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The femora of diapsids have undergone morphological changes related to shifts in postural and locomotor modes, such as the transition from plesiomorphic amniote and diapsid taxa to the apomorphic conditions related to a more erect posture within Archosauriformes. One remarkable clade of Triassic diapsids is the chameleon-like Drepanosauromorpha. This group is known from numerous articulated but heavily compressed skeletons that have the potential to further inform early reptile femoral evolution. For the first time, we describe the three-dimensional osteology of the femora of Drepanosauromorpha, based on undistorted fossils from the Upper Triassic Chinle Formation and Dockum Group of North America. We identify apomorphies and a combination of character states that link these femora to those in crushed specimens of drepanosauromorphs and compare our sample with a range of amniote taxa. Several characteristics of drepanosauromorph femora, including a hemispherical proximal articular surface, prominent asymmetry in the proximodistal length of the tibial condyles, and a deep intercondylar sulcus, are plesiomorphies shared with early diapsids. The femora contrast with those of most diapsids in lacking a crest-like, distally tapering internal trochanter. They bear a ventrolaterally positioned tuberosity on the femoral shaft, resembling the fourth trochanter in Archosauriformes. The reduction of an internal trochanter parallels independent reductions in therapsids and archosauriforms. The presence of a ventrolaterally positioned trochanter is also similar to that of chameleonid squamates. Collectively, these features demonstrate a unique femoral morphology for drepanosauromorphs, and suggest an increased capacity for femoral adduction and protraction relative to most other Permo-Triassic diapsids.
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Silesaurid ( Archosauria: Dinosauriformes ) remains from the base of the Dockum Group ( Late Triassic: Otischalkian ) of Texas provide new insights to the North American record of dinosauriforms
Abstract Silesaurids (Archosauria: Dinosauriformes) are found in Middle to Upper Triassic deposits across Pangea, but few stratigraphic sections record the evolution of the group in one geographic area over millions of years. Here, we describe silesaurid remains from the oldest of the Upper Triassic stratigraphic sequence from the base of the Dockum Group, from the type locality of the Otischalkian faunachronozone. Isolated limb bones diagnostic of silesaurids include humeri, femora, and tibiae of a seemingly uniqueSilesaurus‐like taxon from the same locality (Otis Chalk Quarry 3). The femora consist of four specimens of different lengths that sample the variation of character states associated with ontogeny, also sampled previously in both silesaurids (e.g.,Asilisaurus kongweandSilesaurus opolensis) and within neotheropods within Dinosauria (e.g.,Coelophysis bauri). Our observations of the variation in the silesaurid sample further reinforce the interpretation of high variation of morphological features common in dinosauriforms. Furthermore, we show that overpreparation of bone surfaces has hidden some of this variation in previous interpretations. The tibia growth series shows that the fibular crest of the tibia develops during ontogeny, yet another phylogenetically informative character for dinosaurs and their kin that is at least ontogenetically variable in silesaurids. The presence of silesaurids at the base of the Dockum Group (late Carnian or early Norian) conclusively shows that the group was present near the onset of deposition of Upper Triassic rocks and survived for millions of years in the same geographic area at low latitudes throughout the Late Triassic.
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- Award ID(s):
- 1943286
- PAR ID:
- 10629139
- Publisher / Repository:
- The Anatomical Record
- Date Published:
- Journal Name:
- The Anatomical Record
- ISSN:
- 1932-8486
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/ar.25677
