Eggshell Geochemistry Suggests Endothermy Deeply Rooted in the Dinosauria

The puzzle of dinosaur metabolism has been a subject of debate amongst vertebrate palaeontologists for a very long time.  Numerous studies have been published, drawing on a variety of research methods and lines of enquiry to determine whether the non-avian dinosaurs were warm-blooded like their avian (bird) relatives, or whether they were cold-blooded like today’s crocodilians.  A study published in the journal “Science Advances”, one that looked at the geophysical and chemical properties of dinosaur eggshell, has concluded that non-avian dinosaurs had the ability to metabolically raise their temperatures above their environment – in essence they were endothermic, that is to say “warm-blooded”.

A Thin Cross-section of Fossilised Eggshell Viewed under Cross-polarising Light to Reveal Internal Structure

A dinosaur eggshell fossil in cross-section under a microscope using cross-polarising light.  Eggshell analysis has provided compelling evidence to suggest that dinosaurs were endothermic.  Note scale is 500 microns.

Picture Credit: Robin Dawson/University of Yale

Cold-blooded or Warm-blooded – A Quick Explanation

The terms cold-blooded and warm-blooded are found frequently in articles about dinosaurs.  These terms are very misleading and have been disregarded for a long time by most of the scientific community.  For example, most lizards, regarded as cold-blooded, actually maintain a surprisingly high body temperature in their normal environment during the daytime.  Internal body temperatures around 42 degrees Celsius have been recorded in some species, much higher than the normal 37˚ Celsius associated with our own “warm-blooded” species.  In simple terms, cold-blooded animals (ectotherms), are largely unable to regulate their own body temperature without the assistance of external sources.  Lizards bask in the early morning sun to warm up and then during the heat of the day, they seek shade to help them to keep cool.  In contrast, “warm-blooded” organisms such as mammals and birds (endotherms), are able to maintain a body temperature that is higher than the temperature of the environment.  They can generate their own body heat.  This heat comes from the animal’s metabolism, the chemical reactions that take place in the body (although there are other methods of keeping cool and warming up).

The Debate over Endothermic or Ectothermic Dinosaurs

Where on the spectrum between endothermic and ectothermic are the Dinosauria?  Organisms can demonstrate a range of adaptations to assist them in maintaining an optimal body temperature.

Picture Credit: Everything Dinosaur

Understanding the Metabolism – So What?

Understanding the metabolism of a long extinct group of animals such as the non-avian members of the Dinosauria, can provide valuable insight into all sorts of areas, such as energy requirements, food consumption, behavioural traits and activity levels.  It can also help scientists to understand how extinct animals adapted to a wide range of environments, such as dinosaurs being found at high latitudes, dinosaur fossils being discovered in Antarctica for example.

In this newly published study, the researchers used a technique known as clumped isotope palaeothermometry.  It is based on the fact that the ordering of oxygen and carbon atoms in a fossil eggshell are determined by temperature.  Once the order of the atoms has been plotted, the scientists can calculate the internal body temperature of the egg-layer.

Based on this analysis, the research team were able to demonstrate that potentially, the three major clades of dinosaurs, Ornithischia, Sauropodomorpha and Theropoda, were characterised by warm body temperatures.

Commenting on the significance of this study, lead author of the research Robin Dawson, who conducted the research while she was a doctoral student in geology and geophysics at Yale University stated:

“Dinosaurs sit at an evolutionary point between birds, which are warm-blooded, and reptiles, which are cold-blooded.  Our results suggest that all major groups of dinosaurs had warmer body temperatures than their environment.”

Eggshell ascribed to a troodontid (theropod) tested at 38˚, 27˚, and 28˚ Celsius (100.4, 80.6, and 82.4 degrees Fahrenheit).  Eggshells from the large, duck-billed dinosaur Maiasaura (an ornithischian dinosaur), yielded a temperature of 44˚ Celsius (111.2 degrees Fahrenheit).  Both the troodontid and Maiasaura eggshells were collected from Alberta, Canada.

In addition, the fossilised eggs associated with the oospecies Megaloolithus from the Hateg Formation of Romania tested at 36˚ Celsius (96.8 degrees Fahrenheit).  The taxonomy of the Romanian material remains uncertain.  The eggshells could represent the dwarf titanosaur Magyarosaurus, the much larger titanosaur Paludititan or indeed, the dwarf hadrosauroid Telmatosaurus.  If this fossil material does represent a Sauropodomorph, then these results could suggest that metabolically controlled thermoregulation was the ancestral condition for the Dinosauria.

The Taxonomic Relationships of the Taxa Involved in the Study

The phylogeny of the taxa involved in the study.

Picture Credit: Science Advances

The picture (above), shows living ectotherms in blue, whilst extant endotherms (birds) are shown in orange.  The Maiasaura silhouette represents the major dinosaurian subclade Ornithischia.  The asterisk (*) indicates the uncertainty over the taxonomy of the oospecies Megaloolithus, but the fossil eggshells could represent the dwarf sauropod Magyarosaurus.  The troodontid material is assigned to the Theropoda.

The researchers conducted the same analysis on cold-blooded invertebrate shell fossils (molluscs) from the same locations as the dinosaur eggshells.  This helped the scientists determine the temperature of the local environment — and whether dinosaur body temperatures were higher or lower.

Dawson, now a postdoctoral research associate at the University of Massachusetts-Amherst, explained that  the troodontid samples were as much as 10˚ Celsius (50 degrees Fahrenheit), warmer than their environment, the Maiasaura samples were 15˚ Celsius warmer (59 degrees Fahrenheit) and the Megaloolithus samples were 3 to 6˚  Celsius (37.4-42.8 degrees Fahrenheit) warmer.

She added:

“What we found indicates that the ability to metabolically raise their temperatures above the environment was an early, evolved trait for dinosaurs.”

This new research may have other implications as well.  For instance, the study shows that a dinosaur’s body size and growth rate may not necessarily be a good indicator of body temperature.  The researchers also stated that their findings might add to the ongoing discussion about the role of feathers in early bird evolution.  Dense coats of feathers may have evolved to help insulate the bodies of dinosaurs, secondary functions such as for use in visual displays or as part of adaptations towards powered flight occurred later.

Everything Dinosaur acknowledges the assistance of a press release from Yale University in the compilation of this article.

The scientific paper: “Eggshell geochemistry reveals ancestral metabolic thermoregulation in Dinosauria” by Robin R. Dawson, Daniel J. Field, Pincelli M. Hull, Darla K. Zelenitsky, François Therrien and Hagit P. Affek published in the journal Science Advances.

Rhamphorhynchids – “Good Climbers and Rare Walkers”

One of the great mysteries regarding the Pterosauria may have finally be solved.  Palaeontologists are one “step” closer to better understanding how these flying reptiles moved about on the ground.  Researchers studying six pterosaur trackways preserved in the sandstone that once comprised part of a Late Jurassic beach have been able to examine the locomotive abilities of non-pterodactyloid pterosaurs for the first time.

This is a big deal, tracks of pterosaurs have been known about for some time, but all the trace fossils suggesting tracks up until now were believed to have been made by pterodactyloid pterosaurs, (Pterodactyloidea), essentially flying reptiles with short tails, relatively long metacarpal bones and a fifth toe that is greatly reduced or absent.  Virtually nothing was known about the terrestrial abilities of other types of pterosaur that dominated the skies of the Jurassic, the dimorphodonts, Anurognathidae and the rhamphorhynchids for example.

However, scientists from the remarkable Musée de la Plage aux Ptérosaures, writing in the academic journal “Geobios”, describe six trackways related to three non-pterodactyloid new ichnotaxa and determine that these animals moved quadrupedally and that they were quite at home on the ground.

A Life Reconstruction of a Rhamphorhynchid Pterosaur Walking Across a Beach

The long-tailed Rhamphorhynchus leaves a series of five-toed tracks on the Jurassic beach.

Picture Credit: Mark Witton

“Good Climbers and Bad Walkers”

Over the last two hundred years or so, a variety of theories have been put forward by palaeontologists regarding the way in which these flying reptiles moved about on the ground.  For most of that time, these ideas were based on anatomical analysis of fossil bones.  Trackways preserving evidence of a flying reptile moving about on the ground were exceptionally rare.  Ironically, when such evidence did come to light, such as the trackway found in Wyoming in 1952 (Sundance Formation), these trace fossils received little scientific scrutiny.

The lack of tracks from non-pterodactyloid pterosaurs preserved in the fossil record, led many palaeontologists to believe that these animals rarely left the trees or the water and moved around on land.  When they did, it was thought that they would have been clumsy and slow-moving, very vulnerable to predation.

A Non-pterodactyloid Trackway from the Upper Jurassic (Plage aux Ptérosaures)

Pterosaur trackway (non-pterodactyloid) from south-western France.

Picture Credit: Musée de la Plage aux Ptérosaures/Geobios

“The Pterosaur Beach of Crayssac”

The fossil finds come from the remarkable “la Plage aux Ptérosaures” (the pterosaur beach), located close to the village of Crayssac in the Occitanie region of south-western France.  The site provides a trace fossil record of activity on a Late Jurassic beach around 150 million years ago (lower Tithonian faunal stage).  Both dinosaur and pterosaur trackways are preserved.  The authors of the scientific paper, conclude that the tracks may have been made by rhamphorhynchids and they propose that non-pterodactyloids, at least during the Late Jurassic, were quadrupedal with digitigrade hands and plantigrade to digitigrade feet.  Analysis of the tracks indicates that these animals were good walkers, even if their hind legs were hampered by the uropatagium (the membrane of skin that spanned the back legs).  The idea that these types of flying reptiles were “good climbers but bad walkers”, seems to have been refuted.

The authors state that based on this new study and contrary to current hypotheses, non-pterodactyloid pterosaurs seem to have been good walkers even though their trackways are very rare or unidentified to date.  Each of the trackways is around a metre in length, the individual prints measuring approximately three centimetres long.  Jean-Michel Mazin and his co-author Joane Pouech (from the museum at la Plage aux Ptérosaures), were aware of the significance of these trace fossils as pterodactyloids tracks tend to produce four toe marks in the trace fossil, whereas, non-pterodactyloids had five toes, so five toe marks would be expected in the majority of the hind prints.

Pterosaur expert Mark Witton provides a well-written and comprehensive overview of pterosaur anatomy and discusses the theories associated with their terrestrial locomotion in his excellent book simply entitled “Pterosaurs”.

A review of this publication can be found here: Pterosaurs by Mark Witton – a book review.

The Front Cover of the Comprehensive Book on Pterosaurs by Mark Witton

A very well researched and documented publication from an authority on the Pterosauria.

Picture Credit: Everything Dinosaur

The scientific paper: “The first non-pterodactyloid pterosaurian trackways and the terrestrial ability of non-pterodactyloid pterosaurs” by Jean-Michel Mazin and Joane Pouech published in Geobios.