The Carnian (237 to 227 million years ago) is the first epoch of the Middle Triasssic, and is noted for the appearance and spread of the Dinosaurs and their close relatives, while other groups, such as the Rhynchosaurs, Dicynodonts, and Stereospondyl Amphibians, which had dominated Early and Middle Triassic assemblages, began to decline significantly. However, all known Carnian Dinosaurs to date come from the Southern Hemisphere, with the oldest known Dinosaur from the Northern Hemisphere, the Theropod Lepidus praecisio from the Otis Chalk of Texas, being at most 221 million years old.
Carnian-aged Dinosaurs are known from a number of Southern Hemisphere locations, including Brazil, Argentina, Zimbabwe, and India (today in the Northern Hemisphere, but during the Triassic in the Southern Hemisphere). All of these are from high latitude locations (i.e. they were from a long way from the equator), which has been suggested to indicate that a hostile climate probably stopped them from spreading into other areas, at least until the Carnian Pluvial episode, between 234 and 232 million years ago, during which the global climate shifted, becoming significantly more humid.
This has led palaeontologists to conclude that the first Dinosaurs appeared during the early Carnian (or possibly a little earlier) in the Southern Hemisphere. However, this hypothesis is based upon the absence of Dinosaur fossils from other areas, something which could equally be caused by poor sampling of early Carnian rocks from the Northern Hemisphere. This alternative merits serious consideration, as Carnian deposits are rare in the Northern Hemisphere, and often poorly dated. Furthermore, a number of rock formations in the Northern Hemisphere which have been dated to the early Carnian have produced trace fossils which are attributed to Dinosaurs, strongly indicating their presence in areas where body fossils have not been found.
In a paper published in the Zoological Journal of the Linnean Society on 8 January 2024, David Lovelace and Aaron Kufner of the Department of Geoscience and Geology Museum at the University of Wisconsin-Madison, Adam Fitch, also of the Geology Museum at the University of Wisconsin-Madison, Kristina Curry Rogers of the Biology and Geology departments at Macalester College, Mark Schmitz and Darin Schwartz of the Department of Geosciences at Boise State University, Amanda LeClair-Diaz and Lynette St.Clair of Fort Washakie Schools, Joshua Mann of the Eastern Shoshone Tribal Historic Preservation Office, and Reba Teran, a Shoshone Language Consultant at Wind River Reservation, describe a Sauropodomorph Dinosaur, as well as an indeterminate Silesaurid, from the mid-late Carnian Popo Agie Formation of Wyoming.
The Popo Agie Formation is a Carnian-aged deposit which outcrops across western Wyoming, western Colorado, and Utah. It was laid down in a series of lakes and rivers which are thought to have covered much of what is now the American Southwest at this time. Vertebrate fossils are rare in the Popo Agie Formation, though it has produced Metoposaurid Temnospondyls, Hyperodapedontine Rhynchosaurs, and Loricatan Archosaurs, and has two notable horizons with mass-death assemblages of Metoposaurid and Latiscopid Stereospondyls.
The fossils described by Lovelace et al. come from a site 1 km south of the confluence of the East Fork of the Wind River and Spear Creek called Garrett’s Surprise, in reference to its discoverer, Garrett Johnson, who found the site while working as an undergraduate field assistant on undergraduate field assistant. The discovery was surprising because the surrounding geology is dominated by the Eocene Wind River Formation, with the much older Popo Agie Formation exposed in an erosional gully.
The Sauropodomorph Dinosaur is described from a single isolated left astragalus, with the proximal end of a left femur which shows o clear Saurischian affinities also referred to the same species. This femur fragment was found within 5 m of the original specimen, and both specimens are encrusted with a similar micritic carbonate. The new species is named Ahvaytum bahndooiveche, where 'Ahvaytum' means 'long ago' and 'bahndooiveche' means 'handsome young man', 'Salamander', or 'Dinosaur' in the Shoshone language.
Lovelace et al. note that Western taxonomy has a history deeply rooted in colonialism, with taxa often given names that reflect geographic features, regions, or waterways named by colonizers who did not recognize or validate pre-existing Indigenous names. In recognition of this, the name 'Ahvaytum bahndooiveche' was chosen by a collaborative project involving the Fort Washakie Schools 7th grade cohort of 2022, along with educators, Eastern Shoshone Tribal Historic Preservation Office, and Tribal Elders.
The specimens assigned to Ahvaytum bahndooiveche were recovered from the surface of the upper part of a sandstone layer within the Popo Agie known as the Purple Unit. Uranium/lead analysis of zircons from this layer have yielded ages of between 227.34 and 229.04 million years before the present, with the layer which produced Ahvaytum bahndooiveche no more than 228 million years old. This places the fossils in the early Carnian, only slightly after the Carnian Pluvial Event. Zircon is a volcanic mineral that forms as liquid magma slowly cools to form solid rock. As zircon forms it can incorporate a variety of different elements into its crystal matrix, including uranium but not lead. This is useful as over time uranium decays to form lead, so any lead in a zircon mineral must be the result of the decay of uranium. Since the decay of uranium to lead occurs at a steady rate, it is possible to determine the age of zircons by measuring the ratio of uranium to lead within them.
As well as the specimens assigned to Ahvaytum bahndooiveche, the Purple Unit yielded the distal end of a left humerus (UWGM 7550) and the proximal end of a right femur (UWGM 7407), which Lovelace et al. determined to belong to a Silesaurid Dinosauriform.
Silosaurids have long been considered the sister group to the Dinosaurs. However, a number of recent phylogenetic analyses, including that of Lovelace et al. have been unable to demonstrate that they are a separate clade, less closely related to Saurischian Dinosaurs than Ornithopod Dinosaurs are. This raises the posibility that Silosaurids are Dinosaurs, either being an early diverging group of Ornithpods, a separate group more closely related to Saurischians, or a polyophyletic group, potentially including both plus some in the original Dinosaur-sister-group position (many Silosaurids are known from highly fragmentary remains, so this would not be surprising). If the Silodaurids are Dinosaurs, then they increase the age of the Dinosaurs as a group, as they are present in the Ladinian Epoch (between 241 and 237 million years ago), whereas the oldest known non-Silosaurid Dinosaur fossils all date from the Carnian. Either way, Silosaurids have previously only been known from Southern Hemisphere sights before the discovery of the Garrett's Surprise specimen.
Finally, Lovelace et al. describe a partial foot print from the upper Jelm Formation at Red Wall in Natrona County, Wyoming. This is small, roughly 8.0 x 5.6 cm, and comprises a partial hindlimb print with digits II–IV, with a very faint associated possibly forelimb trace. The pes digits are relatively straight, long, and slender with small acuminate claw impressions. Pads are observable, but not sharply defined. Lovelace et al. consider that this could be assigned to either of the ichnogenera Atreipus or Grallator. The trace is preserved on a slab which has fallen from the Red Wall (a cliff), but can confidently be sourced to a section 1-2 m thick, about 15 m beneath the top of the Jelm Formation, which stratigraphically underlies the Popo Agie Formation.
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