A new carcharodontosaurian theropod (Dinosauria: Saurischia) from the Lower Cretaceous of Thailand

Abstract

The isolated fossil remains of an allosauroid theropod from the Lower Cretaceous Khok Kruat Formation of Khorat, Thailand, are described in this study. Detailed observations support the establishment of a new allosauroid, Siamraptor suwati gen. et sp. nov. This new taxon is based on a composite cranial and postcranial skeleton comprising premaxilla, maxilla, jugal, surangular, prearticular, articular, vertebrae, manual ungual, ischium, tibia, and pedal phalanx. It is distinguished from other allosauroids by characters such as a jugal with straight ventral margin and dorsoventrally deep anterior process below the orbit, a surangular with a deep oval concavity at the posterior end of the lateral shelf and four posterior surangular foramina, a long and narrow groove along the suture between the surangular and the prearticular, an articular with a foramen at the notch of the suture with the prearticular, an anterior cervical vertebra with a pneumatic foramen (so-called ‘pleurocoel’) excavating parapophysis, and cervical and posterior dorsal vertebrae penetrated by a pair of small foramina bilaterally at the base of the neural spine. The presence of a huge number of camerae and pneumatopores in cranial and axial elements reveals a remarkable skeletal pneumatic system in this new taxon. Moreover, the phylogenetic analyses revealed that Siamraptor is a basal taxon of Carcharodontosauria, involving a new sight of the paleobiogeographical context of this group. Siamraptor is the best preserved carcharodontosaurian theropod in Southeast Asia, and it sheds new light on the early evolutionary history of Carcharodontosauria.

A highly pneumatic middle Cretaceous theropod from the British Lower Greensand

“…2009), Sinraptor (Currie & Zhao 1993), Piatnitzkysaurus , such tyrannosauroids as Dilong (Rauhut 2003) and basal carcharodontosaurians such as Siamraptor (Chokchaloemwong et al . 2019) and Concavenator (the latter reflecting the basal condition within allosauroids; Cuesta et al . 2019).…”

“…The presence of a prezygoepipophyseal lamina in the cervical vertebra (effectively separating the lateral and dorsal surfaces of the neural arch) has been recovered as an allosauroid synapomorphy in some analyses (Chokchaloemwong et al . 2019), although it is also present (and developed more prominently) in abelisauroids and megaraptorans (Calvo et al . 2004; Carrano & Sampson 2008) as well as in some megalosauroids (Rauhut et al .…”

“…2004; Chokchaloemwong et al . 2019). However, the cervical parapophyses are not located at the midlength of the centrum, as they are in carcharodontosaurids and Megaraptor (Calvo et al .…”

“…The postzygodiapophyseal lamina of Vectaerovenator is subvertically oriented as it is in carcharodontosaurians (Chokchaloemwong et al . 2019) and megaraptorans (Calvo et al . 2004).…”

“…Based on comparisons with Allosaurus (Madsen 1976), this vertebra is most similar to cervical III in the relative distance between the diapophysis and parapophysis (which increases posteriorly), and in the anterior projection of the prezygapophysis (which projects further anteriorly in the mid‐series and sits atop taller pedicles in the posterior elements; Chokchaloemwong et al . 2019). All neurocentral sutures in this specimen are open, consistent with putative measures of relative morphological immaturity (Brochu 1996; a trait that can be extended to the other recovered axial elements).…”