Tag: condylarth

Eons Roundup 14

It’s been a while, but let’s catch up with some more work I’ve done for PBS Eons:

An illustration of both a fossil example and the reconstructed internal structure of the enigmatic Paleodictyon, a hexagonal network of tunnels in seafloor sediment created by an unknown organism.
Paleodictyon nodosum

The enigmatic Paleodictyon, from “Something Has Been Making This Mark For 500 Million Years”

An illustration of Loxolophus, an extinct mammal related to early ungulates. It somewhat resembles a modern coatimundi, with a slender snout, long legs with plantigrade feet and claw-like hooves, and a long bushy tail. It's reconstructed with a mottled and striped camouflage pattern of brown and cream fur.
Loxolophus hyattianus
An illustration of Arctocyon, an extinct mammal related to early ungulates. It has a dog-like head with large canine teeth, a stocky bear-like body with plantigrade legs and claw-like hooves, and a long tail. It's reconstructed with an orange-brown pelt with white stripes along its limbs.
Arctocyon primaevus
An illustration of Eoconodon, an extinct mammal related to early ungulates. It has a somewhat deer-like head, a rather cat-like body shape, claw-like hooves on its toes, and a long tail. It's reconstructed with a coat of brown fur spotted with white along its back and sides.
Eoconodon coryphaeus

The archaic ungulates Loxolophus, Arctocyon, and Eoconodon, from “How a Mass Extinction Changed Our Brains”

An illustration of Aturia, an extinct relative of modern nautiluses. It's reconstructed with an orange-brown shell patterned with zigzagging darker stripes.
Aturia sp.

And the nautilid Aturia, from “When Nautiloids Met Their Match”

It Came From The Wastebasket #20: Colossally Convoluted Condylarths

“Insectivora” was a wastebasket taxon so bad it had to be revised multiple times, but there’s another particularly infamous case in mammal taxonomy that’s still in the process of being resolved – the “condylarths“.

This group was first created in the early 1880s, during the Bone Wars, and initially was just a subgroup of odd-toed ungulates containing only the phenacodontids. But just a few years later Condylarthra was promoted up to its own order, and groups like the periptychids and hyopsodontids were added in too.

Then over the next few decades various groups were added and removed from the condylarths, most notably with the mesonychids and arctocyonids being brought in from their previous position with the creodonts.

By the mid-20th century the condylarths had become a big convenient dumping ground for any and all “primitive” ungulate-like mammals that didn’t easily fit into any modern groups, ranging in age from the early Paleocene through to the early Oligocene. But it soon became apparent that they had the same problem as the “insectivores” – there weren’t really any unique anatomical features that united all these animals together.

They generally had rounded-cusped molar teeth and hoof-like toes, but they also had rather generalized “primitive mammal” features and a diverse range of ecologies. Some were small herbivores, but others were coati-like or dog-like omnivores, and some were even bear-sized carnivores.

An illustration showing five different "condylarths". On the top row is Hyopsodus, a small guinea-pig-like animal with a long horse-like head; then Meniscotherium, a slightly larger animal that somewhat resembles a capybara; then the much larger Arctocyon, which has a slightly bear-like body, hoofed towes, and a dog-like head. On the bottom row is Ectoconus, a small animal with a tapir-like body and a blunt rectangular snout; then Mesonyx, a much larger dog-like animal with hoofed toes and a long tail.
From left to right, top row: Hyopsodus lepidus (hyopsodontid), Meniscotherium chamense (phenacodontid), Arctocyon primaevus (“arctocyonid”).
Bottom row: Ectoconus ditrigonus (periptychid), Mesonyx obtusidens (mesonychid)

It wasn’t even clear how the various different condylarth groups were actually related to each other. The best guess was that arctocyonids had arisen from within the “insectivores”, with a Protungulatum-like form as the common ancestor of all the other condylarths. Where exactly modern ungulates had then evolved from within the condylarths was also still uncertain.

An image of a diagram from a 1967 academic paper, showing a complicated attempt to figure out the evolutionary relationships of "insectivores", with many different group names linked by arrows. The presence of the "condylarths" Hyopsodontidae and Arctocyonidae is highlighted by red circles. For comparison next to it is the "Pepe Silvia" conspiracy wall meme.
This looks familiar
(Image sources: http://hdl.handle.net/2246/358 & https://knowyourmeme.com/memes/pepe-silvia)

Cladistic analysis in the 1980s began to tackle the confusing pile of assorted condylarths, and showed that they weren’t the single ancestral source of all modern ungulates, but instead a loose collection of several unrelated groups from all over the ungulate evolutionary tree. Arctocyonids, periptychids, and hyopsodontids were placed as early “primitive” lineages, phenacodontids were loosely linked with the ancestors of odd-toed ungulates once again, and mesonychids were considered to be the ancestors of whales.

An image of a diagram from a 1994 academic paper, showing the proposed evolutionary relationships of various "condylarths" with main ungulate groups. It differs majorly from modern understanding by its inclusion of elephants, sirenians, hyraxes, and their extinct relatives, along with showing whales as descending from mesonychids instead of artiodactyls.
…And if you know modern mammal phylogeny you’ll probably see some big problems here. 🐘
(Image source: https://doi.org/10.1017/S2475263000001343)

And, once again paralleling the mess of the “insectivores”, it wasn’t until genetic methods became available in the late 1990s that larger-scale ungulate relationships began to be properly resolved. The paenungulates (elephants, hyraxes, and sirenians), which had been traditionally considered to be a major branch of ungulates, were removed entirely and reclassified as afrotheres. And, along with some new fossil discoveries, whales were recognized as having actually evolved from within the even-toed ungulates instead of from mesonychids.

This shake-up threw the still-problematic “condylarth” classifications back into question – with some “condylarths” turning out to also be afrotheres instead of true ungulates.

Today the actual relationships of the main “condylarth” ungulate families are still in the process of being figured out, and there’s a lot of remaining uncertainty and disagreement about them.

Phenacodontids seem to have mostly maintained their traditional position as early odd-toed ungulates, and hyopsodontids may potentially be part of this group too – possibly as members of the hippomorph lineage, closely related to horses and brontotheres. Arctocyonids might be a wastebasket themselves, with some studies finding them to be a mix of several different archaic ungulate lineages. Periptychids may have links to the even-toed ungulates. The mesonychids, meanwhile, are now generally considered to be a separate order from the traditional “condylarths”, and may be either an early branch of the even-toed ungulates or much more basal ungulates closely related to the “arctocyonids”.

Since the term “condylarth” no longer has any real taxonomic meaning some paleontologists have proposed replacing it with “archaic ungulate” to distance from the historical messiness of the old name. But this hasn’t really caught on, and many papers still use “condylarth” in a very loose sense to refer to an “evolutionary grade” of early ungulates of unclear evolutionary affinities.

A cladogram showing the modern classification of several different "condylarth" families. They're shown as potentially occupying positions throughout the branches of the even-toed and odd-toed ungulates.

And while that’s the last main entry for this month, we’re not quite done yet. There’s still one weekday left in October, and after digging through so many taxonomic garbage cans there’s only one place we can go now.

…See you in the trash heap.