Spectrovenator

Spectrovenator is a small abelisaurid with an estimated body length of 1.42–2.2 metres (4.7–7.2 ft).^[2][3] 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.^[1]

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. In addition to these unique traits, Spectrovenator also 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.^[1]

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.^[1]