Anatomy, systematics, paleoenvironment, growth, and age of the sauropod dinosaurSonorasaurus thompsonifrom the Cretaceous of Arizona, USA

Abstract

Sauropod dinosaurs are rare in the Cretaceous North American fossil record in general and are absent from that record for most of the Late Cretaceous. Sonorasaurus thompsoni from the Turney Ranch Formation of the Bisbee Group of Arizona, USA, potentially represents one of the youngest sauropods before their ca. 30-million-yearlong hiatus from the record. The anatomy of Sonorasaurus has only been briefly described, its taxonomic validity has been questioned, several hypotheses have been proposed regarding its phylogenetic relationships, and its life history, geologic age, and reported paleoenvironment are ambiguous.Herein we assess the systematics, paleoenvironment, life history, and geologic age of Sonorasaurus based on firsthand observation, bone histology, and fieldwork in the holotypic quarry and environs. The validity of S. thompsoni is substantiated by autapomorphies. Cladistic analysis firmly places it within the Brachiosauridae, in contrast to results of some recent analyses. Bone histology suggests that the only known exemplar of Sonorasaurus grew slowly and sporadically compared to other sauropods and was approaching its adult size. In contrast with previous assessments of a coastal/estuarine paleoenvironment for the Turney Ranch Formation, our sedimentological and plant macrofossil data indicate that Sonorasaurus lived in a semiarid, low relief evergreen woodland that received highly variable (perhaps seasonal) precipitation. We obtained detrital zircons from the holotypic quarry for U-Pb dating, which only yielded Barremian-aged and older grains, whereas other radiometric and biostratigraphic data suggest that the sediments at the quarry were deposited near the Albian-Cenomanian boundary.Sonorasaurus is taxonomically valid, represents one of the geologically youngest brachiosaurid sauropods, and inhabited a harsh inland evergreen-dominated woodland environment that limited its growth. A review of other Bisbee Group dinosaurs suggests that its fauna, although poorly sampled, exhibits broad similarity to those from coeval North American horizons, reinforcing the apparent faunal homogeneity at the time.

The earliest known titanosauriform sauropod dinosaur and the evolution of Brachiosauridae

“…Whereas Carballido et al (2015) recovered Padillasaurus as a brachiosaurid, our study places it in Somphospondyli (see below; note that this taxon was described after D’Emic, Foreman & Jud (2016) was accepted for publication). All three analyses place Europasaurus as basal to the other brachiosaurids, although in Carballido et al (2015) Europasaurus lies outside of Titanosauriformes (see below).…”

“…Although we do not dispute that several characters might be “biologically related”, they are not consistent across taxa. For example, D’Emic, Foreman & Jud (2016) highlighted four characters pertaining to aspects of forelimb gracility (characters 42, 45, 50 and 53), arguing that “sauropods with gracile forelimbs tend to have each long bone elongated, not just one segment”. However, a survey of the scores in Mannion et al (2013) shows just how much these scores vary between taxa, including those that are considered as closely related to one another.…”

“…Abydosaurus mcintoshi , from the late Early Cretaceous of the USA (Chure et al, 2010), is also universally considered as a brachiosaurid (Chure et al, 2010; Ksepka & Norell, 2010; D’Emic, 2012; Mannion et al, 2013; Carballido et al, 2015), and the Late Jurassic Portuguese sauropod Lusotitan atalaiensis is probably an additional member of this clade (Antunes & Mateus, 2003; Mannion et al, 2013; D’Emic, Foreman & Jud, 2016; Mocho, Royo-Torres & Ortega, 2016; though see Carballido et al (2015) for a more basal position). Carballido et al (2015) recently described Padillasaurus leivaensis , from the late Early Cretaceous of Colombia, which they recovered within Brachiosauridae; if correctly identified, this would make it the first unambiguous occurrence of a South American brachiosaurid.…”

Redescription of Brachiosaurid Sauropod Dinosaur Material From the Upper Jurassic Morrison Formation, Colorado, USA

“…Lastly, we updated the data matrix of D'Emic et al () to include USNM 5370 and the Potter Creek individual (BYU 9754(4744)/USNM 21903) in the B. altithorax terminal taxon.…”

“…We ran a cladistic analysis using a modification of the matrix presented in D'Emic et al (), using the same software and parameters but coding USNM 5730 and BYU 9754(4744)/USNM 21903 (not including C1–C3 from Potter Creek for reasons described above) as part of the B. altithorax terminal taxon. Addition of these taxa increased the amount of data scored from 36% (44/122 cells scored) to 52% (59/122 cells scored).…”