Cristatusaurus

Cristatusaurus Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Diagram illustrating possible size and skeletal reconstruction combining several fossil specimens
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Spinosauridae
Subfamily:	†Baryonychinae
Genus:	†Cristatusaurus Taquet and Russell, 1998
Species:	†C. lapparenti
Binomial name
†Cristatusaurus lapparenti Taquet and Russell, 1998
Synonyms
Sereno et al., 1998

Cristatusaurus is a

Originally proposed to be an indeterminate species of Baryonyx, the identity of Cristatusaurus has been subject to debate, in part due to the fragmentary nature of its fossils. Some argue that it is probably the same dinosaur as Suchomimus, which has also been found in Niger, in the same sediment layers. In that case the genus Cristatusaurus would have priority, since it was named two months earlier. Others have concluded, however, that Cristatusaurus is a nomen dubium, considering it indistinguishable from both Suchomimus and Baryonyx. Some distinctions between the fossils of Cristatusaurus and Suchomimus have been pointed out, but it is uncertain whether these differences separate the two genera or if they are due to ontogeny (changes in an organism during growth). A recent study differentiated Cristatusaurus from Suchomimus and assigned it as a valid spinosaurid genus, placing the theropod just outside Baryonychinae.

History of research

The first

In a

The fragmentary Cristatusaurus skull material might represent the same taxon as Suchomimus (top) or Baryonyx (bottom)

When describing the

In a 2017 study, Marcos Sales and Cesar Schultz compared the holotype of Cristatusaurus (MNHN GDF 366) to the referred snout of Suchomimus (MNN GDF501). Both of them exhibit a narrow rim across the top of their premaxillae. However, Cristatusaurus's convex secondary palate is clearly visible in side view (situated under the premaxillary teeth), whereas in Suchomimus it is discernible only through cracks on the fossil snout. It was also pointed out that where known, the upward-sloping process of Cristatusaurus's maxilla is narrower than in Suchomimus. The researchers concluded that further study is needed to determine whether these differences are possible autapomorphies (distinguishing features) between the taxa, or if they are the result of ontogenetic (developmental) changes, given that the Cristatusaurus holotype represents a younger individual.^[13] A 2021 study assigned Cristatusaurus just outside of Baryonychinae and differentiated from Suchomimus as a valid genus, supporting its independence as a genus.^[19]

Description

In 2012,

The tip of Cristatusaurus's premaxilla was short and expanded, while the rear end was narrowed near the suture with the maxilla; this rosette-like snout shape was characteristic of spinosaurids. The front of the upper jaw was concave on the bottom, shaped to interlock with what would have been the convex and also enlarged tip of the mandible's dentary bone.^[2][4][22] A thin sagittal crest ran lengthwise on top of the premaxillae, a condition present in Baryonyx and Suchomimus, and very prominent in Angaturama (a possible synonym of Irritator).^[13] Like all spinosaurids, Cristatusaurus's external nares (bony nostrils) were positioned further back on the skull that in typical theropods.^[4][13] Two bony processes extended across the underside of the snout, in a convex structure that formed the animal's secondary palate. This condition is observed in all extant crocodilians, but not in most theropod dinosaurs; however, it was a common trait among spinosaurids.^[13][12]

Cristatusaurus's dental

Spinosaurids were large bipedal carnivores with well-built forelimbs and elongated, crocodile-like skulls. The taxonomic and phylogenetic affinities of the group are subject to active research and debate, given that in comparison to other theropod groups, many of the family's taxa (including Cristatusaurus) are based on relatively poor fossil material.^[22] Traditionally the family has been divided into two subfamilies: Spinosaurinae, which includes genera like Irritator, Spinosaurus, and Oxalaia; and Baryonychinae, which includes Baryonyx and Suchomimus. Although the genus and species placement of Cristatusaurus lapparenti is disputed, its fossils certainly belong to a member of the baryonychinae, because of its more forwardly placed external nostrils; relatively larger first premaxillary teeth; and more closely spaced tooth sockets than in spinosaurines; as well as the presence of fine serrations, in contrast to spinosaurines lacking them entirely.^[22][12][23] However, authors like Sales and Schultz have questioned the monophyly of Baryonychinae (meaning it might be an unnatural group), stating that the South American spinosaurids Angaturama and Irritator represent intermediate forms between Baryonychinae and Spinosaurinae, based on their craniodental (skull and tooth) features. Their cladogram can be seen below.^[13]

Spinosauridae	Baryonyx Cristatusaurus Suchomimus Angaturama Oxalaia Spinosaurus

Paleoecology