Cranial histology of pachycephalosaurs (Ornithischia: Marginocephalia) reveals transitory structures inconsistent with head-butting behavior

Paleobiology, Spring 2004 by Goodwin, Mark B, Horner, John R

Abstract.-Modern histological techniques allow paleontologists to investigate the internal microstructure of bone tissue. We apply high resolution images of histological thin sections from an ontogenetic series (not conspecific) of pachycephalosaurid frontoparietal domes to test the hypothesis that these Late Cretaceous dinosaurs used their heads as battering rams, analogous to the behavior of the bighorn sheep, Ovis canadensis, or as a thermoregulatory device. Our analysis reveals that the internal structure of the pachycephalosaur dome is a dynamic tissue that reflects the changeable expansion and vascularity of the dome throughout ontogeny. The radiating structures within the frontoparietal dome, used previously to support "head-butting" hypotheses, are unexpectedly transitory, diminishing in mature individuals and nearly absent in adult skulls where head-butting behavior is presumed to occur. The unique architecture of the pachycephalosaurid dome is dividable into three distinct Zones. We demonstrate that the relative vascularity, associated tissue structures, and orientation and density of Sharpey's fibers within these Zones are modified during growth. Evidence for an external dome covering in vivo precludes the determination of the final shape of the pachycephalosaur skull. On the basis of these new observations, we propose that cranial display in support of species recognition and communication is a more parsimonious interpretation of the function of the pachycephalosaurid dome. Sexual display behaviors were probably secondary.

Introduction

Pachycephalosaurs (Ornithischia: Marginocephalia), popularly known as dome-headed dinosaurs, are a group of bipedal ornithischian dinosaurs characterized by their distinctive greatly thickened dome above their braincase (Sues and Galton 1987). The majority of Late Cretaceous species occur in the Western Interior of North America and central Asia (Sues 1997). The pachycephalosaurid dome is made up of the fused paired frontals and parietal bones (Fig. 1). The degree of inflation or dome thickness, shape of the dome, and incorporation of the squamosal and lateral cranial elements into the dome have all been used as (1) diagnostic characters of the group (Maryanska and Osmolska 1974; Sereno 1986; Sues and Galton 1987; Maryanska 1990; Sullivan 2000, 2003; Williamson and Carr 2002); (2) support for recognizing sexual dimorphism (Chapman et al. 1981); and (3) relative age assignment of individual specimens (Giffin 1989; Goodwin et al. 1998; Williamson and Carr 2002).

The function of the domed skull of pachycephalosaurid dinosaurs has puzzled paleontologists for decades and continues to stimulate debate on the topic (Carpenter 1997; Horner and Goodwin 1998). Previous studies form the basis for the paleobiological interpretation that internal cranial features in the thickened pachycephalosaurid dome supported head-butting behavior.

What bones in the skull contribute to the pachycephalosaurid dome? This was a question Brown and Schlaikjer asked in their 1943 study describing a new assemblage of pachycephalosaurid dinosaurs (known then as troodonts) from the Western Interior of North America. Brown and Schlaikjer (1943: p.145) confirmed the absence of fused dermal ossifications over the skull roof (contra Gilmore 1931: p. 3) and demonstrated that the pachycephalosaurid dome was made up of the fused frontals and parietal bones. In addition, they recognized the variable density in the internal composition of the dome and described the interior of the dome as having a "radiating structure." This variability in dome density was also noted earlier by Lambe (1918: p. 24) who described the inner organization of the dome as possessing a "columnar structure."

Colbert (1955: p. 195) first suggested, "Perhaps (as a very wild surmise) the skull was used as a sort of battering ram." L. Sprague de Camp (1956: p. 17) popularized this idea a year later in his science fiction story "A Gun for Dinosaur," writing that "the males butt each other with these heads in fighting over the females." Galton (1970: p. 23) referred to pachycephalosaurids as "Dinosaurian Battering Rams" in the title of his 1970 study and proposed that the thick pachycephalosaurid dome was used in the same mode as the horns of fighting rams in the mountain sheep, Ovis canadensis. Galton (1970: p. 31) described the internal texture of the pachycephalosaurid dome as "a radiating series of trabeculae or bony fibers, each of which is perpendicular to the outer surface of the dome" (sensu Brown and Schlaikjer 1943) and suggested that the dome was used for head-butting, acknowledging L. Sprague de Camp's short story. Galton (1971: p. 45) went on to claim "this arrangement is ideal for resisting a force applied against the top of the dome" without further quantification. In a follow-up study of the pachycephalosaurid dome using photoelastic analysis, Sues (1978) stressed a piece of plexiglass in the shape of a pachycephalosaurid dome. This produced a radiating pattern similar to the radiating structure in the coronal-sectioned pachycephalosaurid dome described and figured earlier by Brown and Schlaikjer (1943: p. 143). Sues (1978: p. 465) concluded, "The radiating pattern of the trabeculae and the fact that the trabeculae are oriented in a perpendicular fashion, to the dorsal surface of the frontoparietal invite a functional explanation. Working with the trajectorial theory of bone architecture, the trabeculae could be considered to be compressive resistant." According to Alexander (1997: p. 423), "the bone of the pachycephalosaur dome seems to have had a spongy texture, so it probably deformed enough in an impact to provide useful cushioning." Rigby et al. (1987), Landry (1995), and Reid (1996) suggested that the internal "radiating structures" of the pachycephalosaur dome indicated the dome was more suitable as a heat-exchange organ than for head-butting.