Spectrovenator

Spectrovenator Temporal range: Barremian-Aptian,
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Abelisauridae
Genus:	†Spectrovenator Zaher et al., 2020
Species:	†S. ragei
Binomial name
†Spectrovenator ragei Zaher et al., 2020

Spectrovenator ("ghost hunter", named because the holotype was unexpectedly found under the holotype of Tapuiasaurus and it was found in October) is a genus of basal abelisaurid theropod dinosaur that lived during the early Cretaceous period in what is now Brazil. It is known by a single species, S. ragei, recovered from the Quiricó Formation.

Description

Life reconstruction

Spectrovenator is known from a single specimen, MZSP-PV 833, stored in the Museu de Zoologia da Universidade de São Paulo, and was discovered in the Embira Branca Range, close to the Coração de Jesus Municipality, located in northern Minas Gerais. The specimen consists of a nearly complete skull, partial series of cervicodorsal vertebrae with ribs, a complete sacrum, partial series of caudal vertebrae, and well-preserved hindlimb and hip bones.

The well-preserved skull of Spectrovenator has several features which show it is a transitional taxon between Jurassic and Late Cretaceous abelisaurids. It shares several cranial features with Rugops, another basal abelisaurid, indicating a similar position on the evolutionary tree. More derived taxa possessed adaptations that allowed for a broader chamber for the lower jaw muscles, but Spectrovenator lacked these. Instead, it contained plesiomorphic, or ancestral, traits in the temporal region of the skull. These features are a broad parietal surface between the supratemporal fossae and an elongated squamosal process of the postorbital. These characteristics suggested that Spectrovenator had a weaker bite force than more derived abelisaurids. The authors believe that this lends credence to the theory that bite force increased as abelisaurids evolved. The authors conclude that based on the number of basal traits exhibited by Spectrovenator, only the post-Cenomanian abelisaurids had the necessary adaptations for abelisaurids' characteristic feeding strategy.

Spectrovenator has distinct features in its skull that help differentiate it from the crania of other abelisaurids. These are: the posterior ramus of the maxilla is ornamented with vertically oriented grooves except for the smooth region in front of the maxilla-jugal suture; the lateral surface of the lacrimal is rugose except in the ventral section; a ventrally bowed posterior process of the jugal; a nuchal crest with a thin and smooth dorsal margin; a straight ventral margin of dentaries (part of the lower jaw) with a deep sulcus on the lateral surface; the dorsal margin of the surangular is slightly convex; and a longitudinal ridge along the posteroventral end of the mandibular ramus.

Importantly, Spectrovenator preserves a transversely concave nasal and a row of large foramina on the skull roof. These may represent the presence of blood vessels and nerves, probably related to the passage of the lateral nasal and supraorbital vessels and the trigeminal nerve.

Classification

The authors of the description of Spectrovenator included it in a phylogenetic analysis to test its position within Abelisauridae. Many characters supported its placement within Abelisauridae. The recovery of Spectrovenator as a basal member of Abelisauridae is consistent with its age and morphology. A simplified version of the analysis is shown below.

Abelisauridae	Eoabelisaurus Spectrovenator Rugops Arcovenator Rajasaurus Majungasaurus Indosaurus Abelisaurus Brachyrostra Skorpiovenator Ekrixinatosaurus Ilokelesia Carnotaurini Dahalokely Carnotaurus Aucasaurus Rahiolisaurus