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What did our protomammal ancestors and relatives look like when they were scampering around the Permian world? That's been an extremely difficult question to answer. The tradition, held since the time we used to call them "mammal-like reptiles", has been to go for a more saurian pastiche. The likes of Dimetrodon have often depicted as scaly, befitting their belly-dragging, sprawled posture, with only cynodonts and similar synapsids (that is, members of the wider group from which mammals sprung) getting roughly more mammalian treatments given their proximity to the emergence of the first beasts.
But paleoart keeps pushing the envelope. It's not uncommon to see fuzzier depictions of Dimetrodon these days, or gorgonopsids that look like strange dogs. The fossil record leaves little resolution. The bones of these animals tell us that they're closer to us than to reptiles, but the origin of fur and whiskers is confounded by a lack of fossilized soft tissues from the critical time period. That problem remains, but a new species of exceptionally-preserved protomammal from the 290 million year old rock of Germany adds a little more context to discussions of what early synapsids looked like.
Paleontologist Frederik Spindler have called the little creature Ascendonanus nestleri, several exceptional specimens revealing this synapsid in exceptional detail. The articulated skeletons show Ascendonanus to be lizard-like in overall form, Spindler and colleagues pointing to particular traits of the limbs and claws to suggest this protomammal spent most of its time in the trees. But what really got the attention of other paleontologists is the fact that the Ascendonanus specimens are preserved with traces of their skin.
Finding early synapsids with intact soft tissue is practically unheard of. (A specimen of the multi-horned Estemmenosuchus described in the 1960s was said to have smooth skin, but this hasn't been recently verified.) To get multiple specimens of a synapsid with integument is a big deal. In Ascendonanus, the skin contributed to the lizard-like look. Despite being more closely related to mammals than reptiles, the Ascendonanus fossils "show a regular scale pattern", the researchers write, similar to modern reptiles. This could mean that such a scale pattern was the ancestral condition for the last common ancestor of reptiles and mammals, although more fossils are needed to test the idea.
So what does this mean for our depictions of protomammals? One fossil is too little to speak for them all, but it throws some weight to the traditional theme of early synapsids being more reptile-like in appearance. This was already expected from other lines of evidence, especially given that fur is a sensory structure as well as an insulating one - the origin of fur and whiskers changed the brains of synapsids, and those alterations only show up later in the fossil record. Still, perhaps the most salient lesson Ascendonanus has for us is one that was articulated long ago by Charles Darwin when he pondered the early bird Archaeopteryx. We as yet know little of the prehistoric past, and thinking that we have in any way delivered the last word on the past is laughably premature. Ascendonanus is a reminder that even the most elusive finds can reward paleontological patience.
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Name: Ascendonanus nestleri
Meaning: Ascendonanus means "climbing dwarf", while nestleri honors Knut Nestler.
Age: Permian, about 290 million years ago.
Where in the world?: Saxony, Germany.
What sort of organism?: A protomammal belonging to a group called varanopids.
How much of the organism’s is known?: Several skeletons, including integument.
Reference:
Spindler, F., Werneburg, R., Schneider, J., Luthardt, L., Annacker, V., Rößler, R. 2018. First arboreal "pelycosaurs" (Synapsida: Varanopidae) from the early Permian Chemnitz Fossil Lagerstatte, SE Germany, with a review of varanopid phylogeny. PalZ. doi: 10.1007/s12542-018-0405-9
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