Ahvaytum bahndooiveche: A Sauropodomorph Dinosaur from the mid-late Carnian of Wyoming.

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.

Holotype left astragalus of Ahvaytum bahndooiveche. (UWGM 1975). 3D model in (A) medial, (B) lateral, (C) lateral transparent, (D) posterior transparent, (E) distal, (F) proximal, (I) anterior, and ( J) posterior orthographic views. Photographs in (G) proximal and (H) distal views. Abbreviations: amc, anteromedial corner; ap, ascending process of the astragalus; g, groove; f, foramen; ff, fibular facet; ldn, laterodistal notch (= lateroventral depression); mf, medial fossa; nf, non-articular fossa (= dorsal basin, = semi-elliptical fossa); p, platform; plp, posterolateral process; plr, posterolateral ridge; tf, tibial facet. Diagonal lines indicate broken surfaces. Arrows indicate anterior direction. Scale bar is 1 cm. Lovelace et al. (2025).

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.

A reconstruction of Ahvaytum bahndooiveche as a small Sauropodomoph Dinosaur, along with an audio-explanation of the origin of its name. Lovelace et al. (2025).

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.

Proximal end of a left femur UWGM 7549 (A)–(E) referred to Ahvaytum bahndooiveche. 3D model in (A) anteromedial, (B) posterolateral, (C) proximal, (D) anterolateral, and (E) posteromedial orthographic views. Abbreviations: alt, anterolateral tuber; amt, anteromedial tuber; ce, concave emargination; dlt, dorsolateral trochanter; ft, fossa trochanterica (= facies articularis antitrochanterica); gt, ‘greater trochanter’; pmt, posteromedial tuber; ve, ventral emargination. Arrows indicate anterior direction. Scale bar is 1 cm. Lovelace et al. (2025).

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.

Photographs of Sulcimentisaurian Silesaurid elements from the Garrett’s Surprise locality. Distal end of a left humerus UWGM 7550 (A)–(E) in (A) anterior, (B) posterior, (C) medial, (D) lateral, (E) and distal views. Proximal end of a right femur UWGM 7407 (F)–(J) in (F) proximal, (G) anterolateral, (H) posteromedial, (I) posterolateral, and ( J) anteromedial views. Abbreviations: alt, anterolateral tuber; amt, anteromedial tuber; at, anterior trochanter; dlt, dorsolateral trochanter; ect, ectepicondyle; ent, entepicondyle; g, groove; gt, ‘greater trochanter;’ ipmt, incipient posteromedial tuber; n, notch; rc, radial condyle; uc, ulnar condyle. Arrows point in the anterior direction. Scale bar equals 1 cm. Lovelace et al. (2025).

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. 

UWGM 7435 (left) is an isolated slab containing a single tridactyl pes and possible manus impression attributed to an Atreipus–Grallator plexus tracemaker from the upper Jelm Formation, Natrona County, Wyoming, USA. (A) Digital surface-depth map (right) produced in METASHAPE (v.2.0.3; Agisoft) from surface light-scans demonstrates the depth and toe pad delineations of pes (p) digits II–IV. The manus impression may be present (m?); other than a very slight depression there are no morphological features to confidently identify it as such. Scale bar is in 1-cm increments. Lovelace et al. (2025).

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Skier killed by avalanche in Wyoming.

A skier has died and a second has been injured following an avalanche on Breccia Cliff in Togwotee Pass, Teton County, Wyoming, on Saturday 4 January 2024. The deceased has been identified as Kenneth Goff, 36, from Lander in Fremont County, Wyoming, who is described as having been a nurse and experienced mountaineer. The skiers are reported to have been part of a party of  four who were ascending a slop on skins (strips of material which are attached to the bottom of skis to prevent backsliding, enabling the skier to climb slopes), when they caused a large slab of snow to come loose, burying Goff, and causing injuries to the lower leg of a second skier.

The site of an avalanche which killed a skier and injured a second in Teton County, Wyoming, on Saturday 4 January 2024. Teton County Search and Rescue.

Avalanches are caused by the mechanical failure of snowpacks; essentially when the weight of the snow above a certain point exceeds the carrying capacity of the snow at that point to support its weight. This can happen for two reasons, because more snow falls upslope, causing the weight to rise, or because snow begins to melt downslope, causing the carrying capacity to fall. Avalanches may also be triggered by other events, such as Earthquakes or rockfalls. Contrary to what is often seen in films and on television, avalanches are not usually triggered by loud noises. Because snow forms layers, with each layer typically occurring due to a different snowfall, and having different physical properties, multiple avalanches can occur at the same spot, with the failure of a weaker layer losing to the loss of the snow above it, but other layers below left in place - to potentially fail later.

Diagrammatic representation of an avalanche, showing how layering of snow contributes to these events. Expedition Earth.

Emergency services in Teton County were alerted to the location of the avalanche by an automated call from an iPhone, however, this only provided the location of the incident, not any information about it. This led to multiple rescue teams being deployed, utilising skis, snowmobiles, and a helicopter. In the event, both the helicopter and snowmobiles were forced to turn back due to bad weather, but a party of volunteers on skis was able to reach the location of the avalanche and effect a rescue. Teton County Search and Rescue have warned outdoor sports enthusiasts to be aware of weather conditions when planning expeditions, and also to place limited reliance on automated calls from mobile phones as a way of contacting the emergency services, as these are relatively new technologies and only provide limited information; they advise always backing this up with an actual call or text if possible.

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A possible crown-group Bird from the Late Cretaceous Lance Formation of Wyoming.

The origins and evolution of Mesozoic Birds are now well understood, but the emergence and development of the crown group Birds (a crown group contains all living members of a group, their most recent common ancestor, and everything descended from that ancestor) remains largely clouded in mystery. Most living Bird groups have a very poor fossil record, if they have a fossil record at all, despite Birds being the most diverse group of flying Vertebrates alive today, with more than 10 000 species. Fossils, where known, tend to be extremely fragmentary in nature, with most phylogenies of the group based entirely upon genetic data. This is particularly frustrating as the living Birds are the only group of Dinosaurs to have survived the End Cretaceous Extinction, something which has been taken to imply they had some quality missing in all other Avian and non-Avian Dinosaur groups. However, while crown group Birds are known to arisen before the End of the Cretaceous, they appear to have been at best a minor component of the Cretaceous Fauna, with few-or-no specimens found even in deposits which have produced numerous fossils of extinct Mesozoic Bird groups.

In a paper published in the journal BMC Ecology and Evolution on 9 February 2024, Chase Doran Brownstein of the Department of Ecology and Evolutionary Biology at Yale University and the Stamford Museum and Nature Center, describes a possible crown group Bird from the End Cretaceous Lance Formation of Wyoming.

The specimen, YPM VP 59473, comprises partial skeleton consisting of the complete left quadrate, portions of the skull roof a partially articulated, though very poorly preserved, cervical series, a fragment of the synsacrum, the left humerus, the articulated left radius and ulna, partial left tibiotarsus, and a partial pes. The material is largely disarticulated, but all of the bones are from a young juvenile and no duplicate bones are present, supporting the idea that they came from a single Animal.

Preservation of YPM VP 59473. The blocks containing all bones of the holotype (except for the humerus, tibiotarsus, synsacrum fragment, and large distal pedal phalanx) are shown under light microscopy (a), (e), (h) and with multiple x-ray views of the largest (b), (c), (d), second largest (f), (g), and smallest (i) blocks as rendered in VGStudio, showing the relative placement of bones in the matrix blocks. Brownstein (2024).

Despite the extremely fragmentary nature of the material, Brownstein feels confident in assigning the specimen to the Galloanserae, the group which includes the living Land and Water Fowl, and one of the three groups of Neornithine Birds thought to have diverged before the End of the Cretaceous, with the Palaeognaths and the Neoaves. This diagnosis is on the basis of the clear separation of the otic and squamosal capitula on the quadrate, the presence of a subcapitular tuberculum below the squamosal capitulum on the quadrate, the expansion of the ventral condyles and pterygoid condyle on the quadrate, the humeral head being dorsally offset from the rest of the proximal margin of the humerus, tricipital fossa being deeply excavated, and the dorsal tubercle of the humerus being large and offset from the rest of the proximal margin, all of which traits are typical of Galloanserine Birds, but absent in the various Mesozoic Avian stem groups.

Forelimb of YPM VP 59473. Humerus in (a) posterior, (b) anterior, (c) lateral, and (d) medial views. In (a) and (b), both CT scans and colour images are shown. Radius in (e) anterior, (f) posterior, (g) lateral, (h) medial, and (i) distal views. Ulna in (j) posterior, (k) anterior, (l) lateral, and (m) medial views. Brownstein (2024).

While the presence of a Galloanserine Bird in an End Cretaceous deposit is not unexpected, the presence of the specimen in the Lance Formation is significant in two ways. 

Firstly, because the deposit is from the Northern Hemisphere; phylogenetic studies of Birds based upon genetic data have found that the earliest diverging members of many groups have Southern Hemisphere distributions, which has led to speculation that the Neornithine Birds might have had a Southern Hemisphere origin, and the establishment of YPM VP 59473 adds to a growing body of data which contradicts that, suggesting that Neornithine Birds already had a global distribution in the Late Mesozoic. 

Secondly, unlike other deposits which have yielded Mesozoic Neornithine Birds, the fossils of the Lance Formation are thought to have been buried in situ, rather than being an accumulation deposit. This is important because the deposit has also produced toothed stem-Birds from at least four major clades, as well as Eudromaeosaurian, Alvarezsaurid, Troodontid, and potentially ‘four-winged’ Microraptorine Dinosaurs, all of which are thought to have been ecologically close to Birds. This is significant, as it suggests that the Neornithine Birds were not occupying some ecological niche which protected them from the impacts of the End Cretaceous Extinction, but instead were part of a community of ecologically similar Animals living in similar environments. This undermines the idea that Neornithine Birds were able to survive the End Cretaceous Extinction because they were in some way special, supporting the alternative hypothesis that they survived due to simple luck an important but sometimes overlooked factor in evolutionary biology.

The ecological and temporal origins of living Birds. Left side of the diagram shows the temporal and spatial range extensions and records of key small-bodied non-Avian Theropod clades found in the Lance Formation assemblage, and cladogram at right shows the major clades of stem and crown Birds that survive to or past the End  Cretaceous extinction, with ecologically relevant features that have been considered important to differential Avian survival through that event noted along branches. All clades shown on tree are unambiguously represented in the Lance Formation assemblage, except Neoaves and Paleognathae. Bird illustrations by John Gould. Brownstein (2024).

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Aulacoseira wyomingensis: A new species of freshwater Diatom from a seep near Casper, Wyoming.

Diatoms are single celled algae related to Kelp and Water Moulds. They are encased in silica shells with two valves. During reproduction the cells divide in two, each of which retains one valve of the shell, growing a new opposing valve, which is slightly smaller and fits flush within the older valve. This means that the Diatoms grow smaller with each new generation, until they reach a minimum size, when they undergo a phase of sexual reproduction, giving rise to a new generation of full-sized cells. Members of the Family Aulacoseiraceae have elongate valves with many spines; these spines being able to  interdigitate with other members of the species, enabling them to form long chains. The family currently contains four genera, Aulacoseira, which is known from fossils dating back as far as the Cretaceous and still extant, with a global distribution, Eosira, which is known only from the Eocene of North America, Miosira, which is know from the Miocene of Europe, and Alveolphora, which is known from Miocene and Pliocene deposits across the Northern Hemisphere.

In a paper published in the journal Taxonomy on 8 June 2022, Jeremy Greifenstein, Rachel Shea, and John Patrick Kociolek of the Department of Ecology and Evolutionary Biology at the Museum of Natural History of the University of Colorado Boulder describe a new species of Aulacoseira from a small seep near Casper, Wyoming.

The new species is named Aulacoseira wyomingensis, where 'wyomingensis' means 'from Wyoming'. The new species is described from a series of specimens extracted from a sample collected on 22 August 2021. These Diatoms are cylindrical in shape, and while they can adhere together, chains of longer than two Diatoms have been observed.

Aulacoseira wyomingensis. Scanning electron microscopy. External girdle views of entire frustules. Valves have striae that are disorganised. Column has small ridges. Cingulum is composed of numerous ligulate elements. Spines are small in length and shield-like. Valve on the right in (C) appears to be incompletely formed. Scale bars are 5 µm. Greifenstein et al. (2022).

The valves of Aulacoseira wyomingensis are 7-14 μm in diameter, with faces covered by large areolae (openings) up to 1 μm in diameter. The sides are covered by striations, these having smaller areolae. The central part is covered by ridges. 

Aulacoseira wyomingensis. Scanning electron microscopy. Internal views. (A) Valve view showing areolae and interior of valve. Scale bar is 2.5 µm. (B) Side view showing part of valve interior and exterior. Scale bar is 2.5 µm. (C) Valve view of interior showing ringleiste. Scale bar is 2.5 µm. (D) High magnification view of single areola showing fine hymenate occlusion over opening. Scale bar is 0.3 µm. (E) Side view showing part of the valve interior and exterior. Scale bar is 2.5 µm. Greifenstein et al. (2022).

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Secondary cratering from the Early Permian of Wyoming.

Many Solar System bodies, including our Moon, are covered by enormous numbers of impact craters. On Earth, in contrast, the total number of such craters discovered stands at 208. This is largely due to the Earth's active surface, with the continents and continental shelves being subjected to constant erosion and deposition of sediments, and the ocean floors being constantly recycled through subduction and seafloor spreading. However, the situation is more complicated than it seems at first sight; many of the craters seen on the Moon and other Solar System bodies are in fact secondary craters, formed by debris ejected from larger impact events. Once understood, such craters have been relatively easy to identify, frequently being elliptical rather than circular in shape, shallower than primary craters, and arranged radially around the initial impact crater. The implications of this are debatable, with some planetary scientists arguing that as many as 95% of small craters on some bodies may be secondary in origin, whereas others see them as a insignificant proportion of the total number.

In a paper published in the journal GSA Bulletin on 11 February 2022, Thomas Kenkmann and Louis Müller of the Institute of Earth and Environmental Sciences at Albert-Ludwigs-Universität Freiburg, Independent Consultants Allan Fraser and Doug Cook, Kent Sundell of the School of Science at Casper College, and Auriol Rae, also of the Institute of Earth and Environmental Sciences at Albert-Ludwigs-Universität Freiburg, describe the discovery of a secondary impact field comprising at least 31 craters, and possibly as many as 60 more, in southeastern Wyoming, USA. 

The presence of a field of impact craters in the Rocky Mountains in Wyoming was first reported in 2018 by Thomas Kenkman, Kent Sundall, and Douglas Cook. At the time they reported about 40 circular-to-elliptical structures on a tilted Permian exposure on the northeast flank of the Sheep Mountain, eight of which showed sufficient grain deformation to be confirmed as impact structures. These were initially interpreted as the result of a single large object which broke up as it entered the atmosphere, resulting in a group of closely clustered craters.

The new paper by Kenkman et al. describes the presence of several other craters on Sheep Mountain Ridge, and other exposures of the same age at Wagonhound Ridge, Mule Creek, Fetterman Ridge, Fetterman Road, and Palmer Canyon Road, as well as possible craters at several other locations. These range from 10 to 80 m in diameter, and while many are circular, some are as much as 1.7 times as long as they are wide. All of the structures which are firmly established as being craters lie at the top of the Casper Formation, which is immediately overlain by the Opeche Shale member of the Permian Goose Egg Formation, dating the impacts to about 280 million years ago, making them late Early Permian in origin.

 

Digital elevation model map of southeastern Wyoming, USA, and adjacent areas showing the exposure of Casper Formation and the locations of the secondary craters. Based on the intersection of trajectory fans, the proposed site of the possible primary crater is reconstructed. SM, Sheep Mountain; MC, Mule Creek; FR, Fetterman Ridge; FRX, Fetterman Road; PCR, Palmer Canyon Road; WR, Wagonhound Ridge; BE, Box Elder Canyon; MR, Manning Ridge. Kenkman et al. (2022).

The Casper Formation is made up primarily of aeolian sandstones (i.e. sands laid down in a terrestrial desert or dune environment), although the uppermost portion of the section, where the craters are preserved, represents a marine transgression into this environment, forming a lagoon or sabkha environment (a sabkha being a coastal saltpan regularly refilled by tidal waters and emptied by evaporation). The craters sometimes form pedestals standing above the eroded surface of this formation, resulting from lithification processes of associated with the impacts, such as shock fusion (welding together of particles by a sudden impact), and cementing by a glassy melt formed within the crater.

 

Remote sensing images of selected craters of the different crater fields. (A), (B), (D), and (E) are drone images; (C), (F), and (G) are Google Earth imagery. Crater locations: SM, Sheep Mountain; MC, Mule Creek; FR, Fetterman Ridge; FRX, Fetterman Road; PCR, Palmer Canyon Road. (A) Crater SM-1 has an elliptical outline, a pedestal morphology with a preserved proximal ejecta blanket, and a raised rim. The downrange (northwest) rim shows an overturned ejecta flap. The linear ejecta wall is interpreted as a herringbone pattern. (B) Craters SM-6-3-4-5 form a northwest-southeast–trending radial crater chain. The ovoid crater SM-2 shows an overturned ejecta flap downrange (northwest) and a linear ejecta wall. (C) Craters MC-1 and MC-2 represent eroded craters with very little topography but a concentric fracture pattern. (D) Strongly degraded craters FR-1 and FR-2 contain shock effects along their crater rims. (E) The deeply eroded crater SM-9 is circular and has a bright halo of quartzitic sandstone. (F) Crater FRX-20 is a strongly elliptical landform with a gently rising rim. (G) The western and northern rim of crater PCR-1 exposes steeply dipping rocks. PCR-2 shows an elliptical outline. Kenkman et al. (2022).

The Casper Formation, on the upper surface of which the craters are preserved, has very little surface exposure, 1% of in Converse County and only 4% in Albany County, suggesting that many more craters may be preserved buried beneath the centre. This area lies within the Laramide Mountains, an area within the wider Rocky Mountains where Late Cretaceous to early Eocene (~75-50 million years ago) reverse faulting and folding uplifted an area of Archean basement rocks and the material overlying it. Most of the known craters lie upon the Sheep Mountain flank of the Sheep Mountain anticline, an extended fold-ridge mountain running from northwest-to-southeast, where the Opeche Shale has eroded back revealing the upper surface of the Casper Formation; it has been calculated that about 2 km of overlying rocks have been eroded away here since the onset of the orogeny; strata that are still likely to be in place in other areas, covering up other craters associated with this field.

 

(a) Simplified geological map of the Wyoming state. (b) Aerial photograph of Sheep Mountain anticline (view from the NNW); (c) Geological cross-section through of Sheep Mountain anticline perpendicular to the strike of the average fold axis, and (d) Geological map of the Sheep Mountain anticline area. Amrouch et al. (2010).

Wagonhound Ridge is a similar, and associated, structure to the south of Sheep Mountain, showing slightly less uplift. The Mule Creek, Fetterman Ridge, Fetterman Road, and Palmer Canyon Road exposures are found on the southwestern slope of the basement uplift of the Laramie Mountains in transition to the Shirley Basin, where the Casper Formation has largely been eroded away, but is exposed on several remaining buttes.

The largest cluster of impact-related structures is found on the northeastern flank of the Sheep Mountain, where a series of circular, irregular-shaped, and ellipsoidal have been confirmed as impact craters. These vary in their preservation quality from pristine to heavily eroded, with erosion apparently linked to the recent exposure of structures which were rapidly buried after their formation; i.e. the most eroded structures are located higher on the flanks of the mountain. The most pristine structures show steep crater walls, raised rims with overturned ejecta flaps, and remains of proximal ejecta blankets. These craters have floors are covered by soil and filled with muds derived from the overlying Opeche shales, making it hard to establish their depth-to-diameter ratio. Many of these craters are elliptical-to-ovoid in shape, with their long axes having fairly consistent orientations of 315–328°. The distribution of ejecta around the craters is uneven, with well-developed overturned flaps on their northwestern sides, suggesting they were caused by debris thrown from a primary impact to the southeast. Four of the craters form a chain, with a similar orientation. The more eroded structures further upslope tend also to be more rounded, with an internal ring structures. These often stand proud of the eroded surface, being more resilient to erosion due to the shock-fusion of the sandstone.

Ten possible craters have been found on the exposed surface of the Casper Formation at Wagonhound Ridge. Two of these have been confirmed as definite craters due to elevated rims. These are again filled with soil, and slightly elliptical. 

At Mule Creek a large elliptical crater measuring 56 m by 44 m has an orientation of 284±5°. A second structure, measuring 30 by 27 m is adjacent to this. Neither of these are elevated above the surrounding rock surface, and neither preserves any rim structure or surrounding breccia. It is thought that these represent the lower portions of larger craters that have been mostly eroded away; the larger of them appears to be surrounded by a larger ring at a distance of about 100 m, possibly representing underlying rocks that were consolidated by the impact. This area is cut through by a north-south and northwest-southeast–trending tectonic joint system visible in remote sensing images, which would have served to hasten erosion in this area. This joint system extends about 300 m to the northwest of the main crater, and contains at least nine irregular, soil-filled depressions which might represent further impacts. Another cluster of possible craters, one of which has been confirmed as an impact structure, is found about 2.5 km to the southeast of the main crater at Mule Creek. These structures have crater rims composed of sandstone breccia sealed with chert matrix.

At Fetterman Ridge a series of erosional buttes have exposures of the upper surface of the Casper Formation which have been eroded away from much of the surrounding landscape. One of these, a hill measuring roughly 200 m by 100 m, hosts three impact craters, measuring 30 m by 22 m, 28 m by 17 m, and 10 m by 10 m. The long axes of the two elliptical craters trend west-north-west to east-south-east, although their southern rims are more eroded. The 28 m by 17 m is distinctly elevated on its northwestern rim, with a visible ejecta flap on its western side. Again, the brecciated and fragmented rocks are sealed by microcrystalline silica, making them resistant to erosion. Other buttes to the northwest and southeast show possible additional impact craters, although these are more heavily eroded.

The Fetterman Road cluster comprises six possible craters 7–30 m in diameter, some of which are distinctly eliptical. The most distinct of these is 25 m by 15 m and has an orientation of 296°. The rim of this crater is elevated about a metre above its interior.

Eight possible craters are present at Palmer Canyon Road, about 10 km to the southeast of the Fetterman Road cluster. The two most conspicuous of these measure 42 m by 40 m, and 28 m by 19 m. Again, elevated rims are composed of quartzitic breccia with microcrystalline chert fill.

 

Outcrop-scale observations at the Wyoming crater field. (A) Panoramic view of crater WR-5 that shows very little relief. Note the outcrops along the rim. Persons for scale. (B) Brecciated ejecta. Fragments are partly angular and partly subrounded (SM-34). (C) The wind-scoured crater walls composed of quartzitic sandstone show abundant ventifacts (PCR-001). (D) Chert with flow textures and vesicles is very abundant at most of the craters (SM-36). (E) Breccia with quartzitic matrix (PCR-001). (F) The variegated contact of Casper sandstone and the Opeche Member of the Goose Egg Formation contains a few shocked quartz grains. Kenkman et al. (2022).

All of the discovered craters are on the upper surface of the Casper Formation, and all are in sandstones, but the nature of these sandstones varies slightly from site-to-site, reflecting an environment which was fully sub-aerial in the southwest, passing through a braided-river system into a shallow marine environment with some carbonate deposition in the northeast. The presence of water in the sands in some environments does not appear to have led to degradation of the crater rims. In all cases the craters were buried beneath Opeche Shale Member red beds of the Goose Egg Formation rapidly after their formation.

Brittle deformation, indicative of sudden physical shocks, can be seen on both large and small scales. The crater rims and ejecta all show brecciation (breaking into angular fragments) and brittle deformation, while individual grains are often intensely fractured.

Microstructures related to impact. (A) Crosscutting {1013} and (0001) PDF lamellae in sample from crater SM-19 are decorated by fluid inclusions. (B) {1013} PDF lamellae in sample from crater MC-1. A + B show that shock effects are restricted to the detrital grains while the overgrowth is undeformed. (C) Relatively wide-spaced planar fracture lamellae in sample from crater WR-4. (D) Concussion fractures in adjacent quartz grains emanate from initial grain contacts. (E) Crater SM-28 contains abundant chert layers and chert lumps that are embedded in the sandstone. Some of the chert lumps contain spherical lapilli. For interpretation, see text. (F) Close-up of a spherical lapillus that is interpreted as an accretionary lapillus. The concentric rings are composed of microcrystalline quartz around a dark-colored center. All photomicrographs were taken with crossed polarizers. Kenkman et al. (2022).

However, shocked grains are somewhat rare in the crater sediments, with slides made up from samples taken in the field typically showing only two-or-three shocked grains, and these usually being surrounded by unshocked grains rather than clustered together. Shocked grains were found in all parts of the crater structures, and at a much lower fequency level, outside the craters in undeformed sands, probably indicating their having been blown from craters by winds shortly after their formation. The degree of fracturing implies that these grains were subjected to pressures in excess of 10 gigapascals. 

Cherty (amorphous) silica is present at all sites, often forming the matrix which binds the sand grains together. Investigation of one of the craters at Sheep Mountain found a variety of structures within this chert, including elongated shapes and wavy layers. Within the chert were spherical structures resembling accretionary lapilli; glassy grains which are typically associated with violent volcanic eruptions, formed by the accretion of glassy siliica layers onto grains suspended in hot, turbulent air. 

The Wyoming Crater Field shows a number of features that help the reconstruction of the direction being travelled by impactors. The most obvious of these are oval or elliptical shaped craters, and craters arranged into chains, which gives the orientation of the direction of travel, but not the actual direction. However, direction of travel can be determined by using the following lines of evidence: (1) a steeper crater wall uprange; (2) a preserved overturned ejecta flap downrange, with a preferred deposition of ejecta downrange; (3) an ovoid crater shape with the strongest curvature downrange, and (4) V-shaped herringbone patterns of ejecta pointing up-range. 

The area has been subjected to some deformation since the impact craters formed, but none appears to have had its orientation changed by more than about 5%, enabling the use of craters from different sites to attempt to relocate the site of the original impact. Based upon this, Kenkman et al. suggest that the original impact happened at a site with map co-ordinates close to 41°28′N, 103°59′W; all of the craters lie between 150 km and 200 km from this site.

Modelling of possible trajectories of objects thrown from a primary impact crater suggest that boulders with a diameter of 2 m would need an initial velocity of 3-4 km per second to reach 150-200 km from the initial impact, while 4 m objects could reach this distance with an initial speed of 2 km per second would also fall within this zone. Such objects would deliver energy in the range of 12 to 400 gigajoules when they impacted, arriving at angles of between 45° and 60°.

Kenkman et al. calculate that an object with a density of 2500 kg per cubic meter (typical for many rock types), with a 4 m radius impacting at 1 km per second would create an impact crater about 45 m in diameter. A similar object 2 m in radius would create a crater 25-30 m in diameter, depending on its angle of approach. A range of objects between 1 and 4 m in diameter, travelling at between 500 and 1500 m per second, would generate craters between 8 and 55 m in diameter, with larger impacts releasing more energy and creating more shocked material as a proportion of the impactors mass.

Kenkman et al.'s findings reveal a series of clusters of craters across a wide area of southeast Wyoming, all of which appear to have formed simultaneously about 280 million years ago. These are best explained as secondary craters caused by material thrown from a large primary impact crater. No obviously foreign material was found in any of the craters, making it likely that the impacting material was similar in composition to the rocks of the areas impacted, and reconstructions of the direction of travel suggest the primary impact was between 150 and 200 km from the discovered craters.

Based upon the reconstructed size and impact speeds of the ejecta material, Kenkman et al. predict that the original impactor would have been 2.0-2.7 km in diameter, and to have hit the ground at about 20 km per second, creating a crater 50-65 km in diameter. 

This would place the location of the original impact in Goshen or Laramie counties in Wyoming, or Kimball, Banner, or Cheyenne counties in Nebraska. These areas fall within the Denver Basin, and Permian strata are deeply buried beneath subsequent Mesozoic and Cainozoic deposits. Despite this apparent lack of accessibility, these deposits have been heavily boreholed by hydrocarbons exploration companies, which may enable reconstruction of the original impact site. To date, Kenkman et al. have not found evidence of distorted or missing sedimentary strata which might be associated with such an impact in any well log data examined, but one drill core, 1–35 Hawk Fee, does show a breccia layer at 3023–3066 m below the surface, and Kenkman et al. are hoping to visually inspect this core in the near future. 

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