Super-sized Rodent Re-Writes Family Tree

The skull and jaws from a pair of giant rodents, that represent an extinct species that could have weighed as much as half a tonne, are helping to re-shape the rodent family tree.  Study of these new fossils have led researchers to propose a formal revision of the three known species of the genus Isostylomys into just one species, Isostylomys laurillardi.

Isostylomys laurillardi

These are the best-preserved fossils to date of this extinct group, which was previously known only from skull fragments and individual teeth, the scientists report in a new study, published in the “Journal of Systematic Palaeontology”,

The new fossils of the two rodents, an adult and a juvenile, paint a more complete picture of these extinct and massive rat-like animals.  For example, the fossil discoveries raised questions about how these giant rodents were classified within their genus, and hint that several species that were thought to be related may instead be a single species.  The fossils add to our knowledge regarding giant members of the Dinomyidae family and is helping palaeontologists to reappraise the phylogeny of this once diverse and speciose group of South American mammals.

The Giant Miocene Rodent Isostylomys laurillardi (Adult and Juvenile)

Isostylomys laurillardi (adult and juvenile).

Picture credit: Renzo Vaira/Taylor & Francis

For models and replicas of prehistoric animals: Prehistoric Animal Models and Figures.

Isostylomys laurillardi Fossils from Southern Uruguay

The fossil material comes from the exposed cliffs in the Río de la Plata coastal region of southern Uruguay.  The fossils have been dated to the Miocene Epoch (9.5 to 10 million-years-ago approximately).  The researchers, including lead author, Dr Andres Rinderknecht of the Museo Nacional de Historia Natural (Uruguay), examined the teeth and skulls of fossil specimens, comparing them to the bones and teeth of the largest living rodent the Capybara (Hydrochoerus hydrochaeris).

The research team conclude that, due to similarities in the adult’s and the juvenile’s teeth structure, previously found fossils, which were smaller and thought to belong to a different species, were in fact from the same species.

Skull and Jaw Fossils of Isostylomys laurillardi

Skull in ventral view (A), skull and mandible in left lateral view (B), and mandible in occlusal view (C).

Picture credit: Taylor & Francis

The picture above shows the adult skull in ventral view seen from underneath, (A), and the skull and jaw viewed from the side (B).  Picture (C) shows the jaw in occlusal view, the scale bar is five centimetres.

Revising the Taxonomy

The authors have consequently proposed that members of the subfamily Gyriabrinae could represent juveniles belonging to other subfamilies of Dinomyidae and that three known species of the genus Isostylomys should be merged into just one species, Isostylomys laurillardi.

Commenting on the team’s conclusions, Dr Rinderknecht stated:

“Our study shows how the world’s largest fossil rodents grow and we describe fossil remains of a giant rodent baby and an adult.  Comparing them we conclude that from very young the giant rodents already were very similar to the adults which allows us to deduce that the great majority of the hypotheses before posed were wrong.  The juvenile and the adult analysed here represent some of the largest rodents known to science with some of these animals weighing almost a ton.”

The Giant Incisor of the Adult Isostylomys

MNHN 2187 the giant lower right incisor of (Isostylomys),

Picture credit: Taylor & Francis

The adult remains found consist of an almost complete skull with a partial jaw, while the juvenile’s remains are of a complete lower jaw and the right calcaneum (heel bone).  Almost all previous discoveries of this kind have consisted of isolated teeth, and small fragments of skulls or jaws, which make this discovery some of the best-preserved remains of giant dinomids known to science.

“Making a Giant Rodent: Cranial Anatomy and Ontogenetic Development in the genus Isostylomys (Mammalia, Hystricognathi, Dinomyidae)”.

By Andrés Rinderknecht, Enrique Bostelmann and Martin Ubilla, published by Taylor and Francis.

The scientific paper: Access the scientific paper here.

Visit the award-winning Everything Dinosaur website: Everything Dinosaur.

“Tully Monster” Mystery Not Solved

Back in March 2016, Everything Dinosaur published an article on the enigmatic “Tully Monster” (Tullimonstrum gregarium), a bizarre creature of coastal waters and estuaries, first described in 1966, but not classified until fifty years later.  Trouble is, the proposal that this thirty-centimetre-long, stalk-eyed animal was a vertebrate has been challenged.

Tullimonstrum gregarium

It seems that the mystery surrounding how to classify the State fossil of Illinois (fossils have only been found in the Mazon Creek area of Illinois and nowhere else in the world), has not been solved.

The Bizarre Carboniferous Marine Animal Tullimonstrum gregarium

The Paleo-Creatures Tullimonstrum replica.

Picture credit: Everything Dinosaur/Paleo-Creatures

The Taxonomic Classification of the “Tully Monster”

Two separate recent studies had proposed that the “Tully Monster” was a soft-bodied vertebrate, one that is related to extant jawless fishes such as the very slimy Hagfish and the Lamprey (a fish, perhaps most famous for causing the demise of Henry I, the son of William the Conqueror, who died from a “surfeit of lampreys”.)

To read an article supporting the T. gregarium was a vertebrate theory: Tully Monster Riddle Solved.

Vertebrate Theory Challenged

Palaeobiologists at the University of Pennsylvania have challenged the idea that Tullimonstrum was a primitive fish with a proboscis.  Writing in the academic journal “Palaeontology”, lead author Assistant Professor Lauren Sallan, (Dept. of Earth and Environmental Science) and her co-workers refute the vertebrate hypothesis.

Lauren stated:

​​​​​​​”This animal doesn’t fit easily in classification because it’s so weird.  It has these eyes that are on stalks and it has this pincer at the end of a long proboscis and there’s even disagreement about which way is up.  But the last thing that the Tully monster could be is a fish.”

Sallan and colleagues, which include Robert Sansom (University of Manchester), postdoctoral researcher John Clarke, Zerina Johnason (Natural History Museum, London), Sam Giles (Oxford University), Ivan Sansom of the University of Birmingham and Philippe Janvier of France’s Muséum National d’Histoire Naturelle, postulate that that the two papers which seemingly settled the “Tully Monster” debate are flawed, failing to definitively classify it as a vertebrate.

Considering Enigmatic Fossils

Junior Research Fellow Giles explained:

“It’s important to incorporate all lines of evidence when considering enigmatic fossils: anatomical, preservational and comparative.  Applying that standard to the Tully Monster argues strongly against a vertebrate identity.”

The Ancient Waters of the Mazon Creek Area 300 Million Years Ago

Two “Tully Monsters” in the shallow waters of Mazon Creek.

Picture credit: John Megahan

“Tully Monster” Riddle

The “Tully Monster” has been known since the 1950s, when the first fossils were found in the Mazon Creek fossil beds in central Illinois.  Since then, thousands of specimens have been identified from this locality, but they are not found anywhere else in the world.

Looking at the history of classification, Sallan explained that at first it was thought to be some sort of marine worm, then it was proposed that it was a form of mollusc, like a sea-slug.  Other palaeontologists have argued that this strange little animal has affinities with the Arthropoda, perhaps a distant relative of today’s shrimp.  Then in 2016, two research papers were published proposing that T. gregarium was actually a member of the back-boned group of animals – a vertebrate.

For models and replicas of prehistoric animals: Prehistoric Animal Models, Replicas and Figures.

Evidence for the Vertebrate Theory

One study examined more than 1,200 Tullimonstrum fossils.  In some, the researchers observed a light band running down the creature’s mid-line, which they interpreted as a notochord, a kind of primitive backbone.  They also said it contained other internal organ structures, such as gill sacs, that identified it as a vertebrate, and that the animal’s teeth resembled those of a lamprey.

Assistant Professor Sallan and her colleagues noted that these conclusions are based on a misunderstanding of how fossils in Mazon Creek are preserved.  The “Tully Monster” samples come from what was once a muddy estuarine area and at Mazon Creek, internal soft tissues are very rarely preserved.  Lamprey fossils have been found in the Mazon Creek area and these fossils are markedly different from their supposed relative the Tullimonstrum.

Studying the Eyes

In the other 2016 study, the researchers reported that scanning electron microscope images of the eyes had revealed structures called melanosomes, which produce and store melanin.  That paper’s authors argued that the complex tissue structure they saw in the animals’ eyes indicated it was likely a vertebrate.  The Pennsylvania University led team counter this point by arguing that many arthropods and molluscs, also have complex eyes.

Sallan added:

“Eyes have evolved dozens of times.  It’s not too much of a leap to imagine Tully Monsters could have evolved an eye that resembled a vertebrate eye.”

Based on Sallan and her colleagues’ examination of Tullimonstrum eyes, these creatures in fact possess what is known as a cup eye, a relatively simpler structure that lacks a lens.

“So the problem is, if it does have cup eyes, then it can’t be a vertebrate because all vertebrates either have more complex eyes than that or they secondarily lost them.  But lots of other things have cup eyes, like primitive chordates, molluscs and certain types of worms,” the Assistant Professor stated.

More Than a Thousand Specimens Studied

In this research, not one of more than a thousand fossil specimens studied appeared to possess structures that are believed to be universal in aquatic vertebrates, namely otic capsules, components of the ear that allow animals to balance and a lateral line, a sensory structure along the flanks that enables fishes to orient themselves and to detect their surroundings.

Comparing the Anatomy of Tullimonstrum to Other Creatures

Comparing the “Tully Monster” to other creatures.

Picture credit: Pennsylvania University

Lauren went onto state:

“You would expect at least a handful of the specimens to have preserved these structures.  Not only does this creature have things that should not be preserved in vertebrates, it doesn’t have things that absolutely should be preserved.”

The researchers said that an improper classification of such an unusual species has ripple effects on the larger field of evolution.

“Having this kind of misassignment really affects our understanding of vertebrate evolution and vertebrate diversity at this given time.  It makes it harder to get at how things are changing in response to an ecosystem if you have this outlier.  And though of course there are outliers in the fossil record, there are plenty of weird things and that’s great, if you’re going to make extraordinary claims, you need extraordinary evidence.”

It looks like, for the time being at least the “Tully Monster” has lost its backbone.

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