coats. Sorry, fuzzy dicynodont fans (which sadly includes me). This statement comes with some asterisks, however. Even Benoit et al. do not exclude the possibility that some form of hair could have predated Probainognathia, even if it did not yet form a fur coat or specialized whiskers. While the Msx2 gene controls the density and distribution of hair follicles, it does not control the growth of hair itself, which is tied to different genes (Benoit et al. 2016). It is entirely possible that, instead of specialized whiskers at the front of the snout, the original tactile hairs were instead isolated filaments or strands occurring sporadically across the whole body, a condition that would have been helpful to small, burrowing animals, like Thrinaxodon and Diictodon, to feel their way through dark tunnels. I want to note that this is indeed the condition seen today in the naked mole-rat (Heterocephalus glaber) which has secondarily become almost cold-blooded, but still retains some isolated tactile hairs across the body and tail to aid its burrowing lifestyle. Some artists have indeed been depicting early synapsids with such a naked-mole-rat-look for a long time (good, old Dmitry Bogdanov of Wikimedia fame comes to mind). From this condition, once early probainognaths started trending towards not just a burrowing but also nocturnal life and full warm-bloodedness, the early tactile hairs would have been co-opted into specialized whiskers at the front of the head and isolating fur coats for the rest of the body. If you ascribe to this scenario, then note that such isolated tactile body hairs would only make sense in a burrowing or nocturnal synapsid. Large, above-ground-dwelling animals that were active during the daytime probably had limited or no use for these, so I would still recommend giving the likes of Inostrancevia, Lisowicia and Cynognathus a proper shave.

Isolated tactile hairs along the body could make sense even for a not-wholly-homeothermic, burrowing animal, as evidenced by the modern naked mole-rat. Is it possible such earliest beginnings of hair (which are very unlikely to be preserved, even in mummies) could have been present in some burrowing therapsids like Lystrosaurus? Source: Wikimedia.
 

It is also interesting to note that while probainognaths are the first clade in which all subsequent taxa lack a pineal eye, some individual genera of earlier cynodonts, dicynodonts and therocephalians seem to have convergently also lost this trait at separate points in time (Benoit et al. 2016). Assuming the same mechanisms of a mutated Msx2 gene are at work here, and assuming that some form of hair follicle did exist in the neotherapsid ancestor, I wonder if this meant that these particular genera also independently evolved a denser distribution of hair follicles. So, I am guessing, if you really want to draw a fuzzy dicynodont, only go for the ones known to have definitely lacked a pineal eye, such as Thliptosaurus.

The one fossil piece of evidence for pre-probainognath hairs are coprolites found from the Latest Permian of Russia, which seem to show hair-like filaments alongside half-digested bones of cynodonts and dicynodonts (Bajdek et al. 2015) (the animal that ate and excreted the victims seems to have ominously been a Proterosuchus-like archosauriform). While these are very exciting and suggestive, it cannot be definitively proven that these really are hairs, as the filament-like structures could have also been produced by plants or fungi (Benoit et al. 2016). The case therefore remains ambiguous. I have also heard of rumours that some of the Karoo mummies previously mentioned may not have been as naked as it at first seemed, but so far, that is just it. Rumours.  

The Nose Knows

Another long-standing artistic tradition is equipping theriodont synapsids (gorgonopsians, therocephalians and cynodonts) with a rhinarium. A rhinarium or muzzle is the characteristic mobile, fleshy nose/snout very distinct from the rest of the head that is unique to mammals and not found in any other tetrapods. It’s the wet, leathery nose of your pet cat or dog. At first glance, this might make sense and is a neat way for the artist to make these animals seem more familiar. The skulls of these early therapsids are vaguely dog-like and especially the shape of the front of the snout looks suggestive of a dog-like profile. But the keyword here is profile. The outline of the skull of a gorgonopsian or cynodont looks vaguely like the profile of a living dog, but have you actually compared them to the skull of a dog? There is a night-and-day difference there. Where there is solid bone in the nose region of these animals, in a dog or almost any other therian mammal, there is just nothing. Most mammals in effect only have a single aperture for the nose, with no bone separating the left and right nares (Higashiyama et al. 2019). In contrast, most of everything outside of Theria has clearly separated bony nostrils, and just very “bone-y” snouts in general. This seems to have largely been overlooked by most artists, continuing to equip the likes of Morganucodon with distinct, rodent-like noses, but the fact that this would have massive consequences for the life appearance of these animals was effectively shown by Higashiyama et al. 2019. In their study, they closely looked at the embryological development of the snout across various tetrapods and compared them with fossil synapsids and came to a very significant conclusion: Therian mammals (marsupials + placentals) have completely lost the premaxilla bone, which in all other vertebrates forms the tip of the jaw. 

As can be seen here, all synapsids prior to Mammalia possess a fully intact premaxilla and enclosed, boney nose-ring, much like other early amniotes. Only in therian mammals is the premaxilla completely lost and its function is replaced by an enlarged septomaxilla, which directly correlates with the presence of a rhinarium as an evolutionary novelty in therians. So, no dog-noses for the admittedly dog-like gorgonopsians, therocephalians etc. Source: Higashiyama et al. 2019.

What had long been thought to be the mammalian premaxilla is actually an enlarged septomaxilla, which in other tetrapods is usually a small bone inside the nose. It is the loss of the first bone and the expansion of the latter which allowed the mobile rhinarium to develop in the first place, as a true evolutionary novelty that makes us extremely alien compared to other tetrapods (Higashiyama et al. 2019). Really think about how weird that is for a moment. What does this mean for every animal outside of Theria? The condition is unclear for near-therian Mesozoic mammals like multituberculates (Higashiyama et al. 2019). Monotremes like the echidna and platypus still have an intact premaxilla (at least at an embryonal stage). Their nostrils are leathery but otherwise do not form a special organ separate from the rest of the snout. While an early expansion of the septomaxilla (comparable to what is observed in echidna embryos) can already be seen in early cynodonts and therocephalians, the fact remains that every synapsid outside of crown-group Mammalia retains a fully intact premaxilla even into adulthood, forming a prominent bone ring between the nostrils (Higashiyama et al. 2019). Mammaliaforms like the docodonts and the famous Morganucodon have one, tritylodonts like Kayentatherium have one, every earlier cynodont has one, as does every synapsid that came before them. Their noses must have been immobile and in general quite indistinct from the rest of the head. In other words, their noses would have looked more like those of other tetrapods, such as reptiles and amphibians (Higashiyama et al. 2019). The dog-like heads of some theriodonts are merely an illusion, in the flesh they would have resembled more a reptilian caricature of a dog. Taking Benoit et al. 2016 & 2019 into account, I think it is possible that, starting with Probainognathia, the texture of the nose may have become more distinct, being covered by more sensitive, leathery skin as seen in the echidna and platypus, but that would still not change the fact that most of everything outside of Theria would still have had very “boney” or “shrink-wrapped” immobile noses, with the non-muscular skin adhering close to the shape of the skull’s snout tip. There is surprisingly still little art that accurately reflects this. 

The rhinarium is a structure unique to therian mammals alone that differentiates them from other tetrapods, where the snout is ordinarily formed by the premaxilla. Source: Higashiyama et al. 2019.

 

Can you lend me an ear?

Another typical mammalian trait that has been applied in a rather laissez-faire attitude across various synapsid clades has been the external ear pinna, or “Ohrmuschel”, as it is known in my hood. Like most things discussed so far, this is a purely soft-tissue trait that does not readily fossilize. Though at least one fossil pinna, quite large and mouse-like, is known from the Mesozoic mammals Spinolestes, which lived around 125 million years ago, making it the oldest known example (Martin et al. 2015). Spinolestes was an eutriconodont, a basal clade of theriimorphs (true mammal on the lineage to the marsupials and placentals). As both marsupials and placentals also have pinnae, we can therefore say with reasonable certainty that all theriimorphs would have ancestrally had one too. That includes eutriconodonts, multituberculates, gondwanatheres, dryolestoids and therians. But just that clade? At first glance, monotremes seem to lack this trait, suggesting that, like the rhinarium, this is another unique therian-line adaptation. However, the echidna actually does have pinnae! They are just very small and embedded in the skin of the head (Augee et al. 2006, p. 84). This suggests, at least to me, that they are vestigial, which makes sense, as it is generally thought that the echidna evolved from a platypus-like aquatic ancestor and aquatic mammals tend to reduce their ears for streamlining purposes. It therefore seems likely to me that the last common ancestor of monotremes and therians, meaning the first true crown-group mammal, would have already possessed some form of ear pinnae. 

Illustration of the classic mammaliaform Morganucodon, from a 70s children's book. Prominently featured in such books as the "first mammal", it is often illustrated as here, looking like a shrew or rodent (which are derived placental mammals), with a distinctive, wet button-nose and pointy ears. In reality this animal would in many ways have been more archaic than even a platypus, so we may question such portrayals. Source: McCord 1977.

How does it look outside Mammalia? Probably less optimistic. According to the experts I have asked, the evolution of the external ear pinna is closely linked, for obvious reasons, to that of the typical mammalian inner ear, which was brought about by the distinctive secondary jaw joint that allowed mammals to repurpose two of their jaw bones into ear ossicles. In paleontology, in lieu of milk, the acquisition of this trait is central to distinguishing true mammals from non-mammal synapsids. All earlier mammaliaforms, like Morganucodon or Castorocauda, have the transitional jaw form, where they have both the typical mammalian jaw-joint as well as the earlier tetrapod-typical joint also seen in reptiles. It is possible that the incipient stages of the pinnae may have started somewhere here already, but as their jaw bones were not yet fully integrated into the ear, their hearing was less sensitive than in mammals, so there may not have been a need for them. Everything outside Mammaliaformes still had only the more archaic form of jaw-joint and therefore an even more reptile-like way of hearing. They would have just had simple ear holes with a tympanum right behind the jaw-joint. If you go back far enough, to the pelycosaurs, even ear holes seem to disappear completely, as these animals seem to have picked up vibrations from the ground and air solely with their jaw bones (Olroyd 2022). 

My own attempt at reconstructing Morganucodon (based on the skeletal from Rowe 2022). As you can see, there are no ear pinnae, no distinctive rhinarium for a nose, but added is a (perhaps too prominent) spur on the ankle of the foot (venom optional). An unfamiliar and altogether "archaic" look, but one that seems to be in line with the paleobiology of synapsids before the emergence of true mammals as we know them.
 

In conclusion, I would tend to not give pinnae to anything outside of Mammalia. And even within the crown-group mammals, one has to take into account that burrowing or aquatic lifestyles can lead to a reduction or loss of pinnae.

Greg, could you milk me?

As already alluded to, the most defining mammal trait, when not looking at the skeleton, is the ability to lactate and thereby feed one’s offspring with milk (a few fish and birds also feed their young with milk-like substances secreted from the body, but these are cases of convergent evolution). All living mammals do this in one form or another, so the trait must have already existed when the crown-group Mammalia emerged (though dedicated teats/nipples seem to be unique to the therian-line of mammals, as the monotremes still excrete milk from their bellies as a form of sweat). How old is this elemental link between mother and child? Some paleoartists have stretched lactation all the way back to ancient pelycosaurs like Dimetrodon, though how justified are such projections?

A speculative depiction of Dimetrodon, titled Texan Mama, illustrated by Mike Keesey and first appearing in 2013’s All Your Yesterdays. The animal has the sail covered in a fatty hump, no scales, tufts of hair and feeds its children with proto-milk-like secretions from its skin. How badly has this aged 13 years later?

Even if milk glands do not readily fossilize, we can make some inferences from anatomy and reproductive methods to see how likely it was that a synapsid excreted milk. One of the best starting points are the recently described in-ovo embyros of Lystrosaurus. These show that the chicks of these animals were mostly precocial (able to feed themselves shortly after hatching) and hatched from relatively large eggs, much larger in proportion than those of monotremes (Benoit et al. 2026). The most likely reason for this is that these eggs still contained large yolk sacs, much like turtles or birds, whereas in modern monotremes the lack of a large yolk sac and the resulting small egg size is compensated with lactation from the mother (Benoit et al. 2026). All of this makes it unlikely that Lystrosaurus mothers would have fed their chicks with milk. As Lystrosaurus most likely represents the ancestral condition for therapsids, this is may likely be true for all earlier synapsids (barring convergent evolution of this trait), so, sorry, no Dimetrodon milkshakes for you. At best it seems possible that some of these animals secreted the immediate “ancestor” of milk, a sort of sweat that mothers probably used to coat their leathery-shelled eggs with to protect them from desiccation, as still sometimes observed in monotremes (Benoit et al. 2026).

More intriguing and further up the family tree is the case of Kayentatherium, a tritylodont, a family of superficially rodent-like mammaliamorphs. A famous specimen shows a mother with her nest of about 38 perinates. The large number, curled poses as well as the relatively adult-looking proportions of these offspring were indicated by the original authors as evidence that Kayentatherium still reproduced much like a lizard, laying a large clutch of eggs with precocial offspring able to feed themselves right after hatching (Hoffman et al. 2018), therefore not needing milk. This was further corroborated by the fact that the teeth of some of the offspring already showed signs of use. The authors of the aforementioned Lystrosaurus eggs paper have however called some of these inferences into question. Some modern rodent babies can already be born with wear on their teeth, so this not as clear of a sign for precociality as it at first seems (Benoit et al. 2026). Furthermore, the inferred egg sizes for Kayentatherium must have been exceptionally tiny, much closer to what is seen in modern monotremes, thereby indicating only a very small amount of yolk and a necessity for milk from the mother (Benoit et al. 2026). These arguments are very intriguing, though the sheer size of the clutch to me still suggests a more archaic, reptile-like style of reproduction and I find it hard to imagine feeding up to 40 babies with milk, especially without dedicated teats. For reference, modern monotremes only lay 1 - 2 eggs per breeding season. 

Reconstruction of a Lystrosaurus embryo, preserved likely in-ovo (the soft, leathery eggshell unlikely to be preserved). Proportions of the embryo and likely size of the egg indicate that this animal reproduced much more like a tortoise than a platypus, so extended parental care and milk-feeding is very unlikely. Conditions may vary for other therapsids, however. Source: Benoit et al. 2026.
 

Teeth can also give some indication about the evolution of lactation. Most non-mammal synapsids seem to have had polyphyodont teeth, meaning that, just like reptiles, they continually replaced their teeth throughout their whole life. This in turn indicates that their diet must have stayed mostly the same throughout their whole life, without a distinctive milk-feeding stage. This seems to have been the case all the way up to the early mammaliaform Sinoconodon, which would conflict with the more basal Kayentatherium having had milk. Only with more derived mammaliaforms like the morganucodontans do we find clear evidence of diphyodonty (milk teeth being replaced once by a set of permanent teeth), as well as toothless young that clearly would have needed their mother’s milk to survive (Panciroli et al. 2017).

In summary, lactation probably became the status quo in mammaliaforms like Morganucodon and Castorocauda, but perhaps did not have its origins earlier than, at best, Mammaliamorpha, depending on the parenting style of Kayentatherium. Suckling Thrinaxodon, as seen in Walking with Dinosaurs, are therefore very unlikely.

In the spur of the moment

Instead of deducting traits, how about we add one? One feature that is frequently pointed at in the monotremes to show their supposed “freakiness” are the spurs on the ankles of their hindlimbs. In the adult platypus only the males possess these and they are capable of injecting a painful venom. In the echidna both males and females have these, but neither is venomous. What many laypeople do not seem to be aware of is that these spurs are not a unique oddity of the monotremes but used to be the status quo among mammals as a whole and we are actually the weird ones for having lost this trait.

Venomous ankle-spur of the male platypus. It is very likely one of your distant ancestors had one of these too.

This has been well-documented since Hurum et al. 2006 and still remains the consensus view among Mesozoic mammal-researchers today (pers. comm. Elsa Panciroli). What clearly are platypus-like ankle spurs have been found in a wide variety of specimens from pretty much every major group of Mesozoic mammal, including ones on our side of the family tree, such as multituberculates, eutriconodonts and spalacotheriids. It has also been found in earlier docodont mammaliaforms like Castorocauda (Ji et al. 2006). It is very likely that this was a trait ancestral to most or all Mammaliaformes, with it only being lost in the common ancestor of marsupials and placentals (Hurum et al. 2006).

Ankle-spur on the echidna.

Does this mean every individual pre-therian Mesozoic mammaliaform had them? Well, there is obviously some nuance involved here. For one, some lineages could have obviously secondarily lost it, just like our own ancestors did. For the other, the trait is clearly sexually dimorphic. As mentioned, only male platypodes have them and they use them in battles against other males, while the spur is also only vestigial in adult female echidnas. If a Mesozoic mammal is found without them it could just mean you have found a female specimen. On the other hand, this dimorphism may only be unique to the monotremes. Both male and female echidnas possess the spur, while young female platypodes also have the spur before losing it into adulthood, suggesting that both sexes having spurs was the original condition. Furthermore, the spur’s clear main function is as a venom-delivery system. Even if the echidna lacks venom, its spurs still have a hollow tube like in the platypus’, showing that its ancestors most likely had venom running through there too (Hurum et al. 2006). While platypodes mainly use them to fight each other, they can also use them as defensive weapons against predators. Hurum et al. 2006 speculated that in the earliest Mesozoic mammals this was likely the main function of these spurs, as a defensive weapon against small, predatory dinosaurs. If that is the case then it would make sense for this trait to not have originally been sexually dimorphic, as it would have been useful in daily survival for both males and females.

Don’t take this as gospel

A very rough attempt by me to visually summarize everything we have learned so far, starting with the traits already inherent to early "pelycosaur" synapsids and what each successive clade potentially added or subtracted. The resulting image seems to run against popular speculation of synapsids already looking pretty much like mammals by the Late Permian and instead shows a gradual accumulation of typical "mammalian" traits that protracts itself throughout the whole Triassic and probably into the Jurassic.

Ultimately most of what you have read here is speculative to some degree. I do think we have a better idea now of when certain traits appeared than we did two decades ago, but a lot of this is still based on phylogenetic inference and proxy skeletal-traits. These still leave a lot of room open for hypotheticals and unknown unknowns. To know the life appearance of these animals for certain, we really would need perfectly preserved Lagerstätte specimens from the Triassic or Permian. Here I would like to cite Brian Engh, with whom I had a lively email exchange about exactly this topic and who allowed me to directly quote him (afterwards, please be sure to check out his blog Living Relic Productions):

“Overall, I tend to think a lot of these questions come down to how we're interpreting ecospace for extinct animals for which we sometimes aren't 100% certain of the ecospace they actually inhabited (semiaquatic Kayentatherium definitely falls into this category). While in paleontology people often emphasize phylogenetics over soft tissue or behavioral plasticity, the reality here in real life is that there are lots and lots of examples where animals have adapted their soft tissues or behavior in response to niche radiation or other selective pressures, sometimes very very rapidly, and sometimes in ways that can't be detected in their skeletons or in incomplete fossils. People in paleo love to say "only (therian) mammals have motile facial tissues, reptiles do not!" but that's just not true. Crocodiles and several species of lizards have independently evolved muscular nostrils that can close when they dive. Hump-nosed lizards (Lyriocephalus) can retract their lips when feeding. Toad headed agamas (Phrynocephalus) have mobile cheek flaps. Several chameleons have novel motile ear-like "occipital flaps" for display. And birds show all kinds of weird, muscularized cranial kinesis not clearly indicated by their theropod dinosaur ancestors. Many of these features leave no indication on the skeleton (and with regards to some of the lizards, I've looked), and many have not been studied or published on. So, I tend to regard the rigidness with which paleo people interpret soft tissue adaptation according to (skeletal) phylogenies as an artifact of trusting their own preconceptions rather than actually observing nature. For these reasons I'm a mildly "controversial" artist for being willing to explore those unknown spectra of soft tissue adaptations that must have occurred between known fossil data points. There were almost certainly evolutionary intermediaries of all of these features.”


I think the moral of the story is to think critically when applying soft tissue traits, but to not treat skeletal proxies and phylogeny as absolute gospel. I agree with Brian, in the sense that many paleontologists seem to treat animals like engineers do machines, i.e. “this screw only fits with that socket, this hinge is only suited for such and such weights”, while living organisms show us time and time again that they are not bound to such rigidity, like the proverbial bumblebee that fails to adhere to aerodynamics. To every perceived rule there are exceptions at every corner. A prominent example is the recent finding of the early cynodont Chiniquodon, whose neonatal-line growth-rings suggest that it was born live instead of hatching from an egg (Gaetano et al. 2026). The authors go as far as speculating that live birth may have been the original condition in cynodonts, with monotremes somehow secondarily re-evolving egg-laying (Gaetano et al. 2026). I am aware that there are cases where such athing has been proposed for some squamates, but, as far as I am aware, not in any conclusive manner and if, then only within short time-spans after the first loss of oviparity. Therefore, I find this proposal unlikely that monotremes re-evolved egg-laying somehow after around 100 millions of years of viviparity. I find it far more likely that if Chiniquodon was viviparous (“if” because I am not sure if growth rings on an induvial are enough to say this conclusively compared to more direct evidence like a fossil embryo inside a mother), then it evolved this trait independently of later mammals. We have far more evidence for viviparity evolving repeatedly across vertebrates as a whole and I would not be surprised if far more synapsids evolved live birth independently too, even if the “main line” up to therian mammals was oviparous. The same could be the case for a wide variety of other traits. We can use phylogeny and proxies as a general guideline of what to expect at the base-line, but that does not mean various species could not have evolved certain traits independently ahead of the “mainstream”. Though I remain highly skeptical, maybe there were some shaggy dinocephalians or dicynodonts, maybe some gorgonopsians could snarl, maybe there were some pelycosaurs that could excrete some milk-like substance from their bellies. As long as it makes sense for the lifestyle of the respective animal.

If you liked this and other articles, please consider supporting me on Patreon. I am thankful for any amount, even if it is just 1$, as it will help me at dedicating more time to this blog and related projects. Patrons also gain early access to the draft-versions of these posts and my art.

Related articles:

References:

Posted by at
Labels: , , , ,

No comments:

Post a Comment

Subscribe to: Post Comments (Atom)