Stable Isotopes and the Ecology of Endothiodon

It's been a pretty dry year for dicynodonts so far. There hasn't been much in the way of new dicynodont-centric research to write about, so I've been re-writing few post ideas scribbled drafted before Double Dog Teeth was made to compensate. But fortuitously, a new paper was published at the end of last month to help break up that dry spell! And what's more, it concerns the ecology and lifestyle of one of the strangest dicynodonts out there.

Outside of specialist literature, the diet and lifestyle of dicynodonts is often broadly simplified down to "herbivorous" and rarely elaborated upon. However, there is much more nuance to dicynodont diets than this, as evidenced by the variation in their jaws, teeth, and beaks which implies that many dicynodonts were often specialised in how they acquired and processed their food, and so were almost certainly feeding on different specific foodstuffs. One of the most puzzling in this regard is the large and tuskless-but-toothy Endothiodon bathystoma from the middle Permian of South Africa, Malawi, Mozambique, Tanzania, Zambia, India and Brazil (it got around).

The strange, strange skull of Endothiodon.
Ghedoghedo, public domain.

Endothiodon has been a bit of enigma regarding its diet because its jaws are so unlike any other known dicynodont. It flies in the face of conventional dicynodont anatomy by possessing prominent rows of post-canine teeth in both its upper and lower jaws, that were constantly and rapidly being replaced to boot (and in waves, no less!). Calling them post-canine teeth is a bit of misnomer though, since Endothiodon bathystoma lacks 'canines' (i.e. tusks) altogether,* and indeed the tooth-row in the upper jaw smoothly continues partly onto the back end of the premaxilla for an extra two teeth as if tusks were never there.

Endothiodon still has a beak at the tips of its jaws, but its lower beak is remarkably deep and hooks upwards into a very prominent point that slots into a vaulted palate. What's more, the teeth of Endothiodon seem to change shape as they grow up, going from simple, conical shapes (similar to the postcanines in other dicynodonts that have them) to being compressed and serrated, leading some authors to suggest that Endothiodon were more omnivorous as juveniles, possibly feeding on insects and other invertebrates before transitioning to full herbivory as adults.

(*I specify Endothiodon bathystoma on account of the recently described Tanzanian species Endothiodon tolani actually having (incipient) tusks. Who'd'a' thunk?)

Naturally, the peculiarities of Endothiodon has led to much speculation and analyses of its anatomy to try and figure out just what it was doing with itself. Suggestions for its diet have included using its strange hooked beak for grubbing up roots, vegetables and invertebrates from the soil like a garden hoe, a specialist browser of dense, high quality riparian vegetation, and even a highly specialised diet of conifer cone seeds that it extracted with its strangely shaped jaws and vaulted palate. All of these previous analyses have been derived from the animal's functional morphology alone. A newly released paper by Kévin Rey and colleagues (Rey et al., 2020) published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology tackles this conundrum with a novel technique: stable isotope analysis.

A quick rundown for the uninitiated: stable isotopes are variants of chemical elements that do not decay or alter into another isotope or element over time. These isotopes are present in the living tissues of organisms, and the ratio of isotopes for a particular element depend on the conditions that the organism lived in. Because they don't decay, these isotopes remain in these concentrations even after fossilisation, meaning that they record the isotope values from when the animal was alive. In analysing fossils, the values of oxygen-18 (¹⁸O) and carbon-13 (¹³C) relate to the water-content of animals and the vegetation consumed by them (or in the case of predators, the ¹³C signatures associated with their prey), respectively. These kinds of analyses have probably had the most publicity from their utility in determining the lifestyle and diet of dinosaurs, such as implying semi-aquatic habits for spinosaurs, and the recent identification of niche partitioning between juvenile and adult Deinonychus. As is often the case, this relatively new method for analysing fossils was pioneered with dinosaurs, and has since been trickling its way out into the study of other fossil groups, and here we are with dicynodonts.

Rey et al. (2020) studied the stable isotope values of six Endothiodon specimens spanning different age groups, nine specimens of the more standard dicynodont Tropidostoma, and an unidentified pareiasaur (a parareptile) for good measure. Curiously, Endothiodon was found to have lower δ¹⁸O values consistent with a higher water turnover in its body, meaning Endothiodon was either a frequent drinker or spent a lot of time around water, or was perhaps even semi-aquatic, similar to living hippos. Whichever way, it suggests Endothiodon was closely tied to water and had to occupy habitats where it was constantly available. The difference in oxygen isotope values compared to the pareiasaur is particularly interesting in light of other recent studies that found the stable isotopes of contemporary pareiasaurs, at times suggested to be semi-aquatic, to be more like those of terrestrial animals.

A reconstruction of the Usili Formation in Tanzania by Emilio López-Rolandi, featuring Endothiodon, a gorgonopsian, and the archosauromorph Aenigmastropheus in the foreground. It's a gorgeous painting, but perhaps doesn't represent the kind of habitat where you'd likely find Endothiodon. CC BY 2.5

As for the carbon isotopes, they also record a significantly lower percentage of ¹³C than in Tropidostoma. Now, there is the possibility that such a difference was due to changes in vegetation over time, as Tropidostoma is known from the younger rock layers above those of the studied Endothiodon. However, ¹³C isotope analyses of the little burrowing dicynodont Diictodon from both rock units were available to act as a sort of control to compare them to, and assuming the diet of Diictodon didn't drastically change over that time, they record a decreasing trend in the ¹³C value of plants, meaning that the disparity between Endothiodon and Tropidostoma was likely even greater!

So what does that mean? For one, it could rule out the hypotheses that Endothiodon specialised in conifer cones, roots, tubers, or any other heterotrophic (non-photosynthesising) parts of plants (i.e. the roots, tubers, seeds, and fruits), as these regions are enriched in ¹³C compared to the leaves. Instead, these values may support the hypothesis that Endothiodon was specialised for feeding on soft riparian vegetation, which have lower ¹³C values compared to vegetation found in drier environments away from rivers and other sources of water. This would explain the disparity between Endothiodon and the more typical, browsing Tropidostoma, and is also consistent with the oxygen signatures suggesting a close affinity with water. Altogether, it paints a picture of Endothiodon as a water-dependent herbivore with a preference for soft, probably riparian vegetation that presumably spent much of its time around rivers and lakes, perhaps even in them.

Admittedly, this still doesn't answer the question of why Endothiodon was so weird. Was there a particular kind of riparian vegetation that it was feeding on that led to its peculiar jaws and teeth? Did this require it to feed in a particular way? The prospect of Endothiodon being semi-aquatic and feeding in or under water is tempting, although I have to admit that it doesn't look particularly specialised for doing so. Its nostrils and eyes aren't set particularly high up on the skull, and there's not much in the way of its skeleton to suggest it was a good swimmer, or even designed to be negatively buoyant. That being said, those deep lower jaws and vaulted palate are both features associated with large tongues and suction feeding in other extinct and extant tetrapods that are known or are suggested to feed in water, including some without any aquatic adaptions at all (Deinocheirus, anyone?), so maybe there's something to this after all...?

Of course, that's all frivolous speculation, and I don't want to turn this into a place for pushing unorthodox ideas about dicynodonts, but hey, maybe?

Endothiodon bathystoma, reconstructed as a water-loving browser of riverside vegetation (apologies to any palaeobotanists for whatever vegetative abomination I conjured up here, I am very naive to Permian vegetation).

There's clearly much more to learn about Endothiodon, and a lot more worth writing about as well (I didn't even cover what its inferred jaw mechanics, inner ear, and postcrania suggest about its lifestyle), so Endothiodon will no doubt show up again on the blog some time in the future. There's also still much to learn from stable isotope analyses on dicynodonts. Relatively few species of them have been studied in this way so far, with even fewer having been used to look into their detailed ecologies, and even preliminary studies like this one can turn up some intriguing possibilities.

References

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Rey, K., Day, M. O., Amiot, R., Fourel, F., Luyt, J., Van den Brandt, M. J., Lécuyer, C., Rubidge, B. S. 2019. Oxygen isotopes and ecological inferences of Permian (Guadalupian) tetrapods from the main Karoo Basin of South Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 538.

Rey, K., Day, M. O., Amiot, R., Fourel, F., Luyt, J., Lécuyer, C., Rubidge, B. S. 2020. Stable isotopes (δ¹⁸O and δ¹³C) give new perspective on the ecology and diet of Endothiodon bathystoma (Therapsida, Dicynodontia) from the late Permian of the South African Karoo Basin. Palaeogeography, Palaeoclimatology, Palaeoecology, In press.