Crocodylus acer and the orphans of the genus Crocodylus

Skye McDavid, April 13, 2022

Photo: Skye McDavid

During a recent visit to the American Museum of Natural History, as I wandered through the enormous Hall of Vertebrate Origins, I stumbled upon a skull of a crocodilian. The sign identified it as Crocodylus acer. At the time, I didn't think much of it. The skull superficially resembles those of extant Crocodylus species, and extinct species belonging to modern genera are common. I knew of extinct Crocodylus species such as C. anthropophagus (the "human-eating crocodile") and C. thorbjarnarsoni, so I simply assumed C. acer was one I hadn't heard of. I took a couple of photos and moved on.

It was only later that I learned the story was much more complicated. This is a story that goes back to the Bone Wars, and a story that isn't finished yet. This skull, which may seem unexceptional at first, is the only known specimen of a yet-unnamed and undescribed genus of crocodilians.

The holotype and only known specimen of Crocodylus acer was discovered sometime before 1882 by Charles Hazelius Sternberg (not to be confused with his son Charles Mortram Sternberg), working for Edward Drinker Cope. It was found in the Manti Beds near Manti, Utah. This is part of the Green River Formation, a famous Eocene lagerstätte known for its fish, birds, as well as some of the earliest bats.

Cope determined correctly that this skull, now catalogued AMNH FR 1240, represented a new species, which he named in The American Naturalist in 1882. This description is a perfect example of the sloppy work and rushed descriptions of the Bone Wars. He intended to assign it to the genus Crocodylus, named by Laurenti in 1768. However, he spells it "Crocodilus," and attributes the authorship to Linné. To be fair, this lapsus is fairly common, but that's not the only problem with this paper. The description of Crocodylus acer is extremely brief and does not include a proper diagnosis. There are no measurements given, only a mention that the illustration is "nearly one-third natural size."

Illustration by Edward Drinker Cope

Cope does not give an etymology for the specific epithet acer, but it can be presumed that it comes from the Latin for "sharp".

After the Cope collection was acquired by the American Museum of Natural History, the type of Crocodylus acer was redescribed by Charles C. Mook in 1921. Mook does describe the skull in detail, and notes that Cope made errors in his original description.

Here's where the wrinkles begin though: despite being assigned to the genus Crocodylus by both Cope and Mook, Crocoylus acer isn't actually a crocodile. The first clue is the age: Crocodylus acer is Eocene in age. While some vertebrate genera do survive for tens of millions of years (the lungfish Ceratodus comes to mind), it is quite rare. While crown-group crocodilians such as the alligatoroid Deinosuchus are known as far back as the Late Cretaceous, it seems unlikely that a single genus would survive that long. Modern phylogenetic analysis has found that not only is Crocodylus acer not a crocodile, it's not even that closely related. A 2000 analysis by Christopher Brochu recovered Crocodylus acer as a crocodyloid in a polytomy with Brachyuranochampsa and Crocodylidae.

A 2018 Analysis by Michael Lee and Adam Yates based on both morphology and genetics recovered Crocodylus acer in an even more basal position outside the Longirostres, as the sister taxon to Brachyuranochampsa. Mook himself noted in 1962 that Brachyuranochampsa is very different from Crocodylus. One could argue that "Crocodylus" acer should be lumped into Brachyuranochampsa, and although they are similar and closely related, I would argue that "Crocodylus" acer should be placed in its own genus.

The implications of these two phylogenetic studies go further than Crocodylus acer. Several other species originally assigned to Crocodylus can no longer be considered members of this genus. These include Crocodylus depressifrons, Crocodylus affinis, and Crocodylus megarhinus. But until someone takes off the glass on the display case, these will remain species without a genus: the orphans of Crocodylus.

Diagram of the skull of "Crocodylus" acer by Skye McDavid

Sources

Cope, E. D. (1882). The reptiles of the American Eocene. The American Naturalist, 16(12), 979-993. https://doi.org/10.1086/273224
Cope, E. D. (1883) The Vertebrata of the Tertiary Formations of the West Google Books
Mook, C. C. (1921). The Skull of Crocodilus acer Cope. Bulletin of the American Museum of Natural History 44(11): 117-121 http://hdl.handle.net/2246/1732
Laurenti, J. N. (1768) XV Crocodylus Specimen medicum, exhibens synopsin reptilium emendatam cum experimentis circa venena et antidota reptilium austriacorum. (in Latin)
Brochu, C. A. (2000) Phylogenetic relationships and divergence timing of Crocodylus based on morphology and the fossil record. Copeia. 2000(3): 657-673 https://doi.org/10.1643/0045-8511(2000)000[0657:PRADTO]2.0.CO;2
Cossette, A. P. & Brochu, C. A. (2020) A systematic review of the giant alligatoroid Deinosuchus from the Campanian of North America and its implications for the relationships at the root of Crocodylia. Journal of Vertebrate Paleontology 40:1 https://doi.org/10.1080/02724634.2020.1767638
Lee, M. S. Y. & Yates A. M. (2018) Tip-dating and homoplasy: reconciling the shallow molecular divergences of modern gharials with their long fossil record. Proceedings of the Royal Society B. 285 (1881) https://doi.org/10.1098/rspb.2018.1071
Mook, C. C. (1962) A new species of Brachyuranochampsa (Crocodilia) from the Bridger beds of Wyoming. American Museum Novitates no. 2079 http://hdl.handle.net/2246/3448
American Museum of Natural History Paleobiology Database, FR 1240

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