Homalocephale

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Pachycephalosauridae (disputed)

Genus:Homalocephale

Species:Homalocephale calathocercos

Introduction

Homalocephale was a relatively small, herbivorous dinosaur in Late Cretaceous Mongolia. Homalocephale calathocercos is the only known species in the genus, although its classification is disputed (and some researchers believe it may in fact be a juvenile form of another genus, Prenocephale). It is considered a member of the pachycephalosaurid group, made up of herbivores or omnivores with thick, bony skulls. Paleontologists believe that the group descended from an ancestor in early Jurassic Europe or Asia.

In the Triassic period, the continents of the earth were united in a single land mass, known as Pangaea. Dinosaurs first evolved during this period and began to spread across the globe. In the Jurassic, Pangaea began to split as a result of tectonic forces, giving rise to two supercontinents, Gondwana and Laurasia. As the supercontinents divided, populations became isolated and developed into a variety of species. Homalocephale and its relatives represent a dinosaur lineage that appears in many locations across former Laurasia but does not appear in Gondwana, leading paleontologists to suspect that the group evolved from ancestors that were isolated in the Jurassic after Pangaea began to split apart.

Classification

Scientists utilize two primary methods to classify organisms into different groups. The traditional form of Linnaean taxonomy, or systematics, classifies organisms according to overall physical similarity. An alternate method, called cladistics, creates groups called “clades” that reflect the evolutionary relationships between species.

Systematic analysis places Homalocephale in the order Ornithischia based on its “bird-like” pelvic structure. From there, it is classified in the suborder cerapoda, based largely on dental characteristics. The Pachycephalosauria infraorder contains Homalocephale and other dinosaurs with thickened bony plates on the anterior portion of the skull. However, further classification of Homalocephale is subject to considerable debate; some researchers assign it to the Pachycephalosauridae family, while others give it its own family, Homalocephalidae.

Cladistic analysis also shows uncertainty on the exact classification of Homalocephale, along with many other pachycephalosaurids, which is in part due to a lack of complete specimens. Broadly, it is placed in the clade Marginocephalia, or the “ridged head” dinosaurs, which possessed a shelf of bone extending over the rear portion of the skull. The Pachycephalosauria group is also used, for species that share a thickened skull roof with small ossified sections along the ridge of the skull. Some researchers consider Homalocephale to have split from this group early in its evolutionary development while other representatives developed domed skulls. Others include it within the narrower clade Pachycephalosauridae.

Regardless of which classification system is used, Homalocephale calathocercos is the only recognized species in the genus. However, even this designation has been challenged. Some paleontologists suggest that specimens of Homalocephale may in fact represent juveniles of another pachycephalosaurid, Prenocephale.

Anatomy

Homalocephale was a small dinosaur, measuring between 1.5 and 3 meters (4.9–9.8 ft) in length, almost half of which was accounted for by the animal's long tail. The dinosaur stood roughly 0.8 meters (2.6 ft) at the hip and was relatively lightly built, weighing between 43 and 46 kilograms (93–101 lbs).

As with all pachycephalosaurs, Homalocephale was bipedal, with strong rear legs terminating in three-toed feet with blunt claws. The animal's arms were relatively small and were probably used to maneuver vegetation toward its mouth. The animal had a relatively wide body, with a thick pelvis and a muscular tail strutted with bone to make it useful as a rudder when running at high speed.

The skull of Homalocephale, from which the animal gets its name, was flattened across the top and the bone was thick and reinforced by thick nodes. The animal had large eyes and possessed rows of small, sharp teeth. The top rear of the skull and the cheeks were lined with small, bony spikes and ridges.

Intelligence

Paleontologist James A. Hopson's findings indicate that dinosaurs were similar in intelligence to modern reptiles and not generally as intelligent as mammals and birds living in similar niches. Encephalization quotient (EQ), based on brain and body size, is one way to estimate animal intelligence. Homalocephale probably had an EQ level close to 2.0, placing it mid range along the dinosaur spectrum. Although herbivorous dinosaurs may not have been as intelligent as mammalian herbivores, the predators they faced were similarly less intelligent, and the struggle for survival was likely dominated more by instinct than intelligence.

Reproduction and Population

Paleontologists studying remains of Homalocephale noted that the species had a relatively broad pelvic region and speculated that it may have been viviparous, like mammals and some reptiles. However, no corroborating evidence suggests live birth, and most paleontologists believe that pachycephalosaurs would have been oviparous. Eggs and nests have been found for many groups of dinosaurs, suggesting that egg laying was the primary method of reproduction for the dinosauria.

Variation in the skeletons of some pachycephalosaur species suggests that some species may have displayed sexual dimorphism. Homalocephale is known from only incomplete specimens, and it is uncertain whether males and females of the species differed in appearance. Paleontologists are also uncertain of whether members of this species or other members of the group provided any parental care or cooperated in raising young.

Diet

The teeth of Homalocephale show a considerable amount of wear, indicating that the animal was feeding on tough vegetation. There were a variety of gymnosperms in Late Cretaceous Mongolia, including a variety of conifers and several common cycad species. Homalocephale may also have fed on pteridophytes such as ferns and tree ferns, many of which have thick, tough leaves and stems.

Behavior

The behavior of Homalocephale and other pachycephalosaurs remains a mystery in paleontological circles, particularly concerning the function of the thickened pachycephalosaur skulls. Leading theories suggest that the skulls were a result of sexual selection due to their function in mate competition or attracting breeding partners.

Paleontologists initially thought that the animals may have used their heads for head-butting competitions, similar to behavior observed in big horn sheep (O. canadensis). Paleontologists John R. Horner and Mark Goodwin evaluated the skulls of several pachycephalosaur species and argued that the internal structure of the dome skulls are inconsistent with that expected from a structure designed to withstand direct impact. Goodwin and Horner suggested that the unique skull structures were used for visual displays and species recognition, rather than head-butting. However, later studies showed some evidence that head-butting was not only possible but present.

Other paleontologists have suggested that pachycephalosaurs engaged in flank-butting, using their skulls to strike competitors on the rear flanks, a behavior seen in some hoofed mammals during mating competitions. The flank-butting hypothesis might also explain the relatively wide and thick pelvic bones in pachycephalosaurs, which may have evolved to protect their internal organs from injury during flanking competitions.

Habitat and Other Life Forms

Late Cretaceous Mongolia was a diverse habitat, with many large arid zones blending into dry forest and pockets of tropical habitat. Gymnosperms and ferns were the primary vegetation types, though angiosperms, or flowering plants, were beginning to spread throughout the environment.

Homalocephale was not the only pachycephalosaur in Mongolia, sharing its habitat with the smaller Goyocephale and the larger Prenocephale, which had a domed skull similar to pachycephalosaurs in North America. There were also ankylosaurs among the native herbivore fauna. Both Talarurus and Tarchia lived alongside Homalocephale in the region. The largest herbivores in Mongolia were the segnosaurs, like Segnosaurus and Therizinosaurus, large herbivores with powerful claws that appear to have evolved in Asia.

A variety of predatory dinosaurs lived in Mongolia, most notably the dromaeosaurs or “raptors,” fast moving predators that may have sometimes hunted in packs. Velociraptor and Adasaurus were two of the dromaeosaurs that lived in Mongolia during this period. There were also a number of “ostrich mimic” dinosaurs, which may have been omnivores, including Gallimimus and the large Anserimimus.

Research

The first remains of Homalocephale were discovered in 1974 by Halszka Osmólska and Teresa Maryanska from the Nemegt Formation in Mongolia. The specimen is only known from fragmentary fossils. The skull of Homalocephale is unusual compared to other pachycephalosaurs and, in 2010, paleontologist Nick Longrich published a paper suggesting that flat-headed pachycephalosaurs like Homalocephale may be immature specimens of other “species” displaying domed skulls. If this theory is correct, Homalocephale might be an immature version of the genus Prenocephale, also known from Mongolian remains. However, later studies, including ones published in 2011 and 2017 by researcher David C. Evans and others, provided evidence reaffirming Homalocephale as its own genus.

Bibliography

Briggs, Derek E. G., and Peter R. Crowther. Paleobiology II. Wiley-Blackwell, 2001.

Carpenter, Kenneth. Eggs, Nests and Baby Dinosaurs: A Look at Dinosaur Reproduction. Indiana UP, 1999.

Chinsamay-Turan, Anusuya. Microstructure of Dinosaur Bone: Deciphering Biology with Fine-Scale Techniques. Johns Hopkins UP, 2005.

Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. 3rd ed., Cambridge UP, 2016.

Fastovsky, David E., and David B. Weishampel. Evolution and Extinction of the Dinosaurs. 2nd ed., Cambridge UP, 2007.

Gangloff, Roland A., Fiorillo, Anthony R., and David W. Norton, “The First Pachycephalosaurine (Dinosauria) from the Paleo-Arctic of Alaska and its Paleogeographic Implications.” Journal of Paleontology, vol. 79, 2005, pp. 997–1001.

Goodwin, Mark B., and John R. Horner, “Cranial Histology of Pachycephalosaurs (Ornithischia: Marginocephalia) Reveals Transitory Structures Inconsistent With Head-Butting Behavior.” Paleobiology, vol. 30, 2004, pp. 253–67.

Hamilton, Jason. "Homalocephale." ScienceViews.com, 2010, scienceviews.com/dinosaurs/homalocephale.html. Accessed 28 Apr. 2020.

"Homalocephale." Natural History Museum, www.nhm.ac.uk/discover/dino-directory/homalocephale.html. Accessed 28 Apr. 2020.

Horner, Jack, and James Gorman. How to Build a Dinosaur: Extinction Doesn’t Have to Be Forever. Dutton, 2009.

Horner, John R., and Edwin Dobb. Dinosaur Lives: Unearthing an Evolutionary Saga. Mariner Books, 1998.

Lucas, Spencer G. Dinosaurs: The Textbook. 6th ed., Columbia UP, 2016.

Mangels, John. "Insights from a Hard-Headed Dinosaur: Pachycephalosaur Skulls Shed Light on Growth, Relationships." Cleveland Museum of Natural History, Dec. 2017, www.cmnh.org/science-news/blog/december-2017-(1)/insights-from-a-hard-headed-dinosaur. Accessed 28 Apr. 2020.

Martin, Anthony J. Introduction to the Study of Dinosaurs. 2nd ed., Blackwell, 2006.

Pisani, D., Yates, A. M., Langer, M. C., and M. J. Benton, “A Genus-Level Supertree of the Dinosauria.” Proceedings of the Royal Society of Biology, vol. 269, 2002, pp. 915–21.

Speer, Brian, et al. "Marginocephalia." University of California Museum of Paleontology, ucmp.berkeley.edu/taxa/verts/dinosauria/marginocephalia.php. Accessed 28 Apr. 2020.

Weishampel, David B., et al., editors. The Dinosauria. 3rd. ed., U of California P, 2007