Endothermic or Ectothermic or Somewhere in-between New Research adds to the Debate

The controversial area of vertebrate palaeontology exploring the theory of warm-blooded dinosaurs hots up once again.

The hotly debated (no pun intended), issue of whether dinosaurs were warm-blooded (endothermic) or cold-blooded (ectothermic) continues to attract research and a new paper has been published from a team of U.S. based researchers this week that adds to the amount of data available on this controversial subject.

Warm-blooded dinosaurs?

The debate over the metabolism of dinosaurs has raged for over a hundred years, many scientists in the 19th and early 20th centuries believed that dinosaurs were cold-blooded, that is; they controlled their internal body temperatures by their behaviour.  Basking in the sun to warm up and seeking shade when they needed to cool down.  This sort of behaviour could be seen in extant species such as lizards and snakes,  (reptiles that are around today).  A number of researchers kept lizards and other small reptiles in glass laboratory tanks, altering the temperature of the environment and then monitoring how active or inactive the creatures were.  From their observations, they concluded that cold-blooded reptiles were sluggish when it was cooler and their activity was highly dependent on the surrounding temperature.  It was from studies such as these that the impression of slow, ponderous sluggish dinosaurs came about.

Was Tyrannosaurus rex Warm-Blooded?

The debate over warm-blooded dinosaurs hots up after a new scientific paper exploring endothermy within the Dinosauria is published. Picture credit: Everything Dinosaur.

Picture credit: Everything Dinosaur

Whether an animal is ectothermic (cold-blooded) or endothermic (warm-blooded) is very important as depending on what sort of metabolism an animal has will influence behaviour, activity and even potential intelligence.

So why all the fuss over cold or warm-blooded dinosaurs?

The chemical reactions that take place in an animal’s body work best at set temperatures and if these fluctuate then the body will cease to function efficiently.  Cold-blooded animals like amphibians, snakes and lizards rely on external factors such as sun or outside air temperature to warm their bodies.  When they get too hot they seek shade or shelter.  Warm-blooded animals on the other hand, convert the food they eat into energy to help keep them warm and to maintain the correct body temperature.  When warm-blooded animals get too hot, they have to cool down and various adaptations for cooling down can be seen in mammals today (endothermic creatures).  For example, we humans sweat, dogs pant and elephants wallow in mud or flap their ears to cool the blood that flows through them.

If dinosaurs were warm-blooded then creatures such as Tyrannosaurus rex would have been athletic, active animals with high metabolic rates that would have permitted them to survive in all sorts of climates, even cold, harsh ones at high latitudes such as polar environments.  Palaeontologists, have found dinosaur fossils in Antarctica and at high latitudes in the Northern Hemisphere.  Now although these parts of the world were not the frozen wastelands of today, they were much warmer in the Mesozoic, dinosaurs would still have had to endure months without daylight and freezing temperatures below 0 degrees Celsius.

This new study from scientists at the University of Washington, St Louis appears to confirm that many types of dinosaur were endothermic, or warm-blooded just like mammals or birds.

Dr Herman Pontzer and his colleagues analysed the leg bones of extinct species as well as living animals and calculated the amount of energy required to move about.  In recent research, this team had demonstrated that the energy required to move about, to walk, trot and run was strongly associated with leg length.  The height of the hips above the ground, as well as determining stride pattern and stride length, could be used to predict the observed energy cost of locomotion with a 98% accuracy for a wide range of terrestrial animals.

Dr  Pontzer’s team applied this concept to an analysis of fossilised dinosaur leg bones, calculating the actual volume of leg muscle dinosaurs would have needed to move about efficiently.  In total, fourteen different species of bipedal prehistoric animal or dinosaur were studied.  Animals studied in the research include the early dinosaur Heterodontosaurus, the first bird in the fossil record Archaeopteryx, as well as Triassic saurischian dinosaurs such as Plateosaurus.  Theropods made up the bulk of the extinct animals studied, dinosaurs such as Allosaurus, T. rex, Microraptor and Velociraptor.  The reason why mainly bipedal animals were chosen for this particular study is quite simple; there was less need to make assumptions over weight distribution between bipedal animals as compared to animals which had a four-footed stance.

The findings indicate that these extinct animals had an endothermic metabolism.  That these dinosaurs were warm-blooded, they required too much energy to move about efficiently and that ectothermic bodies could not supply this energy in enough quantities.  This research has been published in the on line scientific journal PLoS One (Public Library of Science).

The team commented:

“These results strongly suggest endothermy for larger bipedal dinosaurs, because other explanations require physiological adaptations or locomotor limitations unseen in living terrestrial vertebrates.”

The research team have indicated that the characteristic of warm-bloodedness evolved very early in the age of reptiles and perhaps this type of metabolism existed in the common ancestor to the dinosaurs and pterosaurs.  The scientists concluded that having an endothermic metabolism and a subsequently high metabolic rate would have contributed to the evolutionary success of the Dinosauria.

Endothermic Dinosaurs

However, not all dinosaurs may have been endothermic, indeed the cold-blooded v warm-blooded dinosaur debate is likely to continue.  For example, Dr Tom Rich of the Museum of Victoria, a palaeontologist famous for excavating dinosaurs at locations such as the famous dinosaur cove and at East Gippsland has commented on these research findings.  Dr Rich and his colleagues are famous for researching into a polar-like paleogeographic environment.  The site of their dinosaur excavations, in the Australian state of Victoria, during the Early Cretaceous would have been much further south, at approximately 75 degrees south.  Dinosaurs such as Leaellynausaura and Atlascopcosaurus would have inhabited a polar forest environment putting up with freezing temperatures and prolonged periods without sunlight.  It has been argued that only endothermic dinosaurs could have survived in these conditions.  It is not known whether these dinosaurs were permanent residents or Summer migrants to the area.

Dr Rich has commented on this new research published in PLoS One, arguing that not all dinosaurs may have been warm-blooded.  The large sauropods would have faced a number of problems maintaining a constant body temperature, and no remains of sauropod-like dinosaurs have been found in polar paleogeographical environments, as far as team members at Everything Dinosaur can recall.  It has been suggested that sauropods such as Diplodocus and the titanosaurs may have been cold-blooded or possibly somewhere in-between an ectothermic and truly endothermic metabolism.

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Aardonyx celestae – The Start of Something Big (Sauropoda)

Scientists have published a report into a recently discovered set of fossilised dinosaur bones that provide clues regarding the evolution of the largest land animals ever to walk the Earth.

A paper written by a joint team of South African and U. S. based scientists on a new dinosaur species, believed to be the ancestor of the sauropods has been published in the online scientific journal Proceedings of the Royal Society of Biology.

This new species of dinosaur has been classified as a basal sauropod, a member of the Sauropodomorpha, a group of primitive herbivores from which the huge giants such as Apatosaurus, Diplodocus and  Brachiosaurus evolved.  This new dinosaur has been named Aardonyx celestae, the name is a combination of Afrikaans and Greek which means “Earth Claw”.

Fossilised Dinosaur Bones

The fossils were found in the Northern Free State of South Africa, near the small town of Senekal, in the Bethlehem region.  The fossils represent at least two individuals, both believed not to be fully grown when they died.  The largest specimen is estimated to have measured approximately 7 metres in length.

The fossils reveal a number of characteristics that indicate that this species may have been a transitional species. Elements of the anatomy are characteristic of the larger, later sauropods.

Matthew Bonnan, a vertebrate palaeontologist at Western Illinois University, one of the authors of the research paper states that this relatively small dinosaur was a sauropod ancestor with features foreshadowing those of its more famous descendants such as Apatosaurus.

Early Jurassic Fossils

The fossils have been dated to the very Early Jurassic, approximately 195 million years ago (Sinemurian faunal stage).  The sauropodomorphs were the first plant-eating dinosaurs, the earliest forms evolved in the Middle Triassic but the fossil record for Triassic sauropodomorphs is extremely poor.  One of the oldest known sauropodomorphs was Saturnalia, fossils of which have been found in Brazil.

During the Triassic and Early Jurassic the continents we know today as South America, Antarctica, Australia and Africa were joined together to form one single land mass – Gondwanaland.  Theoretically, a primitive dinosaur called have walked from South America to South Africa, perhaps the sauropodomorphs evolved in the Southern Hemisphere.

Aardonyx celestae

In images showing this new dinosaur, the bones shaded in white indicate those elements of the skeletons that have been recovered from the dig site.  The discovery of most of the skull has helped scientists to understand the taxonomic relationship between this Early Jurassic dinosaur and its descendants.

When Matthew and his colleagues studied the length of the rib bones and compared these to the length of the forelimbs, they concluded that the front legs of this dinosaur were capable of bearing considerable weight but for the majority of the time this dinosaur may have adopted a bipedal approach.  In essence, Aardonyx may have been a facultative biped, an animal that walked around on its hind legs, but would adopt a quadrupedal stance (all fours) as and when required.

In biology, the ability to undertake an activity by desire rather than obligation is referred to as facultative.  In this case, the bipedal A, celestae could walk on all fours if it chose to do so.  The later much heavier sauropods were so large they rarely if changed their quadrupedal stance.

Pits on the Jawbone

The large number of small pits along the creature’s jawbone suggests that Aardonyx lacked a fleshy cheek that would have constrained how far the creature could open its mouth.  Its wide gape, Bonnan and his colleagues suggest, enabled this dinosaur to grab large mouthfuls of foliage with each bite.  Also, a strip of bone that ran along the base of the peg-like teeth braced them against side-to-side forces produced when the dinosaur stripped foliage from trees.

Commenting on these characteristics, Adam Yates, a palaeontologist at the University of the Wiwatersrand in Johannesburg and lead author in this study stated:

“These features are the first steps on the road to the specialised, bulk-browsing herbivory seen in sauropods.”

The shaded parts of the diagrams supplied indicate those parts of the skull that have been discovered to date.  Scientists hope that these South African fossils will shed more light on the evolution of the sauropods.

Sauropodomorphs

It is hoped that more information on the sauropodomorphs will be revealed when the fossilised remains of Thecodontosaurus are prepared and properly studied.  The best specimen of this English sauropodomorph that dates from the Middle to Late Triassic has not been removed from its rock matrix.  However, a recently awarded Heritage Lottery Fund (HLF) grant will enable a complete preparation process to take place.

To read more about the work on Thecodontosaurus: Bristol’s Dinosaur Thecodontosaurus to Rise Again Thanks to Grant.

The size and shape of muscle attachments on this dinosaur’s femur indicates that the leg muscles were powerful but that the creature was, for the most part, rather slow-moving, a characteristic certainly shared with its extremely large descendants.  For instance, estimates of the top speed of a dinosaur as big as a Brachiosaurus have been as low as 7 kmh, that is slower than most people walk.  However, if you are the size of a three-story house running away is not the best defensive option, not many meat-eaters would be bold enough to tackle a gigantic herbivore such as a Brachiosaurus.

Matthew Bonnan stated:

“This dinosaur wasn’t running a lot.  The overall shape of Aardonyx and particular aspects of its limbs, teeth and jawbone are anatomical hints of things to come in later sauropods.”

Commenting on the discovery, Paul Upchurch, a vertebrate palaeontologist at University College, London said:

“This is a very interesting animal.  The large number of bones recovered from various parts of the skeleton provide a good idea of what the creature might have looked like.”

He went on to add:

“Many of the features expected in a sauropod ancestor are present in Aardonyx, but they’re combined in an unusual mosaic.  For instance, although the creature had a wide gape that would have allowed it to grab large mouthfuls of vegetation, it had a narrow snout, unlike the broad U-shaped snout seen in most true sauropods.  This animal is expected, but in an unexpected sort of way.” 

The Hunt for More Primitive Sauropodomorphs

Scientists have predicted that more primitive sauropodomorphs will be found, hopefully these new South African discoveries will help shed light on the evolution of the Sauropoda and may help palaeontologists to understand the evolution of the ornithischian dinosaurs as well as the saurischian dinosaurs.   The most primitive of all dinosaurs, the ancestor of all that was to follow; was probably a small, gracile, bipedal, meat-eater.

Fossils of primitive members of the Sauropodomorpha will help scientists to learn more about the evolution of different types of dinosaur and the radiation of different forms that occurred during the Triassic/Jurassic.

To view replicas and models of sauropod dinosaurs: Prehistoric Animal Models and Figures.