Hypsilophodon

Kingdom: Animalia

Phylum: Chordata

• Class: Reptilia
• Order: Ornithischia
• Family: Hypsilophodontidae
• Genus: Hypsilophodon
• Species: Hypsilophodon foxii

Introduction

Hypsilophodon was a small- to medium-sized dinosaur that lived in the Early Cretaceous forests of present-day Europe. The species represents a relatively early example of a group of herbivorous dinosaurs that would dominate North America during the Late Cretaceous.

In the Triassic period, the continents were united in a single land mass, known as Pangaea. This single large supercontinent was divided into geographic regions by inland seaways, mountain ranges, and vast arid deserts. The dinosaurs evolved in the Triassic and spread across Pangaea. In the Jurassic and Cretaceous, Pangaea split into two supercontinents, Laurasia in the north and Gondwana in the south, and the dinosaurs isolated on each supercontinent evolved into a variety of unique species.

Hypsilophodon first appeared in the Early Cretaceous, though the first representatives of the family appeared in the Jurassic. Representatives of Hypsilophodon's family lived in North America, Europe, Asia, and South America, indicating that ancestral species were widespread before the break-up of Pangaea.

Classification

Scientists use two primary methods to classify organisms into different groups. Linnaean taxonomy, or “systematics,” classifies organisms hierarchically based on overall physical similarity, while an alternate system called “cladistics” places organisms into groups called “clades” that reflect the evolutionary relationships between species.

Systematic analysis places Hypsilophodon into the order Ornithischia based on the “bird-like” organization of the pelvis (however, by the beginning of the third decade of the twenty-first century, some scientists had begun to debate the traditional division into the two orders of Ornithischia and Saurischia). From there the species is placed in the suborder Cerapoda, a group largely united by features of their teeth, and from there, the species is in the infraorder Ornithopoda, or the “bird-footed” dinosaurs, which all had three-toed feet, beaks, and similar teeth. The family Hypsilophodontidae contains small to medium-sized ornithopods.

Cladistic analysis also assigns Hypsilophodon into the clade Ornithopoda, based largely on features of the animals’ teeth, including an asymmetrical arrangement of enamel on one side of the teeth such that the process of chewing continually sharpens the teeth. The ornithopods also had a hinged jaw that allowed for a more advanced chewing motion.

Anatomy

Hypsilophodon was a relatively small dinosaur, reaching approximately 2 to 2.3 meters (6.6 to 8.2 feet) in length and standing approximately 1.2 meters (3.9 feet) at the hip. Paleontologists estimate that the animal weighed between 25 and 28 kilograms (55 and 62 pounds), making it roughly the weight of a child or a medium-sized dog.

Like all members of its group, Hypsilophodon had a small, triangular head with large eyes. The animal had a small beak, and its mouth was lined with approximately thirty small, ridged teeth, especially in the cheek area, allowing the creature to chew vegetation before swallowing. Hypsilophodon was bipedal with long rear legs ending in four-toed feet. The arms were smaller, with graceful, four-fingered hands ending in small claws.

Hypsilophodon was built for speed but may also have been cryptically colored, allowing it to blend in with low-lying vegetation and hide from predators. Paleontologists are uncertain about the animal's skin covering, but it probably had fine mosaic scales, similar to those found in other ornithopod species.

Intelligence

Since studies of ancient dinosaurs rely on fossil evidence, the most reliable way to assess a dinosaur's intelligence is by measuring the encephalization quotient (EQ). To assess the EQ, scientists compare an animal's brain size or weight, and body size, or weight, to that of similarly sized animals. The resultant ratio provides a value, ranging from zero to as high as 8 on some scales. Dinosaurs assessed as of the beginning of the third decade of the twenty-first century ranged between 0.1 and 5.8, with 0.1 being the lowest intelligence score.

No direct measurement of EQ rating had been taken for Hypsilophodon, though it probably had an EQ value close to the 2.0 mark, making it one of the more intelligent of the herbivorous dinosaurs. Hypsilophodon probably had similar intelligence to large reptiles, like crocodilians, or small birds.

Reproduction & Population

Paleontologists have discovered Hypsilophodon nests containing eggs organized into neatly arranged patterns. This discovery led paleontologists to speculate that the animals guarded and tended their eggs during incubation, possibly sitting on their nests to incubate the developing eggs. It is unknown whether Hypsilophodon parents remained with their young after hatching to provide continued parental care.

Based on the prevalence of Hypsilophodon fossils, paleontologists hypothesize that the species was relatively common and may have lived in dense populations. Size differences between skeletons of Hypsilophodon may represent fossils of different sexes, leading some to speculate that the species may have displayed sexual dimorphism.

Diet

Paleontologists generally believe that Hypsilophodon, like most ornithopods, was herbivorous, though it may occasionally have eaten small animals and/or insects. The cheek teeth of Hypsilophodon, an important feature common to all ornithopods, had evolved to be capable of chewing motion, allowing the animal to grind vegetation before passing it to the stomach, thereby speeding digestion.

Judging by the animal's size and the types of plants discovered in the area, paleontologists surmise that Hypsilophodon would likely have fed primarily on low-growing gymnosperms, like conifers and cycads, and pteridophytes, like ferns and other spore-bearing plants.

Behavior

Paleontologists once believed the Hypsilophodon was an arboreal animal, with long, slender fingers that could help it grasp branches. Paleontologists further believed, after discovering a mineralized plate along the body of the animal, that Hypsilophodon was covered in a bony sheath, providing protection from predators.

Later it was discovered that paleontologists had incorrectly assembled the skeleton and that the animal was instead evolved for a cursorial lifestyle. Paleontologists also later believed that the mineralized sheet found along the side of the animal was actually a sheet of cartilage that helped the animal control its breathing, especially during periods of intense physical exertion. Paleontologists further believed that Hypsilophodon was built for speed and endurance, relying on its long legs and other adaptations to outrun predatory animals.

Paleontologists have discovered groups of Hypsilophodon fossils, raising the possibility that the animal lived and/or traveled in groups. Some paleontologists have suggested that the animals may have lived in family groups, similar to deer or other small hoofed mammals. Alternatively, Hypsilophodon may have been a solitary animal that gathered in groups around shared food sources or a common watering hole, making it appear as though the animal was gathering in herds.

Habitat & Other Life Forms

In the Early Cretaceous, most of Europe was significantly warmer than in the modern period. Vast portions of Europe were covered in lakes, rivers, and floodplains. Gymnosperms were the predominant form of vegetation, though the first angiosperms were beginning to spread throughout the environment. In addition to dinosaurs, there were a variety of small mammals and reptiles living in Europe during this period.

The ornithopods were one of the most successful groups in Laurasia throughout the Cretaceous. The ornithopod Iguanodon was one of the most common dinosaurs of the period, existing throughout Europe. Smaller dinosaurs like Valdosaurus and Vectisaurus were also fairly common in this environment. There were a few larger sauropods in England, like Pelorosaurus and the stegosaur Craterosaurus.

There were also a variety of predatory dinosaurs living in Early Cretaceous England, including the spinosaurid dinosaur Baryonyx and several large theropod predators, relatives of the famous Tyrannosaurus in North America, which included the 7-meter long Neovenator. England was also home to a variety of flying reptiles, called pterosaurs, including Caulkicephalus.

Research

The first Hypsilophodon skeleton was discovered in 1849, but was wrongly classified as a young Iguanodon. In 1869, paleontologist Thomas Huxley placed the 1849 specimen into a new genus, combined with recently uncovered remains containing the animal's distinctive teeth. Since that time, more than twenty full and partial skeletons have been uncovered from the Isle of Wight. Paleontologists Richard Butler and Peter Galton published an extensive study of Hypsilophodon remains in 2008, in which they proposed that the animal was specialized for running. Scientists continued to study the Isle of Wight and the numerous fossils discovered there, which included a focus on and debate around what had caused the deaths of the Hypsilophodon found, particularly in larger numbers, in those areas. In a paper published in 2017, researchers described their hypthothesis regarding this matter based on further study. They suggested that, rather than becoming trapped in quicksand as had traditionally been theorized, the Hypsilophodon, the skeletons of which were spread out over a diverse area and largely intact, had likely been killed and buried during a sequence of flooding events and changes in landscape.

A study published in 2024 explained and examined the findings of a new species of the family Hypsilophodontidae, Vectidromeus insularis. The study revealed previously unrecognized variations in Hypsilophodontid morphology, specifically in dental and limb structures. This new genus, Vectidromeus, was named to reflect these structural and biological differences from Hypsilophodon foxii. These findings draw on previous research and beliefs that Hypsilophodontids were widespread both in location and throughout various historical time periods.

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.

Coram, Robert A., et al. "A Cretaceous Calamity? The Hypsilophodon Bed of the Isle of Wight, Southern England." Geology Today, vol. 33, no. 2, 2017, pp. 66–70.

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.

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.

Hulke, J. W. “An Attempt at a Complete Osteology of Hypsilophodon Foxii, a British Wealden Dinosaur.” Proceedings of the Royal Society, vol. 33, 1881, p. 276.

Longrich, Nicholas R., et al. "Vectidromeus Insularis, a New Hypsilophodontid Dinosaur from the Lower Cretaceous Wessex Formation of the Isle of Wight, England." Cretaceous Research, vol. 154, 2024. ScienceDirect, doi.org/10.1016/j.cretres.2023.105707. Accessed 10 Oct. 2024.

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

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

“The Ornithischian Dinosaurs.” UC Museum of Paleontology, U of California, Berkeley, ucmp.berkeley.edu/diapsids/ornithischia/ornithischia.html. Accessed 10 Oct. 2024.

Seebacher, Frank. “Dinosaur Body Temperatures: the Occurrence of Endothermy and Ectothermy.” Paleobiology, vol. 29, 2003, pp. 105–22.

Varricchio, David J., et al. “First Trace and Body Fossil Evidence of a Burrowing, Denning Dinosaur.” Proceedings of the Royal Society of Biology, vol. 274, 2007, pp. 1361–68.

Weishampel, David B., et al., editors. The Dinosauria. 2nd ed., U of California P, 2004.

Woodward, Holly N., et al. "The Bone Microstructure of Polar “Hypsilophodontid” Dinosaurs from Victoria, Australia." Scientific Reports, vol. 8, 2018. doi.org/10.1038/s41598-018-19362-6. Accessed 10 Oct. 2024.