Australasian Science Magazine

By John Long

The past year has seen some literally enormous palaeontological discoveries, from the dinosaur with the largest BMI to an ichthyosaur that left land to live like a dolphin.

While it seems that most of the world record-holding living animals have been well and truly discovered, the prehistoric world continues to amaze us each year with new finds that break long-held records, or fill in major missing gaps in evolutionary sequences.

Last year saw the announcement of the largest known dinosaur for which a body mass can be accurately calculated. Dreadnoughtus schrani was a sauropod dinosaur that lived around 77 million years ago in Argentina, and reached about 26 metres long and possibly weighed up to 59.3 tonnes (Scientific Reports 4/9/14).

Although single bones of very large dinosaurs that were possibly larger than Dreadnoughtus are known to exist, without enough of a skeleton preserved it’s impossible to determine if the living animals were really as big. It might have a long neck and short tail, or very long front limbs, as in Brachiosaurus, so calculation of its overall body mass really does depend on knowing an animals’s overall skeletal shape.

At the other end of the spectrum we saw one of the smallest known herbivorous horned dinosaurs announced, Aquilops americanus, from Montana, USA, that was about the size of a cat at around 1.6 kg and 70 cm long (PLOS1 10/12/14).

Yet for us palaeontologists, the big finds are always the ones in which a transitional fossil discovery suddenly clarifies the stages in the evolution of a well-known groups of animals.

One such group were the ichthyosaurs, the fish-eating long-snouted marine reptiles that occupied a niche similar to today’s dolphins. They had fish-like tails and paddles like modern cetaceans. To date there was no evidence of how ichthyosaurs transitioned from land into their wholly marine lifestyle until last year’s discovery of a new species, Cartorhynchus lenticarpus, from rocks representing a 248 million-year-old tropical marine setting in China (Nature 5/11/14). The fossils were described by Ryosuke Motani from UC Davis and colleagues, and showed for the first time how ichthyosaurs evolved. Cartorhynchus was also the smallest known ichthyosaur ever found, no longer than an estimated 40 cm.

It had a very short snout unlike the elongate skulls seen in well-known ichthyosaurs, so its skull more closely resembled the ancestral terrestrial reptile. Its bones were also thicker and more robust like a land animal.

Perhaps the most significant thing about Cartorhynchus is its limbs, which had a flexible wrist joint allowing seal-like movements of the large flippers when on land. The shortened body size and flexible flippers suggest it might have come back to land for giving birth to its young as seals do. We know from many well-preserved German ichthyosaur fossils that even the fully marine ichthyosaurs gave birth to live young.

Cartorhynchus evolved just 4 million years after the world’s greatest extinction event at the end of the Permian. It was a time when many groups in the marine realm became extinct, so the time was perfect for newcomers to invade and take over niches left open by the vanished groups.

Another transitional fossil was announced in January that fills the gap between primitive armoured jawed fishes and living fishes (both sharks and bony fishes). Named Janusiscus after the two-faced Roman god, the tiny fish fossils from Siberia, dated at around 415 million years old, reveal a skull roof that looks like an early bony fish (osteichthyan) while the braincase, revealed through micro-CT scanning of the rock containing the fossil, is much more like that of an ancient shark (Nature 12/1/15). Unfortunately this fossil lacks many of the vital bits like jaws, teeth, cheek bones, gill-arches, palate and body skeleton, but there’s still enough of it preserved to change the way we see how modern sharks and bony fishes began to diverge from their last common ancestor.

Who knows what else will be uncovered in museums or from the field. Stay tuned.

John Long is Strategic Professor in Palaeontology at Flinders University and current President of the Society of Vertebrate Paleontology.