A Giant Ichthyosaur from the United Kingdom

Last week, an [article] came out about an ichthyosaur from the United Kingdom. Ichthyosaurs are marine reptiles the lived during the Mesozoic. They had bodies shaped like dolphins and gave birth to their babies instead of laying eggs.

An ichthyosaur. From here.

This article described a few new specimens that were found in the United Kingdom. The specimens are pieces of the lower jaw bone, from slightly different time periods, but all from the Triassic (251-199 million years ago). Now, usually isolated pieces of bone do not make the news, but these lower jaw pieces are HUGE.

Figure 5A from the paper showing one of the jaw pieces.

The authors compared the pieces to other ichthyosaurs of the time and place and found that in one case, the animal could have measured 20-25 meters long. That’s almost the size of a blue whale! In the case of the other specimen, it might have been even larger.

A drawing of Shonisaurus, a related ichthyosaur. By N. Tamura.

Even though large mammals are found in our oceans today, during the Triassic, there was a broad diversity of reptiles ruling the seas.

The Short-Faced Reptile from Connecticut

Last week, a new little reptile was [described] from the Late Triassic (around 212 million years ago) of Connecticut (USA). The specimen was originally found in 1965, but it took almost 30 years before its first description was published in 1993. This new study adds in modern scanning and analysis techniques to get a better idea of what this animal was and how it lived.

A couple of rhynchosaurians with human legs for scale. From Pinterest.

The reptile is a rhyncosaurian, a type of reptile distantly related to crocodiles, dinosaurs, and birds. Rhyncosaurians lived only during the Triassic and were typically small, but some could grow up to 2 meters long. This new rhynchosaur is named Colobops novaportensis, meaning ‘short-faced’ and ‘from New Haven,’ the area where is was discovered. Colobops is interesting because of its small size and because of a few features of its head.

Figure 1A of the paper showing the top of the skull. The front of the skull is pointing up. This image was made using CT scans.

To start, the part of the face in front of the eyes, called the rostrum, is very short. It’s only about a quarter of the length of the skull! That’s very small for a reptile. Its whole skull is only 2.5 cm (1 inch) long. So, this is a very small rhynchosaurian. Secondly, the skull has features that indicate very large muscles for biting. Even though the muscles themselves do not fossilize, the space those muscles occupy and their bony attachment points do fossilize. These bones can give us an estimate of how large the muscles were. Altogether, this species is very small and its bite was much stronger than expected for its size.

An artist reconstruction of Colobops. By M. Hanson.

This study shows that we can always learn more about animals by using up-to-date techniques that were not always available. It also shows how science can occur in a series of steps that build on each other.

A Group of Reptiles Gets a New Head

This week, a [paper] came out describing a new fossil skull. The skull belongs to a reptile group called Drepanosaurs (dre-PAN-o-saurs). This group is well-known as we have several fossils of the bodies. Heads, however, have been harder to find.

A few different drepanosaurs. From here.

These reptiles evolved during the Permian (sometime around 260 million years ago), but did not become a diverse group until the Late Triassic (around 219 million years ago). That means they survived the biggest extinction event this planet has experienced – the Permo-Triassic Extinction Event. This occurred at 250 million years ago and around 90% of all life went extinct.

An approximation of what Earth looked like during the Permo-Triassic Extinction. (Just kidding – it’s Mordor).

The drepanosaurs lived on, though! These little reptiles looked very much like chameleons, but had some unique traits. The new fossil is called Avicranium renestoi (“avi” for bird, “cranium” for head, and “renesto” in honor of a paleontologist who works on this group).

Figure 2 from the paper showing the reconstruction of the skull bones. The long dotted line is the missing portion of the snout. The nose is to the left.

As its name suggests, the skull shows a mix of shapes, some looking very bird-like, and some looking very reptile-like. Let’s start with the bird-like features. The skull has is edentulous (has no teeth). Also, the part of the skull slightly back from the eyes is very domed. This leads to a bird-like braincase and [endocast]. These shapes are very similar to what we find in pterosaurs, modern birds, and some maniraptorans. Some of the features, like the back of the braincase and the ear, are very similar to other early reptiles. So Avicranium has a mix of bird-like and reptile-like features making up its skull.

An analysis of evolutionary relationships puts this group at the very base of the reptile tree. This shows us that some of these ‘bird traits’ evolved way earlier and in unrelated groups. It also shows us that the diversity in body shapes was much higher in the Triassic than we previously thought.

The Earliest Avemetatarsalians

The Triassic (251-199 million years ago) was an interesting time in the history of the Earth. Just after the largest mass extinction in Earth’s history (the Permian Mass Extinction at 252 million years ago), the Triassic was a time when new animals were starting to take over. The weird looking synapsids (distant relatives to mammals) were gone and early archosaurs were emerging.

Fauna of the Late Permian. From Pinterest.

Fauna from the Triassic. By deviantART user Apsaravis.

These archosaurs eventually divided into the Pseudosuchian (archosaurs closer to crocodylians: rauisuchians, thalattosuchians, aetosaurs, modern crocodylians, and more) and Avemetatarsalians (archosaurs closer to birds: pterosaurs, ornithischians, sauropods, theropods, and more). Understanding what these early Avemetetarsalians looked like has been difficult because complete, well-preserved specimens from the Triassic are hard to find. Other animals on these lineages that we do find are already very advanced: for example, early pterosaurs already look like pterosaurs.

An early pterosaur, Dimorphodon. From Wikipedia.

A new [paper] published this week described an early avemetatarsalian that they named Teleocrater rhadinus (Teleo – closed and crater– basin after the closed hip socket, and rhadinos – slender, for the animal’s slender body). It was found in Tanzania and it is from the lower portion of the Middle Triassic rock layers (247-237 million years ago), making it one of the oldest members of this group. The specimen has most of the limbs, some vertebrae, and a bit of the skull.

Figure 2 from the paper showing the Teleocaster specimens. Red bones are present in 1 individual. Blue bones are present in the second individual. Purple bones are present in both.

The bones of Teleocrater tell an interesting story. The way the ankle bones are shaped and the way they fit together is very much the same in this early avemetatarsalian and in early pseudosuchians. This means that the ankle seen in pterosaurs, dinosaurs, and one other group was evolved multiple times instead of just once for the whole group. Other features indicate that these early avemetatarsalians were long-necked, carnivorous, and not built for efficient running.

The authors also ran a phylogenetic analysis (an analysis of evolutionary relationships) and found that Teleocrater belongs to a small group of early avemetatarsalians, now called Aphanosauria, and is one of the earliest members of this group. This study provides much needed information about these early animals, and shows that the diversity of this group was much higher in the Triassic than previously thought.

The Triassic Dome-Headed Archosaur

A new [paper] was published online this week on an archosaur with a fascinating head. The living archosaurs are crocodylians and birds, but the group includes many extinct animals like all the non-avian dinosaurs, pterosaurs, rauisuchians, aetosaurs, and more. All of the animals in this group have teeth in sockets and a couple of new openings in the skull.

nesbitt-cover-2011-bulletin

The archosaurs. From the cover of a different article (Nesbitt 2011).

This new archosaur, Triopticus primus, is interesting because it has a dome-head, just like the Pachycephalosaurus and its relatives, the dome-headed dinosaurs. This archosaur is from the Otis Chalk Formation from the Triassic Period. This is where the story gets interesting.

fig-1b

Figure 1b from the paper showing the dome of the new species.

The Otis Chalk Formation represents a time when archosaurs were evolving very different body shapes. These body shapes included toothless forms, fast predators, armored herbivores, and long-snouted carnivores. Millions of years later during the Jurassic and Cretaceous, when crocodylian relatives and dinosaurs had taken over, these same body shapes were evolved again. This represents major convergent evolution between early archosaurs and their crocodylian and dinosaurian descendants. You may recall that convergent evolution is when different groups evolve the same features independently. The wings of bats, birds, and pterosaurs are also convergent. These Triassic archosaurs evolved a broad diversity of body shapes and it’s fascinating that many of these shapes were evolved again later on.

convergence

Cover art from the article showing how different forms evolved the same body shapes.