Cryolophosaurus (FMNH PR, 1821)

Smith, Nathan D., Makovicky, Peter J., Hammer, William R. & Currie, Philip J., 2007, Osteology of Cryolophosaurus ellioti (Dinosauria: Theropoda) from the Early Jurassic of Antarctica and implications for early theropod evolution, Zoological Journal of the Linnean Society 151 (2), pp. 377-421 : 415-416

OF CRYOLOPHOSAURUS

At an estimated weight of 465 kg, Cryolophosaurus represents the largest known Early Jurassic theropod ( Anderson, Hall-Martin & Russell, 1985). Based on femoral length, Cryolophosaurus rivals the largest known specimens of Ceratosaurus in size ( Madsen & Welles, 2000; Carrano, 2005). Although the neurocentral sutures of all recovered vertebrae (including all regions throughout the axial column: cervical, dorsal, sacral, caudal) of Cryolophosaurus (FMNH PR1821) are closed, the fifth sacral (= ‘caudosacral 1’) lacks any sign of fusion of its centrum to the preceding sacral centrum, and the neurocentral sutures on several of the caudal vertebrae are not obliterated, suggesting that this specimen may not represent a fully mature individual ( Brochu, 1996; Irmis, 2007). More specific inferences regarding the ontogenetic stage of Cryolophosaurus based on patterns of neurocentral suture closure would be tenuous at best, and await integration with histological data (see discussion in Irmis, 2007). Evidence for a significant increase in theropod body size is present almost immediately after the Triassic–Jurassic transition ( Olsen et al., 2002), though these ‘ Dilophosaurus ’-sized animals would have still been considerably smaller than many of their Late Jurassic relatives (e.g. Ceratosaurus , Allosaurus ). The size of Cryolophosaurus suggests that by the end of the Early Jurassic, one or more lineages of theropods had already attained the large size characterized by much of the group’s later evolutionary history.

In addition to its large size, Cryolophosaurus differs from many early theropods in a variety of features throughout its skeleton. The presence of a dorsoventrally high, furrowed transverse cranial crest in Cryolophosaurus is unique among all theropods. However, the construction of this crest, formed by dorsal expansions of the lacrimals which pinch the nasals medially, is similar to the morphology of the posterior portion of the nasolacrimal crests of the Jurassic theropods Dilophosaurus wetherilli (UCMP 77270), ‘ Dilophosaurus ’ sinensis ( Hu, 1993) and Monolophosaurus (IVPP 84019). As stated previously ( Hammer, 1997), the crest of Cryolophosaurus could probably not have functioned in any type of combative role. The diversity of cranial crests present in theropod dinosaurs, including low parasagittal nasal crests (‘ Syntarsus ’ kayentakatae), large blade-like parasagittal nasolacrimal crests ( Dilophosaurus wetherilli ; ‘ Dilophosaurus ’ sinensis ), pneumatically excavated crests (‘ Dilophosaurus ’ sinensis , Monolophosaurus , Guanlong ), nasal horns ( Ceratosaurus , several spinosaurids, Proceratosaurus ) and frontal horns ( Carnotaurus , Majungatholus ), tempts speculation that these structures may be associated with sexual dimorphism, or function in species recognition. The amount of data that must truly be marshalled to support these hypotheses (which often represent population-level processes) remains extremely limited for theropods, however, and we support a view that cautions against over-interpreting the available observational data ( Padian, Horner & Lee, 2005). Regardless, the phylogenetic distribution of cranial crests suggests that cranial ornamentation may have been ubiquitous throughout theropod evolutionary history. Optimizing the presence of nasal crests on the MPTs recovered in the phylogenetic analysis suggests that some type of nasal crest may have been ancestral for Theropoda.

Another unique feature of Cryolophosaurus is the constriction of the infratemporal fenestra by the ventral ramus of the squamosal and the expanded dorsal ramus of the jugal ( Fig. 8 ). Although these elements are not fused, and part of the constriction may be attributed to distortion of the posterior portion of the skull, the constriction of the infratemporal fenestra in Cryolophosaurus is more extreme (particularly with regard to the posterodorsally expanded dorsal ramus of the jugal) than in other basal theropods where this constriction is present [e.g. ‘ Syntarsus ’ kayentakatae ( Rowe, 1989; Tykoski, 1998); Zupaysaurus ( Arcucci & Coria, 2003) ; Dilophosaurus wetherilli [UCMP 37302]; ‘ Dilophosaurus ’ sinensis ( Hu, 1993) ]. The deeply excavated, dorsoventrally low and anteroposteriorly elongate infradiapophyseal fossae of the posterior cervical vertebrae of Cryolophosaurus is also a unique feature among theropods ( Fig. 11 ). However, this morphology is reminiscent of the broadened infradiapophyseal fossae of the cervicals of the poorly known Middle Jurassic Chinese theropod Chuandongocoelurus ( He, 1984) , though the cervical centra of this taxon are also elongated relative to Cryolophosaurus . The slender, elongate cranial processes of the cervical ribs represent an additional apomorphic feature of Cryolophosaurus ( Fig. 11 ). Several other basal theropods possess elongate cranial processes of the cervical ribs, including Coelophysis rhodesiensis ( Raath, 1977) , ‘ Syntarsus ’ kayentakatae and Spinostropheus (FMNH cast of MNN TIG6), though in these taxa the cranial processes protrude at most several centimetres past the anterior articular facets of their respective vertebrae, unlike the extremely elongate condition present in Cryolophosaurus . It is notable that the theropod taxa with similar elongated cranial processes on their cervical ribs appear to bracket Cryolophosaurus phylogenetically ( Fig. 20 ), provided that Spinostropheus represents a basal neoceratosaur ( Sereno et al., 2004). Future assessment of the distribution of these structures (which appear to be very fragile, and often incompletely preserved in many theropod specimens) may provide additional insight into the relationships of Cryolophosaurus .

Based on the results of the phylogenetic analysis, Cryolophosaurus is recovered as being closely related to the Early Jurassic theropods ‘ Dilophosaurus ’ sinensis , Dracovenator and Dilophosaurus wetherilli ( Fig. 20 ), and thus a brief discussion of the features distinguishing Cryolophosaurus from these taxa is warranted. Cryolophosaurus differs from Dracovenator ( Yates, 2005) and Dilophosaurus wetherilli (UCMP 37302; 77270) in possessing a dorsally facing attachment for the m. depressor mandibulae on the retroarticular process. The dorsal ramus of the quadratojugal is also broader anteroposteriorly in Cryolophosaurus than in either Dilophosaurus wetherilli (UCMP 37302) or ‘ Dilophosaurus ’ sinensis ( Hu, 1993) . In the axial column, Cryolophosaurus lacks the well-rimmed posterior pleurocoels present in the posterior cervicals of Dilophosaurus wetherilli (UCMP 77270). The sharp lateral ridge extending from behind the parapophysis in the posterior cervical vertebrae of Cryolophosaurus is not present in Dilophosaurus wetherilli (UCMP 77270). The transverse processes of the fifth sacral (= ‘caudosacral 1’) vertebra of Cryolophosaurus do not extend as far posteriorly as they do in Dilophosaurus wetherilli (UCMP 37302). Many similarities between Cryolophosaurus and Dilophosaurus wetherilli are present in the appendicular skeleton. However, Cryolophosaurus lacks the distinct ‘kink’ along the dorsal margin of the iliac blade that is present in Dilophosaurus wetherilli (UCMP 37302, 77270; Tykoski, 2005), and the supraacetabular shelf of Cryolophosaurus does not extend as far posteriorly as in Dilophosaurus wetherilli (UCMP 37302). The ischium of Cryolophosaurus is also more robust than in Dilophosaurus wetherilli (UCMP 37302; Tykoski, 2005), and bears a prominent ischial rugosity. A trochanteric shelf on the proximal femur is present (though partially damaged) in Crylophosaurus, and has not been reported in Dilophosaurus wetherilli , though this may be due to the relative immaturity of specimens collected to date ( Tykoski, 2005). The tarsus of Cryolophosaurus and Dilophosaurus wetherilli (UCMP 37302, 37303, 77270) are also very similar, with the exception of a small fibular splint located dorsal to the ascending process of the astragalus, although this may be a pathological feature of Cryolophosaurus . In summary, many of the specialized and/or unique morphological features of Cryolophosaurus appear to be concentrated in the cranial skeleton, suggesting that much of the postcranial anatomy of Cryolophosaurus retained a generalized, basal theropod form.

Figures

Figure 8. Portions of the left jugal, postorbital and squamosal of Cryolophosaurus ellioti in medial aspect (A), and interpretive line drawing (B). These elements have been split in half from the left side of the skull.

Figure 11. Several articulated posterior cervical vertebrae and ribs of Cryolophosaurus ellioti in left lateral (A), and left posterolateral (B), aspects (photo courtesy of J. Weinstein).

Figure 20. Strict consensus of 108 MPTs. All trees have length of 833 steps, CI 0.489, RI 0.772. Bootstrap values> 50% are listed to the left of nodes, and Bremer support values> 1 are listed to the right. Several theropod clades are indicated in bold.