#transitional forms

Named after the canine-headed Ancient Egyptian god, Phiomicetus anubis is the first fossil cetacean to discovered, described, and named entirely by a team of Arab paleontologists.

Living during the mid-Eocene, about 43 million years ago, in a shallow sea-covered region that is now part of Egypt's Western Desert, Phiomicetus was an early protocetid – an amphibious foot-powered swimmer, at a transitional point in the evolution of whales from deer-like terrestrial animals to fully aquatic screaming torpedoes.

About 3m long (~10'), it had large jaw muscles and sharp teeth with wear patterns that suggest it was a raptorial hunter grabbing and snapping at prey with powerful bites. It would have probably tackled fairly big prey compared to other protocetids, hunting things like large fish, turtles, and even smaller whales in an ecological role similar to that of modern orcas.

Along with the distantly-related long-snouted Rayanistes it's one of the earliest known whales from Africa, giving us further glimpses at a time period when early cetaceans were first dispersing out from the South Asian subcontinent via the ancient Tethys Sea.

———

Cambrian Explosion #45: Stem-Euarthropoda

The true arthropods – or euarthropods – make up the majority of panarthropods, and include the modern chelicerates (sea spiders, horseshoe crabs, and arachnids), the myriapods (millipedes and centipedes), and the pancrustaceans (crustaceans and insects), along with various completely extinct groups like the trilobites.

The earliest fossil evidence associated with some sort of euarthropod presence are trace fossils dating to only a few million years after the start of the Cambrian (~537 million years ago), and the group's common ancestor is estimated to have split off from the radiodont lineage no more than 550 million years ago in the end of the Ediacaran.

…And this is where things get more complicated.

There's a lot of disagreement about where exactly a lot of early euarthropods fit into the arthropod evolutionary tree, with different basal and "stem" lineages being shuffled around and reclassified by different studies and new discoveries. It's literally a case of "ask ten different paleontologists and you'll get eleven different opinions", so whichever order I present most of the rest of this month's entries in will probably be wrong.

———

While the isoxyids are often considered to represent some of the earliest members of the lineage leading to the euarthropods, in late 2020 another discovery was presented as an arthropod "missing link".

Kylinxia zhangi was found in the Chinese Chengjiang fossil deposits (~518 million years ago). Known from six well-preserved specimens, it was about 5cm long (2") and had a three-part body plan with distinct head, trunk, and pygidium regions, all fully arthrodized into a hard shell.

There were five stalked eyes on its head similar to Opabinia, and it had a pair of spiny serrated front appendages similar to those of radiodonts – but in Kylinxia these appendages curled upwards. Its trunk region resembled that of megacheirans, with many segments, and it had a few pairs of small flap-like limbs at the front followed by many pairs of double-branched biramous limbs with gilled paddle-like structures and jointed legs.

This confusing mixture of anatomical traits seems to help bridge the gap between the "lower stem" euarthropods like the radiodonts and the "upper stem" euarthropods (also sometimes called "deuteropods").

———

Since the last couple of weeks have featured marine mammals, let's have one more! This time not a cetacean but a member of the other group of fully aquatic mammals still alive today: the sirenians.

Although commonly known as "sea-cows" due to their herbivorous grazing habits, sirenians' closest living relatives are actually modern elephants. They're thought to have originated in Africa over 50 million years ago, starting off as pig-like or hippo-like semi-aquatic animals — but they must have been good swimmers capable of crossing oceans very early in their evolutionary history, since some of the earliest known sirenian fossils actually come from the other side of the Atlantic on the Caribbean island of Jamaica.

Sobrarbesiren cardieli here extends some of our knowledge of early four-legged sirenians to Europe, dating to the mid-Eocene about 42 million years ago. Hundreds of bones were found in Northeastern Spain, representing at least six different individuals and giving us a fairly complete idea of this species' anatomy.

It was smaller than modern sea-cows, reaching about 2m long (6'6"), and seems to represent a transitional point between the semi-aquatic ancestral sirenians and fully aquatic later forms. It had a head very similar to its modern relatives, and probably a tail fin, but also still retained small functional hind limbs.

It was initially thought to still be somewhat semi-aquatic and capable of quadrupedal locomotion on land, but a later analysis of its hind limb bones suggests that it may actually have been much more aquatic than that. Its hind legs had a wide range of motion and were probably used for otter-like swimming, undulating the body while paddling, but might not have been capable of supporting its weight on land. So if Sobrarbesiren did still haul out of the water, it may have had to move more like a seal.

———

Mixosaurus cornalianus here was a small early ichthyosaur, only about 0.8-1m long (2'7"-3'3") and generally considered to be transitional between the eel-like swimming style of basal forms and the more dolphin-like later forms. Living during the Middle Triassic, about 242 million years ago, it inhabited a shallow tropical sea that covered what is now the modern border between Switzerland and Italy.

It was previously thought to be a slow swimmer with a low and poorly-developed tail fin, and whether it even had a dorsal fin or not was unclear. But now new specimens with soft tissue impressions have given us a big surprise.

Not only did it actually have a fairly well-developed semilunate tail fin, but it also had a dorsal fin positioned much further forward on its body than expected, giving it a shape similar to some small sharks and representing the current earliest known dorsal fin of any amniote.

Bundles of stiffening collagen fibers inside its fins were very similar to those known from later Jurassic ichthyosaur species, indicating that this adaptation evolved much earlier in the lineage than previously thought. Along with stomach contents showing it mainly ate both cephalopods and small fish – fairly fast-moving prey – this suggests it was a capable open-water swimmer. It wouldn't have been quite as speedy as its much more specialized Jurassic relatives, but it may have still been about as efficient as the small modern sharks it resembled.

———

Exposition Art Blog Maria Bartuszova - Transitional Forms

Maria Bartuszova (1936–1996) was a Slovakian sculptor known for her abstract white plaster sculptures.Her work is included in the Centre Pompidou in Paris, the Slovak National Gallery in Bratislava and the Tate in London. A majority of her sculptures are made of plaster, a material that is preparatory and impermanent by nature. For this reason her sculptures are, by design, tentative, unfinished and transitory. On occasion, when "Bartuszová succumbed to the temptations of using aluminium or bronze, she would immediately undermine their material weight, through either form or subject matter: softening the material, putting it in motion, altering its proportions, and mocking gravity." Her artworks in the second half of the 1960s were influenced by her individual vision of constructive geometric tendencies connected with new materials such as cut aluminium. In 1976 and 1983, together with art historian G. Kladek, she ran workshops for disabled and visually impaired children.

Capinatator praetermissus, an arrow worm from the Mid-Cambrian of Canada (~508 mya). Discovered in the famous Burgess Shale fossil deposits, it was one of the earliest known arrow worms and also much larger than most modern forms, measuring around 10cm in length (4″).

Its mouth was surrounded by 50 hooked spines, which could be extended out to grasp onto its prey -- probably feeding on whatever smaller animals it could catch -- but when not in use these spines would have been kept folded up inside a fleshy “hood” around its head.

Month of Mesozoic Mammals #27: Treetop Origins Juramaia

The other major branch of the therian mammals are the eutherians -- represented today by only the placentals. But although the first eutherians split from their common ancestor with the metatherians all the way back in the mid-Jurassic, more then 160 million years ago, true placentals don’t seem to have actually appeared until right after the end-Cretaceous mass extinction.

There were many other side-branches of the eutherian lineage during the Mesozoic, however, and these last few days of the month will cover some of them.

(Some of these relationships are still under dispute, such as the exact position of leptictidans and Purgatorius and whether they’re actually placentals or not. Eutherian phylogeny is a bit of a mess.)

Juramaia was the earliest known eutherian, living during the Late Jurassic of China (161-156 mya). About 15cm long (6″), it was a shrew-like insectivore with limb anatomy that would have allowed it to climb up trees in a similar manner to modern rats.

Something very similar to it would have been the common ancestor of all later eutherians -- suggesting that the earliest members of the group may have started out as tree-climbers before diversifying into different niches later on.

But despite it being closer related to living placentals than to marsupials, placental-style reproduction hadn’t actually evolved yet and it would have still given birth to tiny undeveloped young.