Evidence for niche partitioning among ground-height browsing sauropods from the Upper Jurassic Morrison Formation of North America

Julia B Mchugh

doi:10.31711/giw.v5i0.20

Abstract

Two tooth-bearing snout fragments from a diplodocid sauropod from the Brushy Basin Member of the Morrison Formation (Upper Jurassic) excavated from the Mygatt-Moore Quarry in Rabbit Valley, Colorado are described. The Mygatt-Moore Quarry has produced thousands of vertebrate fossils from the Brushy Basin Member, with the diplodocid Apatosaurus cf. louisae and the tetanuran Allosaurus fragilis dominating the assemblage. Additionally, remains of another diplodocid, Diplodocus sp., have been found near the quarry within Rabbit Valley. Both specimens in this study preserve eight teeth per alveolar position, as observed through broken surfaces at the gross anatomical level and also through computed tomography (CT) scans. This is inconsistent with the genus Diplodocus sp., which has been previously shown to have a maximum of six teeth per alveolus. The presence of eight replacement teeth per alveolus has previously only been reported in the Cretaceous rebbachisaurid Nigersaurus taqueti, which has been interpreted to have occupied a similar ground-height browsing feeding strategy to both Diplodocus and Apatosaurus. This is the first report of this type of high-count replacement teeth in a diplodocid sauropod from the Morrison Formation. The high number of replacement teeth in a close relative to the contemporaneous Diplodocus provides evidence for niche partitioning among the contemporary ground-height browsing diplodocid sauropods of the Late Jurassic Period in North America.

Highlights

Apatosaurus and Diplodocus are contemporary sauropod genera found within the Upper Jurassic Morrison Formation (Tschopp and others, 2015)
Two tooth-bearing snout fragments from a diplodocid sauropod from the Brushy Basin Member of the Morrison Formation (Upper Jurassic) excavated from the Mygatt-Moore Quarry in Rabbit Valley, Colorado are described
The high number of replacement teeth in a close relative to the contemporaneous Diplodocus provides evidence for niche partitioning among the contemporary ground-height browsing diplodocid sauropods of the Late Jurassic Period in North America

Summary

INTRODUCTION

Apatosaurus and Diplodocus are contemporary sauropod genera found within the Upper Jurassic Morrison Formation (Tschopp and others, 2015). The number of teeth per alveolus within the premaxillae (including both replacement and erupted tooth positions) vary among taxa, with four reported in Camarasaurus sp., six in Diplodocus sp., five to six in Dicraeosaurus hansemanni, and eight in Nigersaurus taqueti (Sereno and others, 2005; D’Emic and others, 2013; Schwarz and others, 2015). Despite the lack of transport evident in the assemblage, fossils often preserve bone surface modifications (e.g., breakage, abrasion, and feeding traces) that suggest a reworked death assemblage by subsequent living individuals, possibly through feeding and trampling (Foster and others, 2018) Both specimens in this study were excavated from the gray mudstone of the Brushy Basin Member of the Morrison Formation at the MMQ using hand-quarrying techniques. Visible at position v is the lingual inflection at the separation of root and crown (figures 5 and 6)

DISCUSSION

CONCLUSIONS