Lesothosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Lesothosauridae

Genus:Lesothosaurus

Species:Lesothosaurus diagnosticus

Introduction

One of the geologically earliest species of dinosaur discovered, Lesothosaurus was a small, bipedal herbivore or omnivore that lived during the Early Jurassic period (201–174 million years ago) in what is now South Africa. Given its age, Lesothosaurus provides paleontologists with a detailed look at a very primitive form of dinosaur and a deeper understanding of the evolution of dinosaurs in general. Although Lesothosaurus died out by the Middle Jurassic (174–163 million years ago), it is believed to have been a precursor species for many other dinosaurs that thrived until the end of the age of dinosaurs 66 million years ago.

It is thought that Lesothosaurus resembled an upright lizard, standing and running on its powerful and elongated hind limbs. The ability to stand on its back legs allowed it to watch for predators and flee if attacked.

Classification

Scientists use two primary methods to classify organisms. Linnaean taxonomy groups organisms according to overall physical similarity, while an alternate method called cladistics attempts to create categories that better reflect the evolutionary relationships between species. The phylogenetic system of classification groups organisms together based on their evolutionary relationships, which are determined by shared physical traits. Each grouping, or clade, consists of an original ancestor and all of its descendents, the original ancestor also being the most recent ancestor of its descendents in the grouping.

Dinosaur species have long been generally classified, based on their hip structure, as either Ornithischia (bird-hipped) or Saurischia (lizard-hipped). (While in the twenty-first century some researchers began to challenge this basic division, it remains commonly in use.) Despite the name, it is from the “lizard-hipped” dinosaurs that modern birds derived. While this may seem illogical, scientifically it is known as convergent evolution, whereby the backward pointing pubis seen in ornithischian dinosaurs and birds actually evolved separately.

Ornithischia is a well-established order of dinosaurs, often sharing anatomical and morphological characteristics. The better-known Ornithischia species include the duck-billed hadrosaurs, the armored and plated ankylosaurs and stegosaurs, the iguanodonids, the tusked heterodontosaurs, the hypsilophodontids, and the thick-skulled pachycephalosaurs. Lesothosaurus is one of the earliest dinosaurs ever discovered and one of the most primitive known ornithischians; therefore exact classification has been debated. It is considered likely that it is the sister taxa from which the Ornithischia order was derived.

Lesothosaurus was a small and very agile species, built for speed and predator avoidance. Initial cladistic classification grouped Lesothosaurus in Fabrosauridae, along with the other primitive Ornithopoda representatives including Fabrosaurus, Scutellosaurus , Trimucrodon, Echinodon, Nanosaurus, and Xiaosaurus. Later cladistic analysis of this grouping, however, identified Fabrosauridae as a monophyletic taxon and it was dissolved. Following this, Lesothosaurus was acknowledged as a basal ornithischian rather than as an ornithopod. However, the phylogenetic positions of dinosaur species such as Lesothosaurus, Pisanosaurus, Fabrosaurus, and Technosaurus have remained controversial.

Anatomy

Unlike the more commonly seen quadrupedal form seen in herbivorous dinosaurs, Lesothosaurus was a bird-hipped cursorial biped. Although Lesothosaurus was a small dinosaur, its legs were strong and its thin hollow bones were built for agility and speed. Its powerful back legs ended in four clawed feet and a small claw on the shin, while its small forearms ended in four clawed hands and a modified stub. Some paleontologists believe that the fifth digit on their hands may have been partially opposable and could have aided grasping movements and feeding.

Despite its likely omnivorous lifestyle, Lesothosaurus more closely resembled the theropod carnivores with it bipedal stance and long tail. Certainly, its small stature bore little resemblance to the massive quadrupedal herbivores that roamed the earth millions of years later. Despite this, characteristics such as its jaw, bird-hipped pubis, and dentition provide evolutionary links to the later ornithischian quadrupeds.

Lesothosaurus relied on speed rather than the defensive structures seen in some other ornithischian species, such as the large tusks of Heterodontosaurus, the dermal plates of Stegosaurus, or the armor of Ankylosaurus. Lesothosaurus had a small skull filled with a number of hollow spaces to maximize its light frame; it also had very large eyes, which would have helped with predator vigilance. Similar to many ornithischians, it possessed a small beaked mouth filled with arrow-shaped teeth capable of interlocking and grinding up foliage.

Intelligence

Scientists have used encephalization quotient (EQ), based on brain and body size, to estimate animal intelligence. Although Lesothosaurus was a very primitive dinosaur, its EQ likely would be between 0.9 and 1.5, placing it in the middle range in terms of intelligence. This would have served it well in terms of avoiding predation.

Reproduction and Population

As the fossil record of species such as Lesothosaurus remains incomplete, it is difficult to estimate population numbers with any degree of accuracy. However, from fossil remains collected so far, the distribution of basal ornithischians such as Lesothosaurus diagnosticus, Pisanosaurus mertii, and Technosaurus smalli appears limited to the Early Jurassic period in South Africa, southern South America, and western North America.

Incomplete records also make it difficult to designate egg fossils to particular species. Despite this, current understanding of the reproductive behavior of dinosaurs indicates that most species were oviparous and laid a number of eggs per clutch. This includes Lesothosaurus and its close relatives, although little evidence has been uncovered supporting highly-developed or sophisticated parental care among most dinosaur species. However, fossil finds from dinosaur nests, communal nesting grounds, and juvenile morphology have hinted at the possibility of advanced parenting behavior.

Diet

Lesothosaurus was originally believed to have been purely herbivorous. It lived in the hot and dry region of what is now Southern Africa some 200 million years ago, and within this semi-arid environment, vegetation consisted of many gymnosperms, or plants that bear naked seeds such as palm-like cycads, ferns, and conifers. Lesothosaurus was an active forager and, given its short height, was likely restricted to feeding on low-lying vegetation such as ferns, horsetails, cycads, and mosses.

The dentition and jaw structure of Lesothosaurus suggest that it possessed an orthal (vertical) pulping feeding strategy, in which vegetation was chewed into a pulp using a vertical grinding movement. Some researchers claim that the keratinous, tooth-lined beak of Lesothosaurus would have been useful for selectively feeding on easily digested soft plant material, such as fruit and new shoots, rather than the more fibrous parts of plants.

Although Lesothosaurus's sharply pointed and grooved teeth were well suited to a diet of vegetation, the similarity of its teeth to that of iguanines and prosauropods also suggests that basal ornithischians may have been capable of omnivorous behavior. This is also supported by the absence of certain anatomical adaptations that many other plant-eating dinosaurs possess, and the fact that a more flexible omnivorous diet would have suited the high energy requirements of this fast-running species. Analysis of dental wear further led many scientists to shift their view of Lesothosaurus from an herbivore to an omnivore.

Behavior

Some paleontologists have hypothesized that Lesothosaurus and similar species may have resembled modern-day gazelles in terms of behavior and filled a similar ecological niche. Lesothosaurus probably used its upright stance and very large eyes as an adaptation for predator vigilance and avoidance, and it may have formed herds for a measure of protection to offspring.

From remains found so far, Lesothosaurus possessed no significant defensive anatomical structures. It thus would have relied on its ability to out-pace predators through agility, vigilance, and speed.

Habitat and Other Life Forms

At one time, all land masses on earth were joined together in one supercontinent called Pangaea. By the beginning of the Jurassic period some 201 million years ago, this super landmass began to develop rifts; and by the end of the Jurassic period, it had separated into the two distinct supercontinents of Gondwana and Laurasia.

The prevailing climate during the Jurassic period was warm and moist, although areas such as Southern Africa were dominated by semi-arid conditions. It was a time before the emergence of angiosperms, or flowering vascular seed plants, and when gymnosperms flourished across the terrestrial landscape. The continued faunal exchange between most areas of the supercontinent also helps to explain the distribution of basal ornithischians such as Lesothosaurus diagnosticus, Pisanosaurus mertii, and Technosaurus smalli in South Africa, southern South America, and western North America.

Although some paleontologists suggest that Lesothosaurus may have existed as early as the Late Triassic (235–201 million years ago), specimens unearthed so far have been located in the Upper Elliot Formation of Mafeteng, a 200-million-year-old geological formation with a rich fossil record of vertebrate fauna. Lesothosaurus shared its semi-arid environment with herbivorous dinosaurs such as Massospondylus, Thecodontosaurus, Lanasaurus, and Heterodontosaurus. A number of carnivorous dinosaurs have been found in the same geological formation, including Megapnosaurus rhodesiensis and Dracovenator regenti, as well as pterosaurs and rauisuchians, or “Rau's crocodiles,” distant cousins of modern crocodiles.

Research

Lesothosaurus diagnosticus was first discovered in Lesotho in southern Africa in 1978, and was named by British paleontologist Peter Galton. Unearthed in the Upper Elliot Formation of Mafeteng, an old geological formation roughly dating back 200 million years, Lesothosaurus is considered one of the most primitive ornithischians ever discovered. Initially described based on few remains, it is now known from at least four skulls and associated partial skeletal remains.

One contentious area of debate in relation to Lesothosaurus is the classification of its very close relative, Fabrosaurus. As this species has been classified based on only partial remains, some paleontologists believe that Fabrosaurus should actually be classified as Lesothosaurus. Other poorly described dinosaurs with similar features, such as Xiaosaurus and Stormbergia dangershoeki, have also been challenged as possibly representing Lesothosaurus in varying stages of growth.

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