Rifling through the ‘wastebaskets’ of palaeontology reveals surprises

0
84

The jaw of a Periptychid, an extinct condylarth mammal. From New Mexico. Photograph: Sarah Shelley

New fossils are announced with delightful regularity. Some of them are novel to science, or they provide information about groups we previously knew little about, while others are just downright weird. But what happens to fossils that have so few identifying features, palaeontologists can’t figure out where they belong in the history of life. Where do they go?

Often, these enigmatic fossils end up as “wastebasket taxa”.

A taxon (plural: taxa) is a taxonomic grouping, such as species, family or order. Wastebasket taxa are the groupings fossils are slung into when no one can figure out where else to put them. It’s more common than you’d think, and usually happens because there aren’t enough skeletal features – called characters by palaeontologists – preserved to show how a specimen is related to other animal groups, or to distinguish it from similar fossils. Sometimes only half a creature survives in the fossil record – and it might be the wrong half for comparisons with its closest relatives. For many fossils, half an animal would be a dream: often only a few scattered bones or teeth make it through the ringer of geological time intact. Some of these remains are so indistinguishable that they are grouped purely by where they were found.

Take for example, the elusive early branches of the modern mammal family tree, after the extinction of the dinosaurs. Let’s start with the ancestors of Carnivora. I say Carnivora – the group to which the likes of cats, dogs, weasels, bears, hyaenas, meerkats and seals belong – to distinguish them from carnivores: any animal that eats meat. Not all the members of Carnivora are carnivores, and not all flesh-eating carnivores are in the family Carnivora. Giant pandas for example (Ailuropoda melanoleuca) are infamous bamboo munchers, yet belong to Carnivora. Crocodiles (like Crocodylus acutus) eat meat, but as I’m sure you’ve noticed, they are not closely related to cats or dogs.

Crocodile, lion and wolf: all carnivores, but only two are in the order Carnivora. Composite: Getty/Barcroft Media/Alamy

The modern family Carnivora descend from somewhere among the mish-mash of extinct animals called miacids. Miacids were long-bodied, had long tails, were pine marten sized or smaller, and some lived in trees while others spent their time on the ground. These creatures are so anatomically similar it’s proven hard to figure out their relationships, both with one another and with later carnivoran groups.

A recent paper describes two fossils originally tossed into the miacid wastebasket, and pulls them out to instate them as early ancestors of the fearsome “bear-dogs”.

Bear-dogs are not to be confused with “dog-bears” – a popular cause of amusement among palaeontology students. Bear-dogs, or amphicyonids to give them their proper name, are members of the Carnivora and related to true dogs, but comprise their own family separate from them. Although now extinct, for over 30 million years amphicyonids predated their way around the Northern continents and Africa, successfully exploiting niches from more slender, wolf-like runners, to big stocky brutes that put the “bear” in “bear-dog”.

Comparison of the jawbone of the newly reclassified Angelarctocyon australis (left) with the jawbone of a larger beardog, Amphicyon riggsi (right), that lived around 22 million years after A. australis. Photograph: Susumu Tomiya/The Field Museum/AP

Previous researchers suggested that one of the specimens, Miacis cognitus, might be related to an early amphicyonid, based on characters in the skeleton. Tomiya and Tseng’s new study involved a full analysis of many miacids and amphicyonids. They found both Miacis cognitus and M. australis were actually early bear-dogs. They renamed them Gustafsonia cognitus and Angelarctocyon australis respectively – just try saying that after a few beers. And so another piece of the carnivoran origin puzzle falls into place.

Appearing a few million years before miacids, the condylarths are an even more cryptic and jumbled mess of mammals. While it’s clear they are the precursors to the hoofed mammals known as ungulates (e.g. horses, cows, hippos), palaeontologists have been flytipping indistinguishable early ungulates into Condylarthra for years.

“Even the simple task of defining a ‘condylarth’ is extremely difficult,” Sarah Shelley, a PhD student at the University of Edinburgh who specialises in winnowing condylarths, tells me. “There don’t appear to be many morphological features of their teeth or skeletons that unite all condylarths together.”

The group Condylarthra was established during the “Bone Wars”: a bitter Victorian rivalry between North American palaeontologists Edward Drinker Cope and Othniel Charles Marsh. Their underhanded squabbles made headlines – and led to some overzealous fossil naming in the constant battle for one-upmanship. Cope named Condylarthra, and people have been trying to sort out the mess ever since.

Othniel Charles Marsh (left) and Edward Drinker Cope (right). Palaeontologists from the late 1800s, their bitter rivalry was known as the “Bone Wars” Photograph: Mathew Brady/ Frederick Gutekunst/Wikimedia Commons/Public Domain

“Resolving the relationships of Condylarthra is key to understanding how severely mammals were affected by the end-Cretaceous mass extinction,” Shelley explains, “and how rapidly they recovered.” The focus of her PhD is salvaging one condylarth in particular: Periptychus. Shelley is in the final stages of her research, and will soon be publishing the results of her in-depth re-examination of this beast from the dawn of modern mammals.

Closer to home but about 130 million years older, British marine reptiles have been getting similar attention from Dean Lomax, Honorary Scientist at The University of Manchester.

“Wastebasket taxa paint an incorrect picture of the stratigraphical and geographical range of a given species,” says Lomax, “they are a major problem.”

Lomax and Massare’s paper, out last month in Papers in Palaeontology, is the result of ongoing work to prise apart the aquatic jumble of ichthyosaurs: fully marine reptiles that thrived in our seas at the same time as dinosaurs dominated the land. Ichthyosaurs successfully exploited niches similar to today’s large fish and marine mammals, such as tuna and dolphins. Since the Victorians began amassing their fossil collections over 200 years ago, a number of anatomically tricksy ichthyosaurs have been lazily chucked in the skip labelled Ichthyosaurus communis.

“Until my recent studies, three Ichthyosaurus species were considered valid,” explained Lomax, “everything else was considered to be I. communis. This included several hundred specimens. So far our reassessments have resulted in the discovery of three new species, I. anningae, I. larkini and I. somersetensis, that were previously considered within the variation of I. communis.”

Dean Lomax examining historical collections of ichthyosaurs in the Natural History Museum in London. Photograph: Dean Lomax

One of the fossils, I. larkini, has been on display in the University of Bristol School of Earth Sciences for decades: “it’s quite amazing – hundreds of people must walk past this skeleton every day,” said Lomax, “including palaeontologists!”

In the world of the taxonomic wastebasket, it takes the patience and expertise of a specialist to figure out what features of the skeleton might redeem a fossil from homogeny. This is why palaeontologists re-appraise fossils, particularly from historical collections. As scientific knowledge is incrementally built upon, there is much information to be gleaned with a pair of fresh eyes and modern technology at our disposal.

You might think that more recent animals like the miacids (60-30 million years old, to a geologist that’s like last week), would have survived the destruction of time better than a 200 million year old ichthyosaur, but annoyingly it doesn’t work that way. Some of the least understood creatures can be quite recent, while some of the best known can be the oldest. It’s down to where the animals lived and died, the composition of the rock, and what’s happened to it since then. It’s also down to chance.

Palaeontologists continue to rifle through the rubbish bins of taxonomy, separating out previously muddled taxa. They hunt for the skeletal markers that give these fossils a place in the family tree of life on earth. It is the painstaking, often tedious work of the specialist, but the result is a clearer picture of the patterns of evolution, and adds to our appreciation of the origins and diversity of life through the many lost worlds of geological time.

References

Lomax, D. R. and Massare, J. M. 2016. Two new species of Ichthyosaurus from the lowermost Jurassic (Hettangian) of Somerset, England. Papers in Palaeontology, http://dx.doi.org/10.1002/spp2.1065

Tomiya S., and Tseng Z. J. 2016 Whence the beardogs? Reappraisal of the Middle to Late Eocene ‘Miacis’ from Texas, USA, and the origin of Amphicyonidae (Mammalia, Carnivora). Royal Society Open Science. DOI: 10.1098/rsos.160518
http://rsos.royalsocietypublishing.org/content/3/10/160518