Thursday, 24 January 2019

Tiny-headed Triassic Reptile Hunted like a Platypus

We like to use metaphors and similes to describe extinct animals. Jurassic puffins, zombie molluscs -  these colourful comparisons help us visualise animals that have no real parallel in the modern day, but there’s always a bit of artistic license involved. Occasionally however, nature produces animals so strikingly similar in their solutions for survival, you’d swear they’d copied one another’s evolutionary homework.

And so it is justified to describe the latest fossil discovery from China as a Triassic reptilian platypus. 

Complete fossil and line drawing of Eretmorhipis carrolldongi. Related to the dolphin-like ichthyosaurs, Eretmorhipis evolved in a world devastated by the mass extinction event at the end of the Permian era. L. Cheng et al, Scientific Reports, Creative Commons 4.0
Called Eretmorhipis carrolldongi, this unlikely monotreme look-alike was described in a study published today in Scientific Reports. They are not related to one another (this new fossil is a reptile, whereas platypuses are mammals) and they are separated by over 240 million years, but both evolved strikingly similar skull shapes to forage for their food underwater.

The long body of Eretmorhipis is superficially almost crocodilian, bestowed with oversized paddle-limbs that would make it at home in any Aardman animation. Running down the spine from shoulder to hip are a series of bony nodules arranged like speedbumps. At one end of the body, the heavy tail tapers to a dagger-tip, while at the other sits a preposterously tiny head. 

Eretmorhipis’ noggin is not only surprising for its small size, but also for the equally miniscule eyes that peeped out of it. When the international team of researchers from China and the US who studied the fossil plotted its eye size against body length, they found their new specimen fell well below other reptiles. In fact the proportions of the eyes were more similar to the mammalian platypus.

Comparison of the skulls of the duckbilled platypus (Ornithorhynchus anatinus), left, and Eretmorhipis carrolldongi on the right. Blue shading indicates cartilage. In the platypus, the bill is sensitive to touch allowing the animal to hunt in low light conditions. L. Cheng et al, Scientific Reports, Creative Commons 4.0
“There are some four-legged vertebrate animals today with unusually small eyes,” says Ryosuke Motani, Professor of Palaeontology at University of California, Davis, and co-author of the study, “relying on another sense to forage in the dusk or darkness - including duck-billed platypus. We now have the oldest record of such small eyes, in Eretmorphipis.”

But the similarities don't end there.

The platypus is often wrongly referred to as ‘duck-billed’, but its snout is quite unlike a bird’s. Rather than a hard structure, the platypus snout is soft and bendy. It is also packed with over 40,000 mechano- and electroreceptors, making it exquisitely sensitive to touch. As they swim through the freshwater rivers and lakes of mainland Australia and Tasmania, these singular mammals close their little eyes and lobeless ears, and their snout feels for food along the river bottom - like a hand reaching out in the dark. It is an incredible adaptation, proving that although the platypus may retain features from their ancient ancestors like laying eggs, other aspects of their anatomy are far from ‘primitive’.

The modern platypus, Ornithorhynchus anatinus, has very small eyes for its body size. It relies on the sensitive snout - which is soft and bendy and packed with electro- and mechano- receptors - to find food at the bottom of rivers and ponds. (Image: Wikipedia)

Exactly how far-fetched is it to compare this new fossil Triassic reptile with the much-loved Australian icon? “They constructed the bill skeletons similarly” says Motani.  Like the platypus, the front of the skull in Eretmorhipis is split in two, forming a pincer shape from the two bones in the front of the face. Motani adds, “it is most likely that Eretmorhipis was duck-billed.” This is a beautiful example of evolutionary convergence, when a completely unrelated animal ends up evolving the same solution to the problem of survival. “We cannot say for sure if it had electroreceptors or not,” Motani continues. “Eretmorhipis was a reptile, so it was probably not as active as the platypus, and it did not have to feed as often. It was a slower swimmer, although it could manoeuvre well using the large flippers.”

The landscape this ancient reptile swam through was unlike anything on earth today. Hubei Province, where its bones were discovered, was a tropical sea in the early Triassic, extending for hundreds of kilometres. It was so shallow you could have walked across most of it - albeit bumping into the occasional duck-billed reptile undulating across the sea-floor in search of shrimps and worms.

Eretmorhipis provides yet another example of the crazy experimentation taking place among vertebrate animals in the Triassic. From giant-headed carnivores to hump-backed Indian rhino-dragons, after the biggest mass extinction of all time at the end of the Permian, the Triassic was a time when surviving lineages explored new ecologies and body plans. Dinosaurs, pterosaurs, marine reptiles and mammals all have their origins in the Triassic. It was an era of evolutionary exploration.

Eretmorhipis lived at time when there were a lot of open opportunities,” explains Motani. “The end-Permian mass extinction destroyed the ecosystem about five million years before, and the ecosystem was finally recovering, providing many open niches. So Eretmorhipis could live just fine despite its strange body design.” 

This new fossil belongs to a group that remains poorly understood. You are probably familiar with ichthyosaurs, the fish-like marine reptiles that populated the oceans of the Triassic, Jurassic and Cretaceous. These reptiles provide a fascinating example of a terrestrial lineage of animals returning to the sea, a trick that the ancestors of cetaceans (dolphin and whales) would repeat to equal effect around 200 million years later, in yet another example of convergent evolution. But tucked in at the base of the ichthyosaur family tree lie their less-well known relatives, the Hupehsuchia. It is to them that Eretmorhipis belongs.

An artist's reconstruction of the closely related hupehsuchian, Hupesuchus. (Image: Wikipedia)
Hupehsuchians are not new, but only a handful are currently known. They were toothless, feeding on soft invertebrates like worms, and ranged from no bigger than an otter, to porpoise-sized animals. Until this new study, Eretmorhipis was only known from headless fossils, leaving researchers in the dark about its special skull adaptations. Eight different species of marine reptile, including four other hupehsuchians, all inhabited the same small area of shallow Triassic sea. It is likely that Eretmorhipis’ tactile foraging method allowed it to occupy a unique niche in the ecosystem, potentially even being nocturnal.

There remain many unanswered questions about these ancient Chinese reptiles, including the purpose of the strange bony bumps along its back. “We unfortunately do not know the exact function,” says Motani, “they were most likely used for multiple purposes, such as thermoregulation and species recognition.” 

Such puzzling creatures are part of what makes palaeontology such a fascinating discipline. The Early Triassic remains, without doubt, one of the most intriguing time-periods of them all. 

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Find the paper here:

Cheng L Motani R, Jiang D-Y, Yan C-B, Tintori A, and Rieppel O. 2019. Early Triassic marine reptile representing the oldest record of unusually small eyes in reptiles indicating  non-visual prey detection. Scientific Reports.

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