Ahvaytum bahndooiveche, Lovelace & Kufner & Fitch & Rogers & Schmitz & Schwartz & LeClair-Diaz & St.Clair & Mann & Teran, 2025

Ahvaytum bahndooiveche sp. nov.

( Fig. 2 View Figure 2 ).

Holotype: UWGM 1975 , isolated left astragalus ( Fig. 2 View Figure 2 ). Referred material: The proximal end of a left femur ( UWGM 7549 ; Fig. 3 View Figure 3 ) is referred to Ahvaytum bahndooiveche due to clear saurischian affinities. This femur was found ex situ within a 5-m radius of the type specimen and encrusted with a similar micritic carbonate.

Etymology: bahndooiveche , from the Shoshone ‘ban·döi·ve·che’ (pronounced ‘bon-do-ee-vee-chee’), meaning ‘water’s young handsome man’ in reference to the colourful salamanders found in the region, and also means ‘dinosaur’ (Supporting Information, Audio S1), which is the meaning used here. The name Ahvaytum bahndooiveche (‘long ago dinosaur’) is the product of a multigenerational collaboration between the Fort Washakie Schools 7th grade cohort (2022), educators, Eastern Shoshone Tribal Historic Preservation Office, and Tribal Elders. Pronunciation guide: bahn as in ‘bonnet’, do-ee as in ‘dewy’, vee as in ‘ivy’, and chee as in ‘cheese’. See the Supporting Information, Audio S1 for recorded pronunciation (recording by RT. LSID urn:lsid:zoobank.org:pub:538C156C-5CC1-4706-8D8A-FFA6757A6590 ).

Locality and horizon: Garrett’s Surprise (after undergraduate field assistant Garrett Johnson who discovered the locality) is a small exposure of the Popo Agie Formation found in an erosional window within the overlying Wind River Formation ( Eocene ), on Wyoming Game and Fish administered lands, c. 1 km south of the confluence of the East Fork of the Wind River and Spear Creek ( Fig. 1A View Figure 1 ). GPS coordinates for Garrett’s Surprise are reposited with the type specimen. Vertebrate material was surface collected and no in situ specimens were recovered during a screening effort of the area; all material is constrained to the mid to upper part of the purple unit of the lower Popo Agie Formation ( Lovelace et al. 2024). The ochre unit is poorly exposed along the fall line where specimens were collected and is unconformably overlain by Eocene deposits of the Wind River Formation [see Supporting Information, Text S 1 (Fieldwork and Site Details)].

Diagnosis: Ahvaytum bahndooiveche shares a combination of features present in sauropodomorph dinosaurs to the exclusion of theropods, herrerasaurs, and ornithischians, including: a flat, roller-shaped distal surface of the astragalus shared with sauropodomorphs but not herrerasaurs and neotheropods ( Marsh et al. 2019); a tibial facet that does not extend on to the anteromedial corner of the astragalus shared with all sauropodomorphs in our study, the silesaurid Lewisuchus admixtus Romer, 1972 , the theropods Lepidus praecisio and Eodromaeus murphi Martinez et al., 2011 , and the herrerasaurs Tawa hallae Nesbitt et al., 2009 and Chindesaurus bryansmalli Long and Murry, 1995 ; a shallow laterodistal notch of the astragalus shared with non-massopodan sauropodomorphs, except Panphagia protos Martinez and Alcober, 2009 and Efraasia minor von Huene, 1875 (sensu Galton 1973) and the herrerasaur T. hallae ; a rounded anterior and medial margin on the proximomedial surface of the astragalus shared with the theropod Eodromaeus murphi and sauropodomorphs, except Saturnalia tupiniquim Langer et al., 1999 and a referred specimen of Buriolestes schultzi Cabreira et al., 2016 (sensu Moro et al. 2024); a posterolateral ridge between the anterior ascending process and the posterolateral process that is always taller than the posterolateral process shared with the herrerasaurids Herrerasaurus ischgualastensis Reig, 1963 and Sanjuansaurus gordilloi Alcober and Martínez, 2010 and sauropodomorphs, except Unaysaurus tolentinoi Leal et al., 2004 . Ahvaytum bahndooiveche is differentiated from all other sauropodomorphs with comparable material by the following autapomorphy: fossa on the medial surface of the astragalus that is open distally. Ahvaytum bahndooiveche is differentiated from Eoraptor lunensis Sereno et al., 1993 , Mbiresaurus raathi Griffin et al., 2022 , P. protos , S. tupiniquim , and a referred specimen of B. schultzi by a proportionally wider astragalus (max. anteroposterior length/ max. transverse width: Ahvaytum = 0.53, Buriolestes ≅ 0.71, Eoraptor ≅ 0.63, Mbiresaurus ≅ 0.64, Panphagia ≅ 0.88, and Saturnalia ≅ 0.73) andbyarelativelywidefibularfacetoccupying more than a quarter of the transverse width of the astragalus. Further differentiated from Eoraptor lunensis by a posteromedial angle on the astragalus. Further differentiated from a referred specimen of B. schultzi ( Moro et al., 2024) by a poorly proximally expanded posteromedial process (= pyramidal process of some authors) and by a relatively tall posterolateral process (= posterior ascending process of some authors). Further differentiated from both B. schultzi and M. raathi by a posterolateral process that is level with the posterior margin of the astragalus in proximal view.

Remarks: The proximal end of a silesaurid femur (UWGM 7407) and the distal end of a probable silesaurid humerus (UWGM 7550; see below) were recovered from the study area precluding the referral of 15 non-diagnostic dinosauromorph elements (UWGM 7434; Supporting Information, Fig. S 1 View Figure 1 in Text S1) to higher taxa. Additional vertebrate material was surface collected from the Garrett’s Surprise locality and identified as non-ornithodiran or indeterminate.

Description: The type specimen of Ahvaytum bahndooiveche (UWGM 1975) is an isolated left astragalus ( Fig. 2 View Figure 2 ). A calcaneum was not recovered and there is no evidence of fusion of these elements. The transverse width of the astragalus is nearly twice its maximum anteroposterior length. The proximal surface is slightly weathered, but the rest of the surface of the element is essentially intact. There is a transverse groove on the anterior surface of the body of the astragalus, which is widespread among dinosauromorphs. The ascending process is separated from the anterior surface by a platform and broken at approximately mid-height. A foramen on the anterior surface of the ascending process extends posterodistally into the body of the astragalus. A posterolateral ridge extends from the ascending process to meet a posterolateral process [= posterior ascending process of Sereno and Arcucci (1994)] along the posterior margin. The anteromedial corner is acute as in other early dinosaurs (e.g. Müller 2021) and projects well beyond the anterolateral condyle. The tibial articular facet is separated from the anterior and medial margins by a wide, rounded lip and does not extend on to the anteromedial corner, which is identical to the condition in several sauropodomorphs (e.g. Müller 2021). The medial surface of the astragalus bears a fossa that opens toward the distal surface, which appears to be autapomorphic among dinosaurs. The posteromedial margin forms a corner, and the proximal margin here is not raised into a pyramidal process such as that found in theropods (e.g. Nesbitt and Ezcurra 2015) or the elevated crest in unaysaurids (e.g. Müller 2021, Ezcurra et al. 2023). Posterior to the ascending process there is a non-articular fossa (= dorsal basin, = semi-elliptical fossa) with a weathered but raised rim along its medial margin. Within this fossa there is a single oblong vascular foramen whose canal is short and anteriorly directed at the base of the ascending process ( Figs. 2C–D View Figure 2 ). The fibular facet occupies just over a quarter of the transverse width of the astragalus and is delineated by a sharp medial margin. A laterodistal notch (= lateroventral depression) is present on the distal surface, presumably for reception of a medial process of the calcaneum, as is the case in Eoraptor lunensis ( Sereno et al., 2012) . An incipient protuberance is present on the distolateral surface of the astragalus just anterior to the laterodistal notch; however, unlike the condition in the unaysaurids Jaklapallisaurus asymmetricus Novas et al., 2010 and Macrocollum itaquii Müller et al., 2018 ( Ezcurra et al. 2023: fig. 3e–g) it does not form a distinct projection.

Referred material: The proximal end of a left femur (UWGM 7549) possesses several dinosaurian and saurischian features but is too incomplete for a referral beyond Saurischia on its own ( Fig. 3 View Figure 3 ). The proximal surface of the femur appears to be slightly abraded revealing trabecular bone and hindering the identification of a transverse groove, such as that seen in UWGM 7407 ( Fig. 4F View Figure 4 ) and widespread among early-diverging sauropodomorphs and dinosauromorphs more generally. There are also several cracks present in this element, one of which passes through the position that would be occupied by a ligament sulcus between the anteromedial and posteromedial tubera. The posteromedial tuber is small and rounded, and the larger anteromedial tuber is also rounded. The anterolateral tuber forms a broad, rounded profile along the anterolateral surface of the femur in proximal view. The head of the femur is offset from the shaft resulting in a concave emargination just ventral to the head, common among all dinosaurs. On the anterolateral surface of the femur, a ridge extends from the ventral margin of the head then merges distally into the shaft forming a ventral emargination, such as that seen in Saturnalia tupiniquim , Nhandumirim waldsangae Marsola et al., 2018 , and some coelophysoids ( Kirmse et al. 2023), but unknown among core ornithischians, exclusive of silesaurids. The shaft of the femur is broken approximately just above the position of the anterior trochanter. The dorsolateral trochanter is present as a rounded rugosity along the lateral side of the femoral shaft and probably missing its distal extent due to the break. The ‘greater trochanter’ is prominent and squared off with a straight margin between it and the head of the femur in anterior/posterior view similar to Herrerasaurus ischigualastensis ( Novas, 1994) and some specimens of Pampadromaeus barberenai Cabreira et al., 2011 and Buriolestes schultzi (Müller, 2022) . The fossa trochanterica (= facies articularis antitrochanterica) along the posterior side of the ‘greater trochanter’ is ventrally (= distally) descended, which is common among dinosaurs ( Nesbitt, 2011).

Ontogenetic assessment: Both the holotype astragalus ( UWGM 1975 ) and referred femur ( UWGM 7549 ) exhibit smooth bone textures and prominent condyles that are consistent with relatively older ontogenetic status in this specimen ( Griffin et al. 2019). Similarly, the referred femur exhibits muscle scars that are robustly developed and also support the notion of an older ontogenetic status for UWGM 7549 . An external fundamental system ( EFS) is absent from the femoral cross-section (see below); its presence would indicate the attainment of skeletal maturity. That said, other osteohistological signatures are consistent with a more advanced ontogenetic status for UWGM 7549 . These data include the presence of endosteal remodelling around the medullary cavity forming an internal fundamental system ( IFS), cortical remodelling that extends into the midcortex alongside a transition to more highly organized and less vascularized primary bone tissue at the periosteal margin, which signals a decrease in primary bone apposition in later ontogeny. A lack of an EFS indicates that UWGM 7549 was still slowly growing at the time of death. In light of these histological details, the absence of lines of arrested growth ( LAG) in UWGM 7549 should not be presumed to indicate an individual less than a single year of age. Instead, this individual had probably progressed beyond earliest ontogeny.