Cranial anatomy and diagnosis ofStygimoloch spinifer(Ornithischia: Pachycephalosauria) with comments on cranial display structures in agonistic behavior

Abstract

A new skull of Stygimoloch spinifer (MPM 8111) from the Upper Cretac eou s Hell Creek Form ation of North Dakota is the most complete specimen discove red to date. It allows much of the skull and braincase of this unusual pachycephalosaurid to be desc ribed for the first time and confirms a suite of diagnostic characters for the species. The skull is long with a vaulted, tran sver sely narrow frontoparietal dome and a robust squamosal forming a prominent posterior shelf. The shelf is ornamented by three to four large, low-angle horn s and mult iple clusters of smaller bony nodes. The orientation of the squamosal is preserved along an unambiguous contact with the frontoparietal suture, allowing definitive determination of the orientation of the squamosal horns . The se cranial features indicate a different mode of agonistic behavior than previously suggested for Stegocera s and Pachycephalosaurus . The high, narr ow dome of S. spinifer is not suited for head-butting, and the orientation of its squamosal horns and ornamental nodes strongly suggest display functions . Several additional specimens are described and referred to S. spinijer. INTRODUCTIONStygimoloch spinifer, an Upper Cretaceous (Maastrichtian) pachycephalosaurid ornithischian from western North America, has a high-domed, transversely narrow skull, distinguished by a unique cluster of well developed horns and bony nodes preserved along the posterodorsal surface of a thickened squamosal shelf. Prior to this report, the published record of Stygimoloch spinijer was limited to two isolated squamosals and one partial cranium. Galton and Sues (1983) named Stygimoloch spinijer from an unusual partial left squamosal ornamented with well-developed horns and robust bony nodes . This specimen (UCMP 119433) was collected from the Hell Creek Formation, McCone County, Montana. Galton and Sues referred an additional squamosal collected by J. B. Hatcher from the Lance Formation, Niobrara County Wyoming (YPM 335) to Stygimoloch spinijer. YPM 335 was first described by Marsh (1896) and refigured by Hatcher et al. (1907) as a dermal ossification of Triceratops. Brown and Schlaikjer (1943) referred this squamosal to Pachycephalosaurus sp . In 1987, Giffin et al. described a tall, narrow. domed pachycephalosaurid skull (MPM 7111) from the Hell Creek Formation of eastern Montana. They noted that it was significantly different from any previously known pachycephalosaurid, and named it a new species, Stenotholus kohleri, primarily on the basis of the markedly different shape of the cranium. Gabriel and Berghaus (1988) formally synonymized Stenotholus kohleri with Stygimoloch spinijer. Their comparison of MPM 7111 and MPM 8111 confirmed that both specimens shared the distinctive frontoparietal dome and cranial features characteristic of S. sp inijer.This study provides, for the first time, an extensive description of the cranial anatomy of Stygimoloch sp inifer and confirms the diagnostic characters for the species. Many aspects of pachycephalosaurid behavioral evolution hav...

Cranial Pathologies in a Specimen of Pachycephalosaurus

“…The dorsal surface of BMR P2001.4.5 is smooth, which is a characteristic of the largest pachycephalosaurid specimens [16] and lacks the distinctive, regular dimpling found in Sphaerothlous or polygonal sulci present on smaller domes [19]. Considering its large size, BMR P2001.4.5 is too broad and large to be attributed to Stygimoloch , which possesses a smaller, mediolaterally narrow dome [8]. However, due to the large frontoparietal dome of Pachycephalosaurus wyomingensis among other pachycephalosaurids, the large size of BMR P2001.4.5 suggests the specimen is referable to Pachycephalosaurus [16].…”

The socio-sexual behaviour of extant archosaurs: implications for understanding dinosaur behaviour

“…Goodwin 1990), an adaptation long thought related to intraspecific combat among males during which two rivals would face each other and engage in head-butting bouts similar to the extant bighorn sheep Ovis canadensis (Colbert 1955;Galton 1970Galton , 1971; see also Snively and Cox 2008). This interpretation was questioned by Sues (1978), Carpenter (1997) and Goodwin et al (1998) who raised objections based upon the minimal contact area between the two opposing heads and the lack of a correcting mechanism, favouring instead a flankbutting model as observed in male African antelope (see Leuthold 1977). In this scenario, rival males would stand roughly parallel facing either each other, or in the same direction, and deliver targeted blows to the sides of the body.…”

“…Type two combat was proposed for Stygimoloch, a genus with squamosal horns along the posterior margins of the dome that could have been employed to inflict localised non-lethal discomfort (Carpenter 1997). Yet another conclusion was reached by Goodwin et al (1998) while evaluating a well-preserved Stygimoloch skull, and braincase. The authors argued against head-to-head contact in this genus for a variety of reasons.…”

“…Brown and Schlaikjer (1943) first suggested the possibility of dimorphs in Stegoceras based on two distinct types of dome, suggesting that the presence of a squamosal shelf Historical Biology 145 may have functioned as a display structure for males, though the caveat was added that the conclusion was purely arbitrary. Goodwin et al (1998) countered that the squamosal shelf was more likely a characteristic of juveniles and therefore not an indication of gender. The strongest and most robust evidence is offered by Chapman et al (1981) who undertook a detailed morphometric analysis of Stegoceras validus domes.…”

“…Furthermore, upon examination the bone that composes the dome in Stygimoloch is highly vascularised and of a pattern that offers no substantial protection from directed impact force. It was concluded that the orientation of the squamosal horns and 'ornamental' nodes suggested a possible display function (Goodwin et al 1998). Goodwin and Horner (2004) examined the cranial histology of the frontoparietal dome with the intention of testing both the head-butting and an older heat dissipation theory and found no support for either.…”