#mammaliaformes

[alphynix]

Early Mammals Had Dark and Dusky Grayish-Brown Fur, Study Suggests | Sci.News

Pelage coloration, which serves numerous functions, is crucial to the evolution of behavior, physiology, and habitat preferences of mammals.

[synapsidgirl]

So... I’ve been very busy. I still am busy, and don’t really have time for drawing. But I thought I’ll let you guys know I didn’t leave this blog completely. So here’s the last completed artwork I did, back in October, when I thought about doing inktober. Well, that didn’t work out. This guy is Miracinonyx trumani, based on, IIRC, Mauricio Anton’s skeletal reconstruction of this guy, and inspired by a photo of a cheetah running at top speed.

And here’s something I started about two or three weeks ago. I’ve been working on it on and off when I have a bit of spare time, but now I really don’t have any time to finish it, so it’ll probably be quite some time before it’s done.

And that’s, of course, another Thylacoleo carnifex, because I’m just never happy with how I restore this guy. Plus, it’s one of my favourite extinct creatures of all time, so I keep coming back to it. All that said, I think this one is going to be my favourite reconstruction of it once I’m finished, unless I royally mess up in the process.

So, there, I’m alive, I just don’t have any spare time for art :(

Oh, and hello to all my new followers! Somehow there’s over 1000 of you now. I don’t know when that happened, but thank you!

[alphynix]

The docodonts were a group of mammaliaformes (close relatives of the earliest true mammals) which lived across North America, Europe, and Asia from the Middle Jurassic to the Early Cretaceous. Originally only known from teeth and jaw fragments they were traditionally thought to be fairly generic shrew-like insectivores, but more recent discoveries of better fossils have revealed they were actually much more diverse, occupying ecological niches ranging from squirrel-like tree-climbers to mole-like diggers to beaver-otter-like swimmers.

Most of the more complete fossil material of these animals comes from the mid-Jurassic of China, but one species from elsewhere is also known from a partial skeleton.

Haldanodon exspectatus here lived in central Portugal during the Late Jurassic, about 155 million years ago. Around 15-20cm long (6-8"), it had small eyes and short chunky well-muscled limbs with the front paws adapted for digging. Since it inhabited a very swampy environment it probably wasn't a pure mole-like burrower – extensive tunnels would have constantly flooded – but it may have instead been a similar sort of semi-aquatic animal to modern platypuses and desmans, foraging for invertebrates in the water and excavating burrows in the banks.

Roughened areas of bone on its snout may also have supported a patch of tough keratinous skin, which would have helped protect its face while digging.

———

Nix Illustration | Tumblr | Pillowfort | Twitter | Patreon

[alphynix]

Month of Mesozoic Mammals #08: Tiny Transitional Hadrocodium

Falling evolutionarily between the docodonts and Mammalia itself, Hadrocodium is an important transitional form in the early mammal family tree. Something very similar to it would have been the common ancestor of all modern mammals.

Living in China during the Early Jurassic (196-189 mya), it was one of the first known mammals to have both an enlarged brain cavity and the characteristic middle ear bones of modern mammals, about 45 million years earlier than such traits were previously thought to have evolved.

It was also one of the smallest mammals of all time, measuring only about 3cm long (1.2″) -- similar in size to the smallest mammals alive today, the bumblebee bat and Etruscan shrew.

[alphynix]

Month of Mesozoic Mammals #16: So Many Gliders Maiopatagium

The haramiyidan featured yesterday was a ground-dwelling animal, but most others in the group were actually highly adapted for tree-climbing. They were very squirrel-like in appearance, with grasping hands and feet and tails that may have been prehensile -- and some took this lifestyle even further, becoming specialized gliders.

Living during the Late Jurassic of China (157-163 mya), Maiopatagium is one of at least four known gliding haramiyidans. It was about 25cm long (10″), around half of which was its long tail, and had a gliding membrane extending between its wrists and ankles. The proportions of its hands and feet were very similar to modern colugos and the feet of bats, which has been interpreted as evidence of the same sort of upside-down roosting behavior.

Its close relative Vilevolodon had rodent-like teeth highly adapted for crushing and grinding, suggesting these haramiyidans were herbivores feeding mainly on seeds and soft plant matter.

And these gliding haramiyidans also contribute to the confusing classification of haramiyidans -- because although Megaconus’ anatomy suggested they might be mammaliaformes, studies of another glider, Arboroharamiya, give a very different result. Its ear bones and jaw show the characteristics of true members of Mammalia, supporting the hypothesis that haramiyidans were actually close relatives (or ancestral to) the multituberculates.

[alphynix]

Month of Mesozoic Mammals #03: Almost-Mammals Adelobasileus & Sinoconodon

The exact line between “highly mammal-like cynodonts” and “actual mammals” is very blurry. The transition was gradual and the fossil record is incomplete, and even the definition of “mammal” varies depending on who you ask. Do we take the strictest possible route and only include everything coming after the most recent common ancestor of all living mammals -- the “crown group” Mammalia itself? Or do we go broader and also include the closely related Mammaliaformes, which already had some of the defining anatomical features of mammals?

(For the purposes of this theme month I’m considering mammaliaformes to count as mammals, but if you prefer the crown group definition then it’ll be a few more days before we reach Mammalia-proper.)

The earliest ancestral mammaliaformes would have looked something like Adelobasileus, a transitional form from the Late Triassic of Texas, USA (221-205 mya). About 10-15cm long (4-6″), it was probably a shrew-like insectivore and may have been close to the start of the hypothetical “nocturnal bottleneck” in mammal evolution -- a point where mammal ancestors are thought to have taken up nighttime activity patterns to avoid competition and predation from early dinosaurs.

Sinoconodon is known from the Early Jurassic of China (196-189 mya). Unlike later mammals it seems to have experienced reptile-like continuous slow growth throughout its lifespan, and had multiple replacements of some of its teeth.

Fossils of several different life stages have been found, averaging at similar sizes to Adelobasileus, but the biggest and longest-lived specimens are estimated to have reached the size of a large brown rat at around 35cm long (1′2″) and 500g in weight (~18oz) -- big enough to be a weasel-like carnivore feeding on small vertebrate prey.

[alphynix]

Month of Mesozoic Mammals #05: Climbing Trees Agilodocodon

Before we get to the actual-Mammalia-mammals, there’s one more group of mammaliaformes who deserve some attention -- the docodonts.

Falling evolutionarily just outside of Mammalia itself, docodonts first appeared in the mid-Jurassic and lasted until the Early Cretaceous. They used to only be known from teeth and jaw fragments and were thought to have been fairly generic shrew-like terrestrial insectivores, but more recent discoveries have shown them to have actually been some of the earliest mammals to specialize into diverse habitats.

Agilodocodon was adapted for climbing around in trees, making it one of the earliest known arboreal mammals (although not the first climbing synapsid). Living in China during the Middle Jurassic (165-161 mya), it measured about 13cm long (5″) and had sharp gripping claws and flexible wrists and ankles similar to modern climbing mammals like tree squirrels.

When it was first described in 2015 it was suggested that its spade-like front teeth were specialized for gnawing bark and feeding on tree sap -- but a later study found that its teeth didn’t really resemble those of any modern sap-eating mammals, and in fact were closer in shape to those of insectivorous marsupials and elephant shrews.

[galswintha]

The earliest-known mammaliaform fossil from Greenland sheds light on origin of mammals | PNAS

https://www.pnas.org/content/early/2020/10/07/2012437117

This study shows a comprehensive analysis of an early mammaliaform dentary, which combines data from comparative anatomy, CT scanning, and FEA. The new fossil fills an important gap in our understanding of mammaliaform evolution showing a transitional stage between triconodont-like molariform pattern of morganucodontids and multicusped pattern of haramiyidans.