Small Meat-Eating Dinosaur Points the way to Dinosaur Diversification

The discovery of a small, meat-eating theropod dinosaur, fossilised in an almost perfect bone preservation state is helping scientists to piece together the evolution of the main types of dinosaur.  This new Triassic dinosaur was discovered during excavations at the famous Ghost Ranch location in New Mexico, (USA).

Theropod Dinosaur

The rocky ridges at Ghost Ranch, consisting of a sequence of Triassic sandstones, is one of the most famous vertebrate fossil sites in the world.  This location has helped provide scientists with an insight into the dawn of the dinosaurs and the flora and fauna that shared the environment with the first types of dinosaur.  Fossil discoveries in and around the Ghost Ranch site formed the basis of the “Walking with Dinosaurs” episode, “New Blood”, the first episode in the ground-breaking BBC TV series.

The Ghost Ranch site was first excavated by George Whitaker and Edwin H. Colbert from the American Museum of Natural History in 1947, their work helped to unearth more than a thousand specimens of the small, sleek dinosaur predator Coelophysis.  Now it looks like Ghost Ranch has provided evidence of another type of Triassic dinosaur predator.

This new genus of dinosaur, named Tawa hallae, after the Native American Hopi tribe’s name for their sun god – Tawa, is estimated to have measured about 2 metres long.  This bipedal animal shows characteristics of the typical theropod lineage that was to produce huge carnivores such as Giganotosaurus and T. rex.  A swift and agile hunter, T. hallae had strong legs, relatively short forelimbs but with strong grasping hands and a jaw lined with curved, sharp teeth.

A Model of the Recently Described Tawa hallae

The Schleich Tawa dinosaur model (T. hallae).

The image shows a replica introduced into the Schleich dinosaur model range.  To view the entire range of Schleich prehistoric animals available from Everything Dinosaur: Schleich Dinosaur and Prehistoric Animal Models.

University of Texas Research

The University of Texas based research team responsible for the discovery and analysis of the fossil bones suggest that this dinosaur provides helpful information on the evolution and diversification of the Dinosauria.  During this time in the Triassic, approximately 215 million years ago, the super-continent Pangaea was at its largest, representing a single landmass that included Australia, Antarctica, the Americas, India and Africa.  A dinosaur would have been able to walk from the South Pole to the North Pole if it had wanted to.

The research team’s paper, published in the scientific journal “Science” suggests that shortly after the first dinosaurs evolved in South America they split into their three main groups the saurischian sauropodomorphs and the theropods, along with the bird-hipped ornithischians.

Dr Sterling Nesbitt, a researcher at the University of Texas in Austin, lead author on the paper, explained that a number of U.S. based scientists were involved in this particular study.  The Tawa hallae bone bed first being excavated in 2004, with more fossil evidence coming to light during a more extensive excavation two years later.  Although a number of theropod skeletons are known from this location, most notably the Coelophysoidea dinosaur Coelophysis; many of the light, hollow fossil bones are crushed.  However, this new discovery is part of a very well-preserved sequence of strata that has yielded articulated specimens which are up to 95% complete.

Commentating on this new dinosaur, Dr Nesbitt stated:

When we saw [the specimen] our jaws dropped.  A lot of these theropods have really hollow bones, so when they get preserved they get really crunched,  but these were in almost perfect condition.”

This new dinosaur genus, is helping to fill a gap in the fossil record.  The skeleton has been dated to around 215 million years ago (mid Norian faunal stage) and puts it only about 15-17 million years later than the very first dinosaurs.  This discovery adds to the evidence that the Dinosauria diversified into its three main component groups shortly after the dinosaurs first evolved.

The finding provides strong evidence for an existing hypothesis that dinosaurs originated in what is now South America, and very soon diverged into their main types and then spread throughout the whole of Pangaea.

Dr David Martill, a palaeontologist from the University of Portsmouth (UK), who was not involved in this study, said this was a “very exciting discovery”.

The Portsmouth University palaeontologist went on to add:

“This… rewrites the evolutionary tree for meat-eating dinosaurs.  This beast shows how important it is to keep going in to the field looking for fossils.  Just one lucky discovery can make such a difference to the way we perceive the evolution of dinosaurs, and any other creature for that matter.”

The discovery of this new, strong-jawed meat-eater may help explain a mystery surrounding the vast numbers of Coelophysis fossils found together.  Coelophysis was a swift-running, bipedal dinosaur with a long tail, long neck and small head.  Scientists have puzzled over why so many of these dinosaurs have been found fossilised together?

An Illustration of the Head of Coelophysis

Picture credit: Everything Dinosaur

Were these animals pack hunters and did they move around in large packs?  Today most vertebrates that move in large numbers are herbivores, scientists have remained curious why so many meat-eaters may have hunted together in the Triassic, perhaps with larger, more powerful carnivores such as Tawa hallae on the prowl it was safer to be part of a large group.  Interestingly, this type of behaviour, not seen in predatory mammals to any great extent, may be part of the explanation why the dinosaurs were able to diversify so rapidly and out compete other terrestrial reptiles that lived at the same time.

Pterosaurs – New Research Provides Support for Orientation of the Pteroid Bone

Pterosaurs or as they are more commonly known, flying reptiles, were not dinosaurs, these remarkable reptiles evolved during the early part of the Mesozoic and they were the first vertebrates to master flight.  Their evolution was all part of the Triassic diversification of the reptiles following the Permian mass extinction event that wiped out approximately 95% of all plant and animal species on the planet.

Pterosaurs

Although, many types of reptiles had evolved “wing-like” structures that enabled them to glide or parachute; it was the pterosaurs that evolved active flight, the ability to control with much more certainty the flying motion.  From early, long-tailed rhamphorhynchoids such as the black bird sized Preondactylus, the pterosaurs dominated the air until the evolution of the first birds.  The last of the pterosaurs such as the giant Quetzalcoatlus with its 12 metre wing span were the largest flying animals known in the fossil record.

An Illustration of the Huge Late Cretaceous Pterosaur Quetzalcoatlus

Picture credit: Everything Dinosaur

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However, with no similar types of animals extant today, scientists have puzzled over a number of mysteries surrounding how these animals, particularly the largest genera, actually flew.  The aerodynamics of the membrane of skin that stretched from the shoulder, to the wing-tip and down to an area near the hind limb were remarkable.  The specialised arms of pterosaurs with their elongated fourth finger supported this double-sided membrane of skin that formed the wing, but how this wing was controlled to enable flight to take place has puzzled scientists for more than 100 years.

The position of a particular bone, the pteroid and how it could have withstood the stresses of flapping flight, as well as helped to direct the shape of the wing membrane has been a cause of much of the debate amongst scientists.  Unfortunately, despite the relatively extensive fossil record of Jurassic and Cretaceous pterosaurs, the incomplete skeletons and their crushed state of preservation has led to confusion over the orientation of the pteroid bone and its role in powered flight.  What was needed to help resolve this issue (or at least add to the debate), were a number of well preserved pterosaur skeletons with wrist bones and limbs that would enable scientists to obtain a 3-dimensional impression of a flying reptile’s wing.  Despite the many superb specimens uncovered from the Santana Formation of Brazil, the fossils found to date were unable to provide conclusive evidence as to the orientation of pteroid bone.

The Pteroid Bone

The pteroid bone is a rod-like bone found exclusive in pterosaurs.  It articulated at the wrist and supported the forewing in front of the inner part of the wing spar.  The function of this bone and its orientation has been a source of contention amongst palaeontologists.  Two contrasting theories have been proposed concerning the orientation of the pteroid bone.  One group of scientists believe that it was orientated medially (towards the body), it pointed towards the body of the animal, leading to a forewing that was relatively narrow.  A second theory suggests that the pteroid bone was directed forwards, away from the body during flight, resulting in a much broader forewing.

A Pair of Pterosaurs (CollectA Deluxe/Supreme Guidraco)

A pair of Guidracos, note the articulated jaws.

Picture credit: Everything Dinosaur

Images the skeleton of a pterodactyloid with a close up of the delicate pteroid bone that projects from the wrist and supports the forewing (propatagium).

In a paper published in the scientific journal “The Proceedings of the Royal Society Biology”, the team of researchers from Bristol University and University College, Dublin (Ireland), used biomechanical analysis and the testing of models in wind tunnels to test the position and orientation of the pteroid bone.  The team discovered that the aerodynamic efficiency of the wing structure was greatly improved if the pteroid bone was positioned in an antero-ventral orientation (forwards and putting downwards).  The lift : drag ratios were dramatically improved when the bone was put in this position when compared to other orientations for this bone.  The team claim that the maximum lift generated is exceptionally high when compared to conventional aerofoils, permitting even the largest pterosaurs to take off and land without difficulty.

Commenting on the sheer size of some of the largest pterosaurs, Colin Palmer, from the University of Bristol and lead author on the paper stated.

“Based on existing fossil evidence, pterosaurs are believed to have had a wing span of up to 12 metres and a weight of between 80 and 250 kilogrammes.”

The absence of any similar sized flying animals today, makes assessments of the flying capabilities of large pterosaurs very difficult.  The wing membranes of bats are also made of skin, but how they are supported by bones is very different to that which is seen in pterosaurs.  The bat wing is supported by all the fingers except the first digit, the thumb and often extends beyond the back leg to reach the tail.  Even the largest species of bats alive today do not have wingspans approaching anything near as large as some of the pterosaurs.

Commentating on the importance of understanding the structure and performance of pterosaur wings, a spokesperson said:

“It affects the speed at which they could fly, which could tell you about the type of life they led, where they lived, and possibly even what they ate.  They are the biggest animals ever take to the skies.”

Such a light and delicate looking wing being able to provide powered flight to an animal with the dimensions of a small aircraft is a remarkable evolutionary achievement.  The pterosaurs were around for something like 150 million years and they were the first back-boned animals to achieve powered flight.  Further studies will no doubt be undertaken to explore their flight capabilities, perhaps focusing not just on the aerodynamic structures but analysing the brains and senses of these animals that enabled them to control and manipulate their huge wings.

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