Zephyrosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Parksosauridae/Thescelosauridae

Genus:Zephyrosaurus

Species:Zephyrosaurus schaffi

Introduction

Zephyrosaurus was a small, herbivorous dinosaur that lived in Early Cretaceous North America. A member of a successful group of small herbivores, Zephyrosaurus and its relatives would become the most dominant herbivores in North America by the middle of the Cretaceous period.

In the Triassic period, the continents of the earth were united in a single land mass known as Pangaea. The dinosaurs evolved and spread during this period, crossing into every continent. During the Jurassic and Early Cretaceous, Pangaea split into two land masses, Laurasia and Gondwana, in the north and south, respectively. Dinosaurs that evolved and spread during the Triassic were isolated after the breakup of Pangaea and evolved into unique lineages.

Zephyrosaurus was part of a lineage of dinosaurs that were common in the Jurassic and spread around the world before the breakup of Pangaea. Relatives of Zephyrosaurus persisted into the Early Cretaceous when the lineage was being replaced by related groups that became the dominant terrestrial herbivores.

Classification

Paleontologists use two major systems for classifying organisms. Linnaean taxonomy, also known as “systematics,” uses overall physical similarity to place organisms into different groups. By contrast, cladistics seeks to create groups of organisms based on evolutionary relationships; these groups are called “clades.” Cladistic analysis focuses on important key traits and attempts to trace the inheritance of a trait among descendants of a common ancestor.

Systematic analysis places Zephyrosaurus into the order Ornithischia, or the “bird-hipped” dinosaurs, a group with reversed pelvic bones (though the longstanding division of dinosaurs between Ornithischia and Saurischia has been challenged by some scientists). From there, Zephyrosaurus is placed into the suborders Cerapoda based on the enamel of its teeth. More specific classification is subject to dispute, but the type species of the Zephyrosaurus genus is Zephyrosaurus schaffi.

Cladistic analysis has introduced even more instability in the classification of Zephyrosaurus and similar species. Many systems placed the species in the clade Ornithopoda, a group largely marked by the development of cheek teeth for chewing. Initially, most scientists then placed Zephyrosaurus in the clade Heterodontosauridae, based largely on the presence of enlarged canine tusks; and then into the Hypsilophodontidae group. However, in the twenty-first century most paleontologists came to see the hypsilophodont clade as invalid. Some researchers suggest Zephyrosaurus and related species should not be considered part of the clade Ornithopoda at all, and research continues to seek to better understand the distribution of species. Analysis in the 2010s supported the placement of Zephyrosaurus in the Orodrominae clade, itself within the larger clade Parksosauridae (sometimes considered a synonym of Thescelosauridae), but disputes remained.

Anatomy

Zephyrosaurus was a small herbivore, ranging from 1 to 2 meters (3.2–6.5 ft) long, with a long tail and an extended neck. The animal stood on its hind legs and could reach a height of 2 meters (6.5 ft). Zephyrosaurus was sturdily built, with adult weights of up to 30 or 40 kilograms (66–88 lbs).

Because only partial fossils exist, the exact anatomy of Zephyrosaurus remains unknown. The species had masticating teeth in its cheeks as well as enlarged, tusk-like teeth. The eyes were large compared to the skull, and evidence exists that some species were able to adjust their jaws to handle large food items, similar to the jaw structure seen in some modern birds. Zephyrosaurus also had sturdy rear legs, with five-toed feet modified with blunt claws for traction. The front legs had five-fingered hands with more delicate claws. The animal's tail was long and sturdy and probably used for balance.

Some members of the Ornithopoda clade have been discovered with small protuberances of the skin that may have been protofeathers, modified scales that proceeded true feathers. Alternatively, Zephyrosaurus might have been covered with pebbled, mosaic scales, as found in skin impressions of hadrosaurs, distant ancestors that lived in the Late Cretaceous.

Intelligence

Animal intelligence can be estimated by a measurement known as encephalization quotient (EQ), based on relative size of the brain and body. Zephyrosaurus and other hypsilophodontids probably had EQ ratings in the range of 1.5 to 2.0, placing them mid-range in relation to other dinosaurs. While some paleontologists have suggested that Zephyrosaurus and other hypsilophodontids filled a similar niche to the small and mid-sized antelopes in Africa, they were likely slightly less intelligent than these species and more similar in intelligence to small birds.

Reproduction and Population

Paleontologists believe that most dinosaurs were oviparous, rather than viviparous, meaning that they reproduced by laying eggs rather than giving live birth. Nests for some ornithopods have been found with as many as twenty eggs, indicating that some species laid eggs in multiple batches.

Dinosaur nests were generally formed from dirt and other debris. A few species similar to Zephyrosaurus have been discovered in underground burrows and may also have reproduced by laying eggs in subterranean nests. As Zephyrosaurus is known from only a few incomplete specimens, paleontologists are uncertain as to the animal's population size and have been unable to determine if males and females differed in size and/or appearance. Later members of the ornithopod group are believed to have gathered into herds, but Zephyrosaurus may also have been solitary, uniting only for reproduction.

Diet

Zephyrosaurus and related species were herbivorous, as illustrated by their masticating cheek teeth, and probably fed on a wide variety of plant life available in Early Cretaceous North America, including dozens of varieties of ferns and other spore-bearing plants, as well as conifers, cycads, and other species. Some paleontologists have suggested that certain species may have been omnivorous, feeding on eggs, small reptiles, and amphibians, in addition to plant matter. Their jaws may have been able to adjust to accommodate unusually-sized food items, which may have been useful for handling large mouthfuls of plant matter or for eating small animals.

Behavior

The species once referred to as hypsilophodontids are one of the most poorly understood dinosaur groups, as indicated by the lack of consensus on their classification and relationships, and paleontologists have found insufficient evidence to create a detailed picture of their behavior. However, paleontologists believe that many species similar to Zephyrosaurus were solitary herbivores, living in small groups rather than gathering into herds. The creatures have often been compared to the small African antelopes.

Paleontologists have found the remains of a middle Cretaceous species similar to Zephyrosaurus, called Oryctodromeus, which is the first dinosaur thought to have lived in subterranean burrows. The discovery led some paleontologists to suspect that the burrowing trend may have been more widespread within the clade. Species like Zephyrosaurus may also have utilized burrows for defense or possibly for breeding. Expeditions have recovered similar burrows from Cretaceous and Jurassic fossil beds in Australia that may have belonged to burrowing dinosaurs.

Habitat and Other Life Forms

During the Early Cretaceous, North America was warmer than in the modern period, and much of the center of the continent was covered by the Western Interior Seaway, surrounded by shallow flood plains. North America was beginning to split from Europe during this period.

The rich North American forests were dominated by such gymnosperms as conifers, cycads, and ferns, as well as by their spore-bearing relatives. The flowering plants, or angiosperms, evolved during the Cretaceous and began to spread around the continents. North America was home to a wide variety of small reptiles, amphibians, and small mammals, many similar to modern species. The skies were dominated by pterosaurs, flying reptiles that lived alongside the dinosaurs in the Mesozoic.

Cretaceous North America was home to a diverse range of dinosaurs, including several species of sauropods, such as Cedarosaurus and Sonorasaurus, which were smaller relatives of the giant sauropods of the previous Jurassic period.

Although the “duck-billed” hadrosaurs had not yet evolved, ancestors such as Eolambia and iguanodonts filled a similar niche in the environment. In addition to Zephyrosaurus, other similar species lived in the environment, forming an important part of the small herbivore fauna.

Predatory dinosaurs were abundant as well, including the large dromaeosaurid Utahraptor, one of the largest predators of the period. Smaller predators like Nedcolbertia were common in the environment, too. Paleontologists have also found the remains of several large armored dinosaurs like Gastonia from Early Cretaceous fossil beds, relatives of the later ankylosaurs.

Research

Zephyrosaurus was described by paleontologist Hans-Dieter Sues in 1980, from fossils found in sediments in Montana. The species was known only from a single skull and limb fragment until additional specimens were discovered in 2002–2003.

Zephyrosaurus belongs to one of the most poorly understood dinosaur groups, which has undergone considerable revision through cladistic analysis. Though superficially related to the ornithopod groups, members of the group once known as hypsilophodontids are increasingly believed to actually represent a separate branch from a common ancestor. The twenty-first century introduction of the family or clade Parksosauridae (or Thescelosauridae) to include Zephyrosaurus—and distinct from several other branches once considered hypsilophodontids—indicates this trend. Many paleontologists also believe that the small dinosaurs in this group may have been more common than their occurrence in the fossil record suggests.

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