The First Detailed Analysis of the Stapes In Triassic Cynodonts

The smallest bone in the human body and how it evolved has been the subject of a major research project conducted by scientists at the University of Witwatersrand (Johannesburg, South Africa).  That small bone is called the stapes and it forms part of the three bones of the middle ear, the malleus, incus and the stapes which together are known as the ossicles.  All modern mammals possess these three bones, which are also called the hammer, anvil and the stirrup, these names relate to their shapes, as the stapes for example, resembles a stirrup, the support for a rider’s foot.

Mammalian Evolution

A Diagram Showing the Shape of a Extant Mammal’s Stapes

The three middle ear bones of a modern mammal.

Picture credit: Everything Dinosaur

University of Witwatersand Research

A team of scientists from the University of Witwatersand’s Evolutionary Studies Institute which includes Dr Leandro Gaetano and Professor Fernando Abdala have completed the first detailed comparative analysis of ancient ear bones of Triassic cynodonts, ancient synapsids that are ancestral to the Mammalia.  Until this study was carried out, it had long been thought that the stapes showed no differences between species.  However, in this new research, published in the academic journal PLoS One, the researchers map variations in the morphology of this bone, even amongst animals of the same species.

Commenting on their findings, Dr Gaetano stated:

“No one has really paid attention to this small bone before.  In studying this ear bone of Triassic Cynodonts, the forerunners of mammals, including humans, over the past two years we now start to see these differences.”

The Hearing Mechanism of Reptiles

Reptiles have a different hearing mechanism when compared to mammals.  As the synapsid clade evolved and modern mammals came about, they evolved more sophisticated and sensitive hearing.  Scientists believe that the three middle ear bones gave the early mammals, which were probably nocturnal, an improved ability to detect high-frequency sounds – useful if you spent your waking hours in the dark and you relied on your hearing to detect prey as well as to sense danger.  Bones in the reptilian jaw, the articular (lower jaw) and the quadrate (upper jaw) evolved into the middle ear bones, connecting to the stapes and forming the ossicles.

Mammalian Middle Ear (Evolution)

The evolution of the mammalian middle ear.

Picture credit: Everything Dinosaur

Embryo Studies and Transitory Fossils

Embryo studies and the discovery of a number of transitory fossils have provided evidence for this, however, in this study it was the stapes as found in many Triassic cynodonts which lived between 250 and 22o million years ago, that was central to this new analysis.  The stapes is the only ear bone in mammalian ancestors, the evolution of the middle ear configuration as found in modern Mammalia had not yet occurred when the animals featured in this study were alive.

The Variation in the Shape of the Stapes of Triassic Cynodonts

Shape of the stapes bone in different cynodonts studied.

Picture credit: Witwatersrand University

The picture above shows morphological variation in the stapes of Triassic gomphodont cynodonts.

A, Diademodon; B, Trirachodon; C and D, Massetognathus.

Below is the ventral view of the skull of a cynodont showing the position of the stapes.

Dr Gaetano explained:

“Few contributions studied the stapes in cynodonts and it has been historically regarded as a conservative element, showing no difference among species.  Surprisingly, we discovered that there are noticeable variations in the morphology of this bone, even within representatives of the same species.

This research is helping to unravel a mystery surrounding the origins of the middle ear bones and their configuration.  Professor Abdala suggests that the sound waves in cynodonts  were transmitted to the inner ear from an eardrum at the posterior part of the lower jaw through the stapes and quadrate bones.  The research is on-going, utilising the extensive Permian and Triassic vertebrate fossil record found in South Africa (the biota of the Karoo basin).

The Next Stage of the Research Project

The next stage is to try to calculate the impact on hearing ability of the differently shaped stapes bones that the team have identified.  In addition, the scientists will focus on ontogenic changes (changes in the shape of the stapes from the youngest to the oldest animal in one extinct species).

For replicas and models of ancient reptiles and other prehistoric creatures: CollectA Deluxe Prehistoric Life Figures.

Pterosaur Rostrum Discovered on the Isle of Wight

Thanks to sharp-eyed, local fossil collector Will Thurbin, a fragment of bone from an Early Cretaceous pterosaur has established that a another member of the Ornithocheiridae flew over the skies of what was to become the Isle of Wight.  Whilst searching for fossils along Chilton Chine beach on the south-west coast of the Isle of Wight, Mr Thurbin spotted a strange looking, well worn pebble that when examined more closely showed traces of eroded teeth

Strange Fossil Discovery

Unsure of what the object was, he brought the specimen to the Dinosaur Isle museum on the island so that the experts there could examine it.  After consulting Dr Dave Martill (School of Earth and Environmental Sciences at the University of Portsmouth), an expert in vertebrate fossils from the Isle of Wight, the find was identified as the tip of the rostrum the upper jaw of a Pterosaur genus known as Coloborhynchus.

To give readers an idea of which part of the animal the fossil belongs, we have taken the excellent model of Guidraco made by CollectA, another ornithocheirid, but this time from China and used this replica to show you where the rostrum is located.

The tip of the upper jaw.

Picture credit: Everything Dinosaur

CollectA Deluxe Pterosaur Figure

The picture (above), shows the rostrum of a CollectA Supreme/Deluxe pterosaur model.  To view the CollectA Supreme/Deluxe range: CollectA Deluxe/Supreme Prehistoric Animal Figures.

Although very worn, the fossil still retained enough detail to provide the scientists with the opportunity to identify the family (Ornithocheiridae) and the pterosaur genus.  The specimen, which is just a few centimetres long has gone on display at the Dinosaur Isle Museum.

The Isle of Wight Coloborhynchus Pterosaur Fossil

Posterior view for Isle of Wight fossil.

Picture credit: Isle of Wight Council

The fossil may not look much, but this fossil fragment, the tip from the upper jaw (rostrum), represents the world’s oldest example of the Coloborhynchus genus.  It pre-dates earlier Coloborhynchus fossils by around ten million years.  In all likelihood it is a new species, but that’s the trouble with the Pterosauria, lots of species have been named from just fragments of fossil material.  This tends to “muddy the waters somewhat” when it comes to Pterosauria taxonomy, let’s look into this in a bit more detail.

The Problem with those “English Pterosaurs”

Coloborhynchus belongs to a very enigmatic family of pterosaurs, the Ornithocheiridae.  The fossil record for the ornithocheirids is a bit of a “curates egg”, that is to say, that the fossil record is good in parts.  Thanks to beautifully preserved specimens from China and South America, this family is amongst the best known of all the flying reptiles, but it has only been in the last twenty-five years or so that these specimens have come to light.  Before that much of what we knew (or thought we knew) about these widespread Cretaceous pterosaurs came from the study of extremely fragmentary specimens found in southern England.

Poorly Preserved Specimens

These very poorly preserved fossils probably would not get a lot of attention these days, but back in the latter part of the 19th century these remains were studied by some of the most eminent scientists around at the time.  The likes of Sir Richard Owen and Harry Govier Seeley examined and described these specimens, as a result, a range of different genera were erected, most of them now regarded as nomen dubium.  Much of these fossil were excavated from the Cambridge Greensand of southern England, marine deposits laid down in the Cretaceous, with most of the material dating from around 105 to 115 million years ago.

Three-dimensional Fossil Material

The fossils are preserved in three-dimensions, just like the Isle of Wight Coloborhynchus specimen but they are the remains of pterosaurs that died far out to sea.  The corpses were scavenged, the bones once they had sunk to the bottom of the sea became encrusted with shelled animals such as barnacles and they were drilled into by marine worms.  These bones were eventually buried only to be exposed again by violent storms and finally buried as part of the fossil record several million years after the flying reptile had actually died.  As a result, these fossils are extremely difficult to interpret, let alone assign to a new species.

Coloborhynchus clavirostris Holotype Fossil Material (Hastings Group)

Rostrum from the Hastings Group (West Sussex). A Coloborhynchus rostrum.

Picture credit: Natural History Museum

The picture above shows one of the pterosaur fossils from the 19th century.  This is the holotype for C. clavirostris, (A) = anterior view (view from the front), with (B) a line drawing of the same view.  The fossil is viewed from the side, a left lateral view (C) with a corresponding line drawing (D).  Numbers and arrows indicated teeth sockets (alveoli) and individual teeth.

Scale bar = 1cm.

Coloborhynchus Rostrum

When the Isle of Wight fossil is compared to the holotype fossil material (both seen in anterior view), these two specimens look very similar, but it was the position, orientation of the alveoli (teeth sockets) that aided identification.

The Isle of Wight Fossil Material with Teeth Sockets Labelled (anterior view)

Teeth sockets can clearly be seen, it was the orientation, shape and position of the teeth sockets that led to the Coloborhynchus identification.

Picture credit: Isle of Wight Council with additional annotation by Everything Dinosaur

Pterosaur Piscifores

It is very likely that these toothy pterosaurs were mainly fish-eaters and they lived on the coast, or at least in estuarine environments.  The paper detailing this 2014 discovery has just been published in the Proceedings of the Geologists Association.

Intriguingly, in 2014, two fragmentary pieces of pterosaur rostrum were found on the Isle of Wight.  They were found at different locations and they are shaped differently.  The second fossil, donated by Mr Glyn Watson (Nottinghamshire), is currently being researched in order to identify the pterosaur family.  This too, is likely to be a new species, although whether a species can be assigned from the rostrum alone has yet to be determined.

An Illustration of Coloborhynchus

An illustration of the pterosaur called Colobohynchus (C. clavirostris).

Picture credit: Mark Witton

 Coloborhynchus was certainly a sizeable pterosaur, although not the biggest member of the Ornithocheiridae.  Size estimates are difficult to calculate based on fragmentary material, but a maximum wingspan between four and six metres has been cited by a number of authors.