Month: June 2020

Diictodon

Even before Lystrosaurus briefly took over the world, the dicynodont synapsids were a highly successful group. These herbivorous beaked-and-tusked mammal-cousins were abundant all over the supercontinent of Pangaea during the Late Permian and occupied a wide variety of ecological niches, ranging from rat-sized to almost bear-sized. (And later during the Triassic some got even bigger.)

Towards the smaller end of that size range were species like Diictodon. Living around 259-254 million years ago in Southern Africa (but with fossils also found in northern China, suggesting a much larger geographic range) this dicynodont grew up to about 45cm long (1’6″) and was a gopher-like creature adapted for digging, with a tubular body and short muscular limbs.

It was a very common animal, making up around half of all vertebrate fossils in some locations. Numerous preserved spiral-shaped burrows have been found concentrated in small areas, going down as much as 1.5m (5′) into the ground.

Several different species have been named within the Diictodon genus, but currently they’ve all been lumped together under the single name of Diictodon feliceps. There’s a lot of anatomical variation between specimens, though, with some notably being smaller and lacking the distinctive tusks seen in others – which may be evidence of sexual dimorphism, with the tuskless individuals possibly being females. (Although differences in inner ear anatomy may also indicate they were a separate species entirely, in which case female D. feliceps might instead be represented by fossils showing smaller tusks.)

I’ve illustrated one of the tuskless forms here, since they don’t generally get as much attention as the tusked ones. It’s also speculatively fluffy and iridescent similar to modern golden moles.

Groenlandaspis

The armor-headed placoderms were the dominant fish during the Devonian period, evolving a wonderfully diverse range of shapes and sizes, and occupying ecological niches in both marine and freshwater habitats.

Groenlandaspis antarctica here lived during the mid-to-late Devonian, about 383 million years ago, in the Oates Land region of Antarctica – at that time located further north than it is today, with the local climate being warm and subtropical.

It was a moderately-sized river-dwelling placoderm, around 50cm long (1’8″), and its bony armor formed a sort of pyramid shape with wing-like projections at its sides, a structure that would have acted as a hydrofoil and made it an efficient swimmer. Most of the armor plates were rigidly fused together, except for a hinge point between its head and thorax that allowed it to open its jaws, but unlike its more famous relative Dunkleosteus it couldn’t gape its mouth open particularly wide. It may have been a bottom-feeder, grubbing around in muddy riverbeds and using its small but strong jaws to crush hard-shelled prey.

Various other species of the Groenlandaspis genus have been found all around the world, but there’s something incredibly rare and special about Groenlandaspis antarctica in particular:

We actually know what color it was.

Preserved pigment cells in its fossils indicate that it was red on top and silvery-white on its underside in a countershaded pattern, camouflaging it in the murky silty waters of the ancient Antarctic rivers.

…And also made it look a bit like a prehistoric goldfish.

Hyopsodus

Back during the early Eocene, around 50 million years ago, global temperatures were much warmer than today, and in North America tropical and subtropical rainforests extended as far as Alaska.

And one of the most abundant animals in these balmy ecosystems was a small mammal called Hyopsodus, an early type of ungulate that was probably part of the perissodactyl lineage, closely related to the ancestors of modern horses.

Many different species of this genus have been discovered, ranging from rat-sized to cat-sized. Remains of Hyopsodus account for up to 30% of fossils in some locations, with tens of thousands of specimens known – although most of them are isolated teeth and jaw fragments.

(The illustration here depicts Hyopsodus wortmani, a 30cm/12″ long species which lived about 50-46 million years ago across the Western and Southern USA.)

More substantial skeletal remains of this little mammal are very rare, and initially seemed to show a long weasel-like body that resulted in Hyopsodus being given the nickname of “tube-sheep”. But more recent specimens have given us a better idea of its proportions, and it wasn’t really tubular at all. Instead it was probably built more like a cavy or a hyrax, with a more chunky body and a spine held more strongly curved.

Its teeth suggest it was a generalist omnivore, probably mainly eating a mixture of vegetation, fruits, seeds, insects, and occasionally smaller animals, and while its limbs were proportionally short it was likely still quite an agile fast-moving animal. It also appears to have had some ability to dig, and may have sheltered in burrows similarly to modern groundhogs.

But one of the most surprising things about the “tube-sheep” comes from studies of its braincase via CT scans of its skull. Its brain was unusually large for its size, and had enlarged areas associated with good senses of smell and hearing – and notably one sound-processing region (known as the inferior colliculus) was developed to a degree similar to those seen in echolocating animals.

Analysis of its ear bones suggest it wasn’t highly specialized for echolocation like bats, but may have still been capable of a more basic shrew-like version, using it for close-range navigation.

Ophthalmothule

The cryptoclidids were fairly standard-looking plesiosaurs, with long necks and small heads – but those tiny skull bones were also rather fragile and so there’s very little good fossil material of their heads, making it difficult to figure out both their feeding ecology and their exact evolutionary relationships.

But a recently-discovered specimen from the Svalbard archipelago actually preserved a mostly-complete skeleton, including an unusually intact skull.

Given the name Ophthalmothule cryostea (meaning “frozen bones of the Northern eye”), this cryptoclidid lived about 145 million years ago, right at the boundary between the Jurassic and the Cretaceous.

It measured around 5m long (16’5″) and had proportionally huge eyes that faced upwards on its head – an adaptation for seeing in low-light underwater conditions, maximizing the amount of light reaching it from above.

Those big dark-adapted eyes suggest it may have been nocturnal, or spent a lot of time diving into very deep waters in search of food. Its skull had weak jaw muscles and delicate teeth, and its gut region contained a lot of fine gravelly sediment, so it probably mainly grubbed around for small soft-bodied prey on the sea floor.

At that point in time Svalbard would have been a little further south than it is today, at a subarctic latitude, but the area would have still experienced particularly long nights during the winter. So it’s possible Ophthalmothule also developed such big sensitive eyes to help it survive through those darker seasons.

Qianshanornis

Many modern predatory birds have enlarged claws on their second toes, similar to those of their paravian dinosaur ancestors – with seriemas being a particularly good example.

Seriemas are part of a lineage known as cariamiformes, highly terrestrial birds that were widespread across most of the world but are today represented today by only two living species in South America. During the Cenozoic this group repeatedly evolved into large predatory flightless forms like the the phorusrhacids and bathornithids, and were probably the closest avians ever got to recreating the “carnivorous theropod” body plan and ecological niche.

And yet none of them ever seem to have experimented with more dromaeosaurid-like claws.

…With one known exception.

Qianshanornis rapax here lived in East China during the mid-Paleocene, about 63 million years ago. It was a small cariamiform, probably around 30cm tall (1″), and is only known from fragmentary fossil material – but part of those fragments was a fairly well-preserved foot. And the bones of its second toe were unlike any other known Cenozoic bird, shaped incredibly similarly to those of dromaeosaurids and suggesting it may have had the same sort of big hyperextendible “sickle claw”.

While it had sturdy legs and short wings, and probably spent a lot of time walking on the ground like other cariamiformes, it was probably also still a fairly strong flier based on the known anatomy of its arms and shoulders.

Unfortunately, though, its head and claws were entirely missing, so without more fossil discoveries it’s hard to say anything definite about its ecology. I’ve restored it here based on other predatory cariamiformes, but since it was also closely related to a herbivorous species it’s not clear whether Qianshanornis was truly a dromaeosaur-mimic or if something else was going on with that unique second toe.