Bactrosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Hadrosauridae

Genus:Bactrosaurus

Species:Bactrosaurus johnsoni

Introduction

First unearthed in a region known as Inner Mongolia in China, Bactrosaurus johnsoni belongs to the duck-billed family of dinosaurs known as the hadrosaurs (“bulky lizards”). The hadrosaurs are famed for their unique anatomy, specifically their duck-billed skulls topped with bold crests. Bactrosaurus was among the earliest hadrosaur fossils found, but its family has provided some of the most numerous fossils in the history of paleontology. Believed to have been herd animals, many hadrosaur fossils, including Bactrosaurus, have been found in groups, often with full skeletons and even skin intact. Bactrosaurus and its relatives have yielded important discoveries regarding dinosaur reproduction and socialization.

Bactrosaurus and its Asian relatives also bear close resemblance to hadrosaur fossils found in North America, which supports the popular theory that two continents were connected during the Late Cretaceous.

Classification

The classification system commonly used for dinosaurs and other animals is the hierarchical Linnaean system, developed by Carl Linnaeus in 1735. This system comprises different tiers of classification (class, order, genus, and species) based on shared characteristics within each grouping. Species, the lowest tier, is based on the most specific shared traits. Later, the biologist Ernst Haeckel added the phylum tier, which linked animals according to common ancestry.

Bactrosaurus is classified based on the findings of six partial skeletons. Using the Linnaean system, these skeletons have yielded ample evidence to place Bactrosaurus in the Ornithischia order of dinosaurs, which comprises those dinosaurs who more closely resemble birds than lizards in skeletal structure. Specifically, dinosaurs in this order had a pelvic (or hip) structure in which the bones point down and back toward the tail rather than forward. They also tended to possess bird-like feet and beak-shaped mouths with rows of teeth set higher in their cheeks and designed for plant-eating.

The Ornithischia order breaks down into several dinosaur families according to more specific physical characteristics. Bactrosaurus is assigned to the Hadrosauridae family because of its flat duck-billed skull, toothless beak, and abundant teeth set back in its cheek, as well as its bipedal stance, four-fingered hands, and notable cranial crest. Hadrosaurids may be broken down into the subfamilies of lambeosaurines and saurolophines, depending on the nature of their crest. Though the Bactrosaurus fossils lack a full crest, the dinosaur is believed to belong to the lambeosaurines, which possessed hollow crests atop their skulls.

The hadrosaurid family is a well-populated family, and many thousands of fossils have been found, evidencing at least two dozen genera within the family. These dinosaurs lived primarily in Asia and North America, but also in South America and Europe. Bactrosaurus hails from the Asian branch of the family, and is named for the club-shaped tips of its spinal vertebrae. “Bactro” refers to the word “baktron,” meaning “club,” while the species name, “johnsoni,” derives from the quarry in which the Bactrosaurus fossils were found, the Johnson Quarry in the Iren Dabasu Formation in China.

Scientists may also use another classification system, called phylogenetic or cladistics. This system classifies animals in family trees, called cladograms, grouped by common ancestry. Each split in the tree creates a new group based on shared physical traits. Within this system, Hadrosauridae comprise one clade, or branch, on the dinosaur family tree. This means that all hadrosaurs, including Bactrosaurus, share a common ancestor within the larger family of ornithopods. The Hadrosauridae clade branches into smaller, more specific groups, of which Bactrosaurus belongs to the Lambeosaurinae subclade.

Anatomy

Bactrosaurus belongs to the large family of hadrosaurs, which are known for the distinctive duck-billed skulls and bird-like hips and feet. Bactrosaurus blends elements of the two sub-families of the hadrosaurs. It was primarily bipedal, though it could walk on four feet, using both its hind legs and forearms. Its skull has a characteristically flat, beaked shape with a cranial ridge that indicates it likely had the crest of many of the hadrosaurs.

Its teeth were set back in its cheeks, rather than in the beak, and it featured small, plant-chewing teeth stacked as many as three deep. Bactrosaurus, though it possessed the bird-like feet, hands and pelvis of the ornithischia, appears to have had a sturdier, fuller torso than many of its relations. Its name makes note of another unique feature, the club-shaped spines along its back, which may have supported a sail. Most hadrosaurs had hundreds of teeth (up to 2,000), but Bactrosaurus likely had fewer than its successors. All hadrosaur teeth were self-replenishing so that when one set was worn down or broken, a new set rose to replace it.

Scientific tradition has long held that dinosaurs were ectothermic, or cold-blooded, as most modern reptiles are. Recent studies, however, challenge this conclusion. The bipedal nature of many dinosaurs, including Bactrosaurus, suggests that dinosaurs were prone to more activity and motion than their reptilian ancestors and suggests that some may have been endothermic, or warm-blooded. Some scientists suggest that dinosaurs may have been neither warm- nor cold-blooded, but something in between.

Intelligence

Slower-moving herbivores tend to fall lower on the EQ scale than their faster, carnivorous, predatory counterparts. However, the hadrosaurs as a group (as well as other ornithopods) appears to have had larger brain-to-body mass ratios and keeners senses, placing them higher on the intelligence scale than most other herbivores. They likely ranked between 0.8 and 1.5.

Reproduction and Population

Though specific demographic information for Bactrosaurus is limited to the six partial skeletons found in China, the abundance of fossil evidence for the hadrosaurid family indicates that the family and its distinct species likely flourished. Large fossil fields found in North America and Asia suggest that hadrosaurs traveled in herds that numbered in the hundreds, and scientists suspect that this likely was true of Bactrosaurus as well.

Existing evidence suggests that most dinosaurs, including hadrosaurs and Bactrosaurus, were oviparous. This means that they reproduced by laying eggs. In fact, the fossil remains of hadrosaurs have contributed a great deal to the study of dinosaur reproduction and nesting. Hadrosaur nesting sites have yielded important egg finds, demonstrating not only the oviparous nature of some (if not all) dinosaurs, but also the nesting tendencies of certain genera.

Group nesting sites suggest that herds laid and watched over eggs together, and shell and skeletal evidence reveals that hatchlings likely remained in the nest for a period of time and were fed by their parents or other adult hadrosaurs. Some geographic evidence suggests that hadrosaurs traveled to specific nesting sites to lay and tend their eggs, and that these sites were used by cycling groups of parents that returned to the same ground each year.

Diet

As evidenced by an abundance of flat and spoon-shaped teeth, Bactrosaurus was undoubtedly an herbivore. In fact, the large number of hadrosaurs and herding behavior indicate that Bactrosaurus and its relatives were very efficient and successful plant-eaters. Located at the back of mouth, in the cheek, rather than in the forefront of the beak and designed for grinding, Bactrosaurus teeth likely chewed up all manner of foliage, including branches, seeds, seed pods, and needles as well as leaves. The skeletons of most hadrosaurs reveal that the family likely grazed on low-growing plant life rather than foraging for high-growing vegetation in trees. Like many herbivores, Bactrosaurus probably spent a good deal of its time foraging for food. As Bactrosaurus lived in the Late Cretaceous when plant life was diversifying, they fed both on gymnosperms, such as conifers, and angiosperms, the flowering plants that flourished in the Late Cretaceous.

Behavior

The abundance of fossil evidence found for Bactrosaurus and the larger family of hadrosaurs suggests that these dinosaurs were highly socialized herd animals. In the case of Bactrosaurus, six partial skeletons ranging from juvenile to adult were found together, and many other hadrosaur finds have comprised the remains of multiple specimens. These fossils and their widespread geographic locations indicate that hadrosaurs such as Bactrosaurus traveled in large groups and migrated across the continents. For this reason, hadrosaurs from an earlier time period are often called the “sheep” of the Mesozoic Era.

These North American relatives of the Bactrosaurus are believed to have traveled in groups that numbered in the hundreds. Most migratory dinosaurs, such as hadrosaurs, likely moved from place to place in search of food and may have traveled with other plant-eating, herding species. The large nasal passages of Bactrosaurus and the suspected hollow tubing of the dinosaur have led some scientists to believe that they—and hadrosaurs in general—had the ability to make a wide range of unique sounds, ranging from high to low pitches. These sounds may have been used to identify herd members, to communicate, and to attract mates.

The anatomy of Bactrosaurus suggests that the dinosaur was bipedal but could have walked on all fours as well. The dinosaur's grazing tendencies likely kept it on all fours while eating or searching for food, but longer distance travel or the need for escape would have sprung Bactrosaurus into bipedal mode, lending it more speed.

Habitat and Other Life Forms

During the latest Triassic and Early Jurassic periods (between 230 and 174 million years ago), the great supercontinent of Pangaea had split apart into two landmasses, Laurasia and Gondwana. Eventually, during the Late Jurassic, further division occurred, and four distinct biogeographic provinces began to develop: West Laurasia (North America); West Gondwana (Africa and South America); Central Laurasia (Europe); and East Laurasia (Asia), plus East Gondwana (Antarctica, Madagascar, India, and Australia).

In the Cretaceous, though the climate remained warmer than today, global shifts in climate and sea levels were changing the land and the life that lived there. Bactrosaurus and its hadrosaurid relations ranged across the Northern Hemisphere, largely in North America and Asia, but the migratory patterns of their ancestors had taken them as far as South America, Europe, and Antarctica. No fossil remains of hadrosaurs have been found in Africa or Australia as of yet.

As with other animal and plant life, the Late Cretaceous brought on a great period of diversification in the hadrosaur family. Evidence suggests Bactrosaurus and its relations also benefited from the sudden flourishing of flowering plants across the globe. Their success at foraging on angiosperms and other plant life may have contributed to the decline in some other herbivore species. They likely roamed the grasslands and woodlands of their environment.

Just as herbivores like Bactrosaurus flourished in the Late Cretaceous, so too did its predators, carnivorous dinosaurs such as members of the genus Tarbosaurus.

Research

In 1933, American paleontologist Charles W. Gilmore made the first description of Bactrosaurus fossils, based on six incomplete skeletons found in an ancient rock formation known as the Iren Dabasu Formation in what is now the Gobi Desert of China. These known fossils include the dinosaur's spine and vertebrae, arms, legs, pelvic bone, and skull. The partial skull reveals evidence of the roots of a cranial crest, as is found in other hadrosaurs, and much of the rest of Bactrosaurus has been reconstructed based on study of its close relatives. Bactrosaurus was among the earliest finds of the hadrosaur, or duck-billed, family of dinosaurs. Earlier finds include Edmontosaurus regalis, a successor hadrosaur (that lived between 70 and 65 million years ago) found in 1917 in the Horseshoe Canyon Formation of Alberta, Canada, and Lambeosaurus lambei, a contemporary of Bactrosaurus, found a decade before in the Dinosaur Park Formation in Southern Alberta.

Thus far, the closest relative of Bactrosaurus appears to have been a very recent find in Asia, Levnesovia transoxiana, which was unearthed in Uzbekistan in 2009 by Russian paleontologist Lev Nesov. Levnesovia transoxiana appears to be more ancient than Bactrosaurus and also differs in that it possesses a tall sagittal crest, or ridge, atop the skull and lacks the distinctive club-shaped tips on its vertebrae that characterize Bactrosaurus.

Other more recent research has led to the discovery of Charonosaurus jiayinensis, another successor found in 2000 in the Yuliangze Formation of China; the approximate contemporary Shantungosaurus giganteus, found in 1973 in the Wangshi Group of China; and Gilmoreosaurus mongoliensis, another close relative found in the same geologic formation in 1979. Gilmoreosaurus mongoliensis is believed to be possibly the earliest iteration of a hadrosaurid fossil, dating back as far as 99 million years ago.

In 2016, the US returned an almost complete Bactrosaurus skeleton to Mongolia that had been previously smuggled out of the country.

Bibliography

Carpenter, Kenneth. Eggs, Nests, and Baby Dinosaurs. Indiana UP, 1999.

Cox, Barry, et al. Simon & Schuster Encyclopedia of Dinosaurs & Prehistoric Creatures. Simon & Schuster, 1999.

"Dinobase." University of Bristol, dinobase.gly.bris.ac.uk. Accessed 15 Apr. 2020.

Fedde, Corey. "Bone Smugglers: US Returns Stolen Dinosaur Fossils to Mongolia, Again." The Christian Science Monitor, 7 Apr. 2016, www.csmonitor.com/Business/2016/0407/Bone-smugglers-US-returns-stolen-dinosaur-fossils-to-Mongolia-again. Accessed 15 Apr. 2020.

Jerison, H. J. “Brain Evolution and Dinosaur Brains.” American Naturalist, vol. 103, 1969, pp. 575–88.

Lund, Eric K., and Terry A. Gates. “A Historical and Biogeographical Examination of Hadrosaurian Dinosaurs.” New Mexico Museum of Natural History and Science Bulletin, vol. 35, 2006.

"The Paleontology Portal." UC Museum of Paleontology, University of California, Berkeley, paleoportal.org/. Accessed 15 Apr. 2020.

Richardson, Hazel. Dinosaurs and Prehistoric Life. DK Publishing, 2003.

Weishampel, David B., et al, editors. Dinosauria. University of California Press, 2007.

Wright, Rebekah A. "Were Dinosaurs Bird Brains." University of Arizona Geosciences, 2002, http://www.geo.arizona.edu/~rwright/abs2002.html. Accessed 15 Mar 2014.

Zimmer, Carl. “Dinosaurs.” Discover, vol. 26, no. 4, 2005, p. 32–39. MasterFILE Complete, search.ebscohost.com/login.aspx?direct=true&db=f6h&AN=16382092&site=eds-live. Accessed 15 Apr. 2020.