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| Psittacotherium |
Such is the case with the taeniodonts. These were first identified as a distinct group of fossil mammals in 1876 by Edward Drinker Cope, so we've known about them for a long time. He placed them, based on perceived similarities to hedgehogs, as a suborder of the larger group then called the Insectivora. It eventually became clear that this wasn't valid, not least because "Insectivora" was one of the orders that had to be scrapped once we began to use genetic data to uncover deeper evolutionary relationships.
What was less clear was what they actually were.
A large part of the problem is that they lived early on. Really early on. The last of them died out around 40 million years ago, towards the end of the Eocene epoch, but their heyday was even earlier, during the very first epoch of the Age of Mammals, the Paleocene. This is early enough that it doesn't follow that any given mammal fossil is necessarily a placental, marsupial, or monotreme - other options still existed.
In fact, this doesn't turn out to be the case for taeniodonts, at least according to our best current understanding. We have enough features from the fossils to work out that they were more closely related to the placental lineage than to the marsupial one, and a 2022 study confirmed that they probably were literal placental mammals. (That is to say, they are descended from the last common ancestor of all living placentals, and thus fit inside that particular family tree). Even so, we can't tell which of the four subgroups within the living placentals they belong to, and there's every chance that the answer is "none of them".
What's significant about the taeniodonts is that they were amongst the first mammals to reach large body sizes. Psittacotherium, for instance, was about the weight of a Great Dane, albeit with a more compact body and short legs... and it first appeared over 62 million years ago, just 4 million years after the K-Pg impact that wiped out the non-avian dinosaurs. (The name, incidentally, literally translates as "parrot-beast" on account of the shape of the head).
The fossil record of taeniodonts is sparse, partly because they lived so long ago, but mainly because they were either inherently rare or, perhaps more likely, lived in arid environments that weren't good for forming fossils. It has also been suggested that the local processes by which animals ended up in particular fossil beds did not favour larger-bodied specimens, giving us a false signal - although why this would be the case over such a broad area is not obvious.
Even so, we know that, within 3 million years of the K-Pg impact, there were at least three genera, already different enough from one another to be placed into two families. One of those families consisted of relatively small animals with some adaptations for eating tough food, while the other included the larger animals, such as Psittacotherium. These latter had muscular forelimbs with powerful claws suitable for digging burrows.
The taeniodonts were unique to North America, an island continent at the time. Most of the early fossils come from New Mexico, Colorado, Utah, and Wyoming, with just a few fragments found further north, in Montana and North Dakota. However, there is reason to suppose that the taeniodonts were successful, and spread across more of the continent than their limited fossil remains suggest. Indeed, by the later Paleocene, they had reached places as far afield as California, Texas, and Maryland... and it isn't as if they would have had much competition, given the absence of much else of similar size at the time.
However, the oldest known taeniodont fossil isn't from the US at all. This belongs to Schowalteria, which is generally considered primitive enough to belong to neither of the two widely recognised families within the group. It was first described in 2003, based on a fossil discovered in southern Alberta and dates back, not to the Paleocene, but to the Late Cretaceous - a few million years before the K-Pg impact. Which is to say that it lived at the same time as the likes of Triceratops and Ankylosaurus.
Significantly, this is not only the oldest known taeniodont, but also the most northerly. The absence of anything of similar age from richer deposits in the US suggests that the animals headed south in the millions of years either side of the K-Pg extinction event, expanding across the continent once dinosaurs no longer dominated the fauna.
However, there's no obvious reason why at least some of them shouldn't have remained behind in Canada. The Paleocene was not noticeably colder than the preceding epoch and was much warmer than it is today - Alberta's climate may not have been too different from present-day northern California at the time, so it was hardly inhospitable. Of course, as you get closer to the poles there is the issue of long winter nights to deal with, which might explain why taeniodonts didn't fancy going any further north, but that wouldn't be an issue with staying put.
Well, we now have an answer: they didn't all leave. Earlier this year, a couple of fossils discovered near Calgary were formally described for the first time. They date from the Late Paleocene, 60 to 61 million years ago, and thus perhaps ten million years after the time of Schowalteria, and easily contemporaneous with many US taenidonts. The fossils consist solely of teeth, which limits how much we can say, but they are sufficiently distinctive and well-preserved to place them, if not in a particular species at least in genera: Psittacotherium and Huerfanodon.
These two genera belong to different families so, not only can we say that taeniodonts did survive in Canada after the K-Pg impact, but representatives of both the branches within the group did so. The creatures were even more widespread across the continent than their limited fossil remains had led us to believe. It's all part of the story of the rapid expansion of new kinds of mammal across the world once the dinosaurs were out of the way, a story repeated across the continents of the Earth.
[Picture by "ДиБгд" from Wikimedia Commons.]
I wondered why a North American taxon had a so Chinese-sounding name as Huerfanodon, but it turns out it's actually Spanish (and that the "huer-" part is just one syllable, not two as I'd assumed).
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