Milk Sweat Since 110MYA

“Q: could you explain a bit of the what-the-fuckery that is monotremes? how did they evolve, and how have they survived for so long? they separated from the rest of mammals so long ago, and everything else that separated there is extinct, but not monotremes. is there a reason for that, some kind of adaptation?”

— Anonymous

ALRIGHT MY FRIENDS hold on to your egg-laying horses because this will be a convoluted and likely uncomfortable ride.

Photo from Pinterest

Monotremes, represented these days by platypuses and echidnas, are super goddamn weird mammals. They’re an ungodly patchwork of highly specialized, definitively mammalian traits mixed with some equally primitive reptilian-esque ones, which result in a hot mess that gives me a headache.

For example, monotremes:

• Lay eggs, but produce milk from mammary glands to feed their young. Not in a normal way, but in a “leaking out of your pores” kind of way.

• Their X chromosomes resemble those of birds more than they do ours, and they have a cloaca. Why

• Have tribosphenic molars and inner ear bones incorporated into the skull, both incontrovertible traits of therian mammals. Finally, proof that they are-

• JUST KIDDING it turns out both of those things could have convergently evolved!! haha! HILARIOUS

• Their gait is closer to the sprawling synapsid gait than the erect therian one, I can only assume because they are going for a cool and retro look

• Though platypus venom is derived from proteins also found in the immune systems of therian mammals, guess who else uses those proteins in venom?

• REPTILES.

• AUGH

Photo from Understanding Evolution

To add insult to injury here, the fossil record for monotremes is fairly garbage, with the absolute earliest examples (Teinolophus and Steropodon - shown below) already being recognizably similar to extant monotremes. These two species are dated to the Early Cretaceous, but based on genetic and molecular data it is thought that monotremes diverged from all other mammals at the very least in the Early Jurassic, and potentially as early as the Late Triassic.

Photo from Australian Museum

Of course, it is almost impossible that we’ll ever stumble across the fossil remains of an actual direct ancestor to extant taxa, due to the rarity of fossilization in general. The fossil monotremes that we do have are from a period of highly successful radiation - not only have they been found in several locales across Australia and New Guinea, but a single ancestral platypus tooth was found in South America, proving that monotremes were once fairly widespread across the Gondwana supercontinent. 

So, we have no proof of how and when monotremes came to be, and a lot of their evolutionary history is just straight up missing. The ones that have made it this far likely did so because they managed to grab a hyper-specific niche that live-bearing mammals couldn’t steal from them. Until more fossils are found, the rest is just conjecture.

tl;dr: monotremes are the ultimate wannabe-Triassic mammalian hipsters and nobody knows why we still put up with them

---

SOURCES YO

The evidence lines up in early mammal evolution (2011)

Molecules, morphology, and ecology indicate a recent, amphibious ancestry for echidnas (2009)

Genome analysis of the platypus reveals unique signatures of evolution (2008)

The Platypus Is in Its Place: Nuclear Genes and Indels Confirm the Sister Group Relation of Monotremes and Therians (2005)

Independent Origins of Middle Ear Bones in Monotremes and Therians (2005)

Review of the monotreme fossil record and comparison of palaeontological and molecular data (2003)

http://www.ucmp.berkeley.edu/mammal/monotremefr.html

Fluffosaurus rex

“Q: did t. rex’s have feathers? i’ve been getting conflicting answers”

— Anonymous

I hate to have to tell you this, but the reason you are getting conflicting answers is because the answer itself is unresolved. But never fear, my friend, you came to the right place! I will gladly muddy the situation further by explaining why. No, no, don’t thank me. I am here to serve.

The main problem is that so far no large, fossilized skin impressions attributed to T. rex have been discovered, which is why we can’t say for certain if they were or were not covered in scales, feathers, or some ungodly mix of both.

Photo from SAURIAN

Without direct evidence, reconstructions of T. rex are forced to rely on skin impressions and fossils from other dinosaurs, with the likelihood of similarities being based on how closely they’re related; whether or not the species is an ancestor or descendant of the tyrannosauroids; whether they are the same size, same environment, same niche… you get the idea. It’s complicated.

And we do have many skin impressions of dinosaurs that were definitively scaly. Stegosaurs, allosaurs, ankylosaurs and many other species have had soft tissue impressions found, all with scales. It is not completely unreasonable to say that T. rex could also have been entirely scaled.

Original photo from Getty Images

Of course, this is all without taking into account Dilong and Yutyrannus. Dilong was a small, basal tyrannosaur - an ancestor to T. rex - and in 2004 a fossil was discovered with preserved filamentous protofeathers.

Photo from Xu et al 2004

Then, in 2012, Yutyrannus huali was discovered, a tyrannosaur from the Early Cretaceous with definitive impressions of feathers. Also, much closer T. rex in both size and time.

Photo from Xu et al 2012

At this point, even without significant soft tissue impressions from T. rex itself, it is becoming increasingly unlikely that T. rex was entirely scaled. It is surrounded on all sides, evolutionarily speaking, with feathery species of theropods. It is still possible, of course, since the feathers of its ancestors could have secondarily lost for any number of very valid reasons, but it is just as possible, if not more so, that it would have been covered in some kind of feathery integument.

Anyway, I hope this cleared absolutely nothing up for you, as it has for me. You’re welcome.