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Expanded dorsal ribs in the Late Triassic pseudosuchian reptile Euscolosuchus olseni


Scheyer, Torsten M; Sues, Hans-Dieter (2016). Expanded dorsal ribs in the Late Triassic pseudosuchian reptile Euscolosuchus olseni. Journal of Vertebrate Paleontology:e1248768.

Abstract

The presence of expanded dorsal ribs is an unusual feature found only in a small number of tetrapod taxa. This condition, characterized by broadening of much of the entire rib shaft, differs from one where only a portion of the rib shaft is expanded or bears an uncinate process (e.g., birds, crocodylians, and Sphenodon; Jenkins, 1970). Recently, there has been renewed interest in the structure and development of dorsal ribs among amniotes in connection with the evolutionary development of the carapace in turtles (Lyson et al., 2013; Schoch and Sues, 2015). Reinterpretation of the anteroposteriorly broadened dorsal ribs in the Middle Permian putative stem turtle Eunotosaurus (Lyson et al., 2013) and the discovery of similar ribs in the more derived Triassic stem turtles Odontochelys (Li et al., 2008) and Pappochelys (Schoch and Sues, 2015) suggest that this feature represented an important step in the early evolution of the turtle carapace. Sues (1992) described a series of dissociated cervical and dorsal osteoderms and an incomplete dorsal vertebra of a new archosaurian reptile, Euscolosuchus olseni, from the Vinita Formation (formerly Turkey Branch Formation; Upper Triassic: Carnian) in the Richmond Basin of the Newark Supergroup at Midlothian, Virginia. Sues et al. (1994) have discussed the fossil locality and its geological context. Based on apomorphic features of the osteoderms, which formed two rows of dermal armor over at least the neck and trunk, Sues (1992) suggested referral of Euscolosuchus olseni to Pseudosuchia (Crurotarsi). A nearly complete rib was found together with disarticulated but still associated osteoderms and a dorsal vertebra of Euscolosuchus olseni. It was not included in the original description by Sues (1992) because it was not recognized as a rib at the time. In addition, a proximal end of another rib was recovered on an earlier occasion, but it articulates almost perfectly with the left transverse process of the dorsal vertebra. Referral of these elements to Euscolosuchus olseni is further supported by the distinctive external ornamentation on part of the rib shaft, which is indistinguishable from that on the external surfaces of the osteoderms and comprises round to oval pits of various sizes bounded by low ridges. The ribs clearly differ in their structure from those of Doswellia kaltenbachi, an unusual, heavily armored non-archosaurian archosauriform recovered from the Falling Creek Formation (Carnian) of Hanover County, Virginia (Dilkes and Sues, 2009). We here present a description of the two rib fragments referred to Euscolosuchus olseni and review their possible.

Abstract

The presence of expanded dorsal ribs is an unusual feature found only in a small number of tetrapod taxa. This condition, characterized by broadening of much of the entire rib shaft, differs from one where only a portion of the rib shaft is expanded or bears an uncinate process (e.g., birds, crocodylians, and Sphenodon; Jenkins, 1970). Recently, there has been renewed interest in the structure and development of dorsal ribs among amniotes in connection with the evolutionary development of the carapace in turtles (Lyson et al., 2013; Schoch and Sues, 2015). Reinterpretation of the anteroposteriorly broadened dorsal ribs in the Middle Permian putative stem turtle Eunotosaurus (Lyson et al., 2013) and the discovery of similar ribs in the more derived Triassic stem turtles Odontochelys (Li et al., 2008) and Pappochelys (Schoch and Sues, 2015) suggest that this feature represented an important step in the early evolution of the turtle carapace. Sues (1992) described a series of dissociated cervical and dorsal osteoderms and an incomplete dorsal vertebra of a new archosaurian reptile, Euscolosuchus olseni, from the Vinita Formation (formerly Turkey Branch Formation; Upper Triassic: Carnian) in the Richmond Basin of the Newark Supergroup at Midlothian, Virginia. Sues et al. (1994) have discussed the fossil locality and its geological context. Based on apomorphic features of the osteoderms, which formed two rows of dermal armor over at least the neck and trunk, Sues (1992) suggested referral of Euscolosuchus olseni to Pseudosuchia (Crurotarsi). A nearly complete rib was found together with disarticulated but still associated osteoderms and a dorsal vertebra of Euscolosuchus olseni. It was not included in the original description by Sues (1992) because it was not recognized as a rib at the time. In addition, a proximal end of another rib was recovered on an earlier occasion, but it articulates almost perfectly with the left transverse process of the dorsal vertebra. Referral of these elements to Euscolosuchus olseni is further supported by the distinctive external ornamentation on part of the rib shaft, which is indistinguishable from that on the external surfaces of the osteoderms and comprises round to oval pits of various sizes bounded by low ridges. The ribs clearly differ in their structure from those of Doswellia kaltenbachi, an unusual, heavily armored non-archosaurian archosauriform recovered from the Falling Creek Formation (Carnian) of Hanover County, Virginia (Dilkes and Sues, 2009). We here present a description of the two rib fragments referred to Euscolosuchus olseni and review their possible.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Paleontological Institute and Museum
Dewey Decimal Classification:560 Fossils & prehistoric life
Language:English
Date:9 September 2016
Deposited On:16 Dec 2016 10:01
Last Modified:08 Dec 2017 21:28
Publisher:Taylor & Francis
ISSN:0272-4634
Publisher DOI:https://doi.org/10.1080/02724634.2017.1248768

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