A multitaxic bonebed near the Carboniferous-Permian boundary (Halgaito Formation, Cutler Group) in Valley of the Gods, Utah, USA: Vertebrate paleontology and taphonomy

摘要

The Carboniferous Permian (C-P) transition records a shift in the composition and environmental setting of tropical flora and vertebrate assemblages across western and central Pangea. Here we report the discovery of a rare, multitaxic bonebed in the lower Halgaito Formation (Cutler Group) in Valley of the Gods and its vicinity (San Juan County), southeast Utah, USA. The assemblage, which comprises carcasses and disarticulated bones and teeth, preserves aquatic and semi-terrestrial elements including xenacanth chondrichthyans, actinopterygians, sagenodontid lungfish, and the temnospondyl amphibian Eryops, together with terrestrial taxa including the synapsids, Ophiacodon navajovicus, Edaphosaunrus, and Sphenacodon, and a hitherto undescribed araeoscelidan reptile. Sedimentological, paleontological, and strontium isotopic evidence indicates the bonebed formed as a slackwater deposit at the confluence of a major freshwater stream channel and its tributary. The deposit probably formed through late stage flooding of a likely bottlenecked fluvial system, consistent with increasingly episodic or seasonal precipitation, concentrating carcasses of immature and mature shore-dwelling animals. Comparisons with vertebrate assemblages in the Cutler Group of New Mexico, USA suggest that the assemblage is correlative with the lower or middle assemblage of the El Cobre Canyon Formation (Cobrean and Coyotean Land Vertebrate Faunachrons; LVFs), and as such, is of latest Carboniferous age. Our findings strengthen the hypothesis that dryland-adapted vertebrate communities proliferated earlier in westernmost Pangea, well before earliest Permian time, compared to other tropical regions.