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Fossil Friday: Time Wanderers Debunk Popular Scenario of Mammalian Evolution

July 5, 2024, 6:37 AM
Photo credit: 5of7, CC BY-SA 2.0 , via Wikimedia Commons.

This Fossil Friday we will have a look at out-of-place fossils of mammal-like reptiles such as the shrew-sized Chronoperates paradoxus from the Early Tertiary (Late Paleocene) Paskapoo Formation of Alberta in Canada. I still remember very vividly, when I was still a graduate student at the University of Tübingen the sensational new discovery of this fossil hit the news in 1992 (Hecht 1992, Novacek 1992), which we discussed in the vertebrate phylogeny course of the late great Dr. Gerhard Mickoleit. What was so special with this find?

In their original description of the tiny fossil jaw fragment, Fox et al. (1992a) documented several unique features of the lower jaw and its dentition that are exclusively characteristic of primitive mammal-like reptiles of the order Therapsida and the suborder Cynodontia (both taxa were later redefined in cladistic classification to include true mammals). Therapsids first appeared in the Early Permian 280 million years ago and were believed to have become extinct in the Middle Jurassic about 160 million years ago. Therefore, this Paleocene fossil would have been 100 million years younger than the previous youngest fossil record of therapsids and double their stratigraphic range, implying a long ghost lineage. This is why the relict genus was named Chronoperates, which means time wanderer in Greek.

Immediately Contested

The determination of Chronoperates as a therapsid was immediately contested by vertebrate paleontologist Hans-Dieter Sues (2006), but defended by the original authors (Fox et al. 1992b), who emphasized that Sues had not presented any synapomorphies in support of his alternative mammalian determination, and also had never studied the actual specimen. Indeed, the main argument by Sues was a very vague appeal to the only subtle differences between the teeth of Chronoperates and those of Cretaceous symmetrodont mammals (also see Novacek 1992), which was a possibility that was actually already addressed and refuted in the original description, as well as the false and misleading claim that the therapsid features could rather be artefacts of preservation. Nevertheless, some other scientists also doubted a therapsid affinity but explicitly admitted that they “don’t have any alternative to offer” (Hopson quoted in Hecht 1992). McKenna & Bell (1997: 43) likewise did not believe in the non-mammalian cynodont identity, but rather suggested a “dubious” position, possibly among basal holotherians, which are Triassic and Jurassic stem mammals (see Naish 2006b). Naish (2006b) also mentioned that “Meng et al. (2003) noted that the medial dentary scar seen in Chronoperates might not house post-dentary bones, as Fox et al. proposed, but instead a persisting Meckel’s cartilage. Now, if Chronoperates did possess a Meckel’s cartilage, this would be a first for a post-Mesozoic synapsid, and would further support ideas that Chronoperates is actually a late-surviving basal mammal” as suggested by McKenna & Bell.

Until today the therapsid affinity of Chronoperates is generally considered to be debunked, and Chronoperates is listed as potential symmetrodont mammal in Wikipedia, all based on a single one-page article with some unsubstantiated sweeping comments by a scientist, who never studied the actual fossil himself. Nobody ever bothered to look at this sensational fossil again. The only reason why the skeptical view still prevailed, is that the fossil would be very much out-of-place and contradicting the expectations from the mainstream Darwinian scenario of mammal origins. So much for unbiased scientific quest for truth. If some facts don’t fit the narrative, just deny them and sweep them under the rug.

Other Out-of-Place Fossils

In his two-part blog post, vertebrate paleontologist Darren Naish (2006a, 2006b) also discussed some other out-of-place fossils of mammal-like reptiles (non-mammalia therapsids):

In 1915 several fragmentary fossil bones were discovered in Early Cretaceous sediments from Queensland in Australia, which exhibited remarkable similarities with the extinct Dicynodontia, a clade of Permian and Triassic herbivore non-mammalian therapsids that lacked any fossil record beyond the Triassic-Jurassic boundary. The surprising dicynodont affinity was later indeed strongly confirmed by Thulborn & Turner (2003), who meticulously demonstrated that neither the dating nor the phylogenetic position can be reasonably disputed. This is a big deal, because this fossil record of dicynodonts is again “something like 100 million years younger than the previously known youngest members of the group” (Naish 2006a). This arguably proves that a similar ghost lineage for Chronoperates would be well within the realm of possibility and no reason to reject its determination as therapsid. So, it is hardly surprising that just a few years later, Agnolin et al. (2010) claimed that the Australian bones may rather have belonged to an extinct group of crocodyliforms called Baurusuchia, just because of unspecified striking similarities without any further explanation or documentation, while Knutsen & Oerlemans (2020) disputed the dating (instead suggesting a Plio-Pleistocene age) and the determination all-together, but this time the bones were claimed to belong to a diprotodontid marsupial and suddenly found “no features diagnostic of dicynodonts.” Thulborn & Turner (2003) on the other hand had unequivocally stated that this fossil “clearly does not represent … any of the big marsupials such as Diprotodon … The only identification supported by positive evidence, in the form of demonstrable similarities and diagnostic features, is ‘dicynodont.’” As happens all too often, apparently everything goes by the board when the goal is to make inconvenient fossils fit the preferred story. It is sobering to realize how dubious and sloppy evolutionary biology actually works when you look a bit closer behind the pompous facade of the so-called “undeniable scientific facts.”

Simpson (1928) described the docodont Peraiocynodon inexpectatus from the Early Cretaceous Purbeck Limestone of England, which was confirmed as a docodont by Averianov (2004). The species name inexpectatus of course alludes to the unexpected young age of this fossil, because Docodonta are Mesozoic mammaliaforms that abruptly appeared in the fossil record of the Middle Jurassic and were believed to have gone extinct in the Late Jurassic. Maschenko et al. (2002) described with Sibirotherium another late surviving docodont from the Early Cretaceous of West Siberia, later complemented by another genus Khorotherium from the Early Cretaceous of Yakutia as the youngest known docodont (Averianov et al. 2018). The previously described assumed docodont Reigitherium from the Late Cretaceous of Patagonia (Pascual et al. 2000), was later recognized as a real mammal of the dryolestoid order Meridiolestida (Rougier et al. 2003, 2011, Harper et al. 2018), which may well be correct.

Finally, there is the unnamed Saint Bathans mammal from the Early Miocene (18.7-15.9 mya) of New Zealand (Worthy et al. 2006), which is known from three fragmentary specimens and seems to represent a basal mammaliaform outside the crown group clade of monotremes, marsupial and placental mammals, and even basal of the extinct multituberculate branch. It is technically not a non-mammalian therapsid but also not a crown group mammal. Therefore, the discovery of such a primitive stem mammal in Miocene layers was highly unexpected and surprising (Naish 2006b).

Survivors or Relicts

All these late survivors or relicts (sometimes called Lazarus taxa) are not just some weird scientific curiosities, but actually demonstrate a general problem with the common evolutionary narrative about mammalian origins, which claims that mammal-like reptiles disappeared because they were outcompeted by the modern true mammals. Mammal-like reptiles like pelycosaurs and therapsids ruled the Permian and Triassic periods, and some therapsids like the herbivorous Tritylodontidae existed till the Jurassic, such as the featured Kayentatherium from the Early Jurassic of Arizona. All these alleged early ancestors of mammals were thought to have been wiped out by the more modern real mammals (Mammaliaformes), which originated in the Triassic but did not diversify before the Late Jurassic. This widely accepted replacement theory was overturned for good, when more than 250 tritylodontid teeth were discovered in Early Cretaceous layers of Japan (Matsuoka et al. 2016), proving that tritylodontids survived 30 million years longer than previously believed. Tritylodontids demonstrably coexisted for millions of years with more modern mammaliaforms that partly even occupied the same herbivorous niches. The crude Darwinist presumption of more advanced descendants outcompeting their primitive ancestors turned out to be wrong once again. The take home message is this: Darwinian stories are exactly that — just fancy stories rooted in wishful thinking rather than in hard science.

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