#stem-turtle

Repostober Day 8

Elasmosaurids are often depicted with noodly snake-like or swan-like necks, but they were probably actually quite stiff and inflexible in life. And while we know from fossilized gut contents that they ate relatively small prey like fish, crustaceans, and cephalopods, exactly how they used their distinctive long necks is still uncertain.

There’s some variation in the sizes and shapes of their teeth, so it’s likely each species was specialized for slightly different feeding styles -- we’ve even found a filter-feeding one! -- and the recently-named Leivanectes bernardoi here adds in a little more diversity, too.

Living about 115-112 million years ago during the mid-Cretaceous of Colombia, Leivanectes would have been fairly large at around 9m long (29′6″), slightly bigger than the other elasmosaurid species known from the same ancient marine deposits. It had a reduced number of teeth in its jaws, but these teeth were also proportionally larger, suggesting that it may have been tackling bigger tougher prey than its relatives.

Unfortunately it’s currently only known from a single partial skull, so we don’t have any other clues about its ecology.

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Panzhousaurus rotundirostris, a sauropterygian marine reptile from the mid-Triassic of southwestern China (~245 mya), living just a few million years after the devastating Permian-Triassic mass extinction. This small marine reptile was only about 40cm long (1′4″) and is known from a single near-complete skeleton.

Although it was a distant evolutionary cousin to plesiosaurs (and even more distantly to modern turtles), it was actually most closely related to an early sauropterygian lineage known as the pachypleurosaurs -- a group of small lizard-like aquatic reptiles with tiny heads, long necks, and paddle-like limbs.

Paludidraco multidentatus from the Late Triassic of Spain (~237-227 mya).

This 3m long (9′10″) animal was a member of the nothosaurs, a group of semi-aquatic seal-like marine reptiles that were closely related to plesiosaurs (and both were also evolutionary cousins to modern turtles).

It had long slender jaws full of numerous tiny teeth, creating an interlocking comb that was probably used for filter feeding -- scooping up mouthfuls of fine-grained sediment from the seafloor and filtering out small invertebrates or soft plant matter.

The bones of its skeleton were also highly thickened and dense, a condition known as pachyostosis that provided ballast to weigh it down in the water. This would have made it a slow and unmaneuverable swimmer, but a very energy-efficient one, using its natural neutral buoyancy to hover or walk along the seabed.

Almost-Living Fossils Month #20 -- Some Very Spiky Turtles

The meiolaniformes were a group of terrestrial turtles that first appeared in the fossil record in the Early Cretaceous, around 125 million years ago. Although they were originally thought to be cryptodirans, more recent studies suggest they weren’t actually quite true turtles at all, instead being close evolutionary cousins to them in a much older and more “primitive” lineage that may go back as far as the Triassic.

They’re known mainly from South America and Oceania, but they may have had a more global distribution during the Cretaceous, with some fossils from the northern continents sometimes being classified as members of the group. However, only the South American meiolaniformes seem to have actually survived through the end-Cretaceous extinction.

The most distinctive meiolaniformes were the heavily armored meiolaniids, which first appeared in Patagonia during the Early Eocene (~48 mya). With large horns on their heads and thorn-like spikes along their long tails, they seem to have convergently evolved to fill the same sort of large-herbivore-tank niche as ankylosaurs and glyptodonts.

They also had fairly large nasal cavities, which might indicate a well-developed sense of smell -- or may have been an adaptation for regulating the heat and moisture content of each breath, similar to the complex noses of ankylosaurs.

The South American meiolaniformes all went extinct around the end of the Eocene (~33 mya), but some meiolaniids had already dispersed across to Australia via Antarctica (before the continents had fully separated, and before Antarctica had frozen over) and they continued to survive there for most of the rest of the Cenozoic. They even went on to spread to various islands around Oceania, suggesting they were able to float and swim like modern giant tortoises.

The largest Australian meiolaniids reached sizes of around 2.5m long (8′2″), making them some of the biggest of all known terrestrial turtles. These giant forms went extinct in the Late Pleistocene, around 50,000 years ago, alongside much of the other Australian megafauna.

A few smaller varieties hung on in smaller islands to the east, with one of the latest-surviving species being Meiolania platyceps on Lord Howe island. It was only about half the size of its biggest Australian relatives -- an example of insular dwarfism -- and lived into the Late Holocene just 3000-2000 years ago.

Meiolania species on other islands seem to have gone extinct after the arrival of humans. But Lord Howe Island appears to have never been inhabited prior to European settlement in the late 1700s, so it’s unclear why this last of the meiolaniformes disappeared.

Mauriciosaurus fernandezi, a polycotylid plesiosaur from the Late Cretaceous of Mexico (~94-89 mya). About 1.9m long (6′3″) with a flipper-span of 1.5m (4′11″), it’s known from a near-complete skeleton with preserved soft tissue impressions. The fossil shows evidence of rows of very tiny scales, the skin outlines of the flippers, and also a thick layer of insulating blubbery fat.