|Giant, Oligocene rhinocerotoids Paraceratherium transouralicum engage in some early morning flirting. Because, in rhino speak, playing hard to get involves shoulder barges and head-butts.|
The obligatory note on nomenclatureIt almost seems tradition that any article or paper on indricotherines requires an aside on their confused taxonomy. As has been the case for decades now, the taxonomy and systematic nomenclature of these giant rhinocerotoids are a matter of ongoing discussion. It is widely appreciated that several giant indricotherine species from roughly contemporaneous Oligocene Asian sediments can be identified, but how many species they represent, and how they are related to each other, is not clear. At least seven generic titles and many more species names have been given to the largest of these animals over the years (Indricotherium and Baluchitherium are perhaps the most famous generic labels), but some authors (e.g. Lucas and Sobus 1989, Prothero 2013) tidy all or most of these taxa into three species of the oldest established genus, Paraceratherium. Arguments persist, however, that at least one other, perhaps slightly smaller genus existed, Dzungariotherium (Qiu and Wang 2007). Geologically older indricotherine genera such as the Eocene Urtinotherium are also wrapped into these discussions as remains attributed to the Oligocene genera are sometimes argued as having greater affinity to these older taxa (Prothero 2013).
This confusion is sometimes framed as a 'lumper/splitter' philosophical distinction, but it does not help that the fossil record of these giant rhinocerotoids is far from exemplar: giant indricotherine specimens can be fragmentary, of starkly contrasting size with one another, and many suffer from distortion. The fact that 20th century indricotherine science developed with Asian and American teams working largely in isolation, with limited access to certain specimens and literature, has also contributed to the confused history of this group. Those interested in the history of indricothere taxonomy should check out Prothero (2013) for an overview. For now, it will serve us to simply state that the best known, biggest and most famous of these animals currently resides as the taxonomic address of Paraceratherium transouralicum. This is the species most of us think of as 'the' giant indriotherine as well as the taxon that has carried both the Indricotherium and Baluchitherium label at one time or another. It's also the focus of most artwork of giant rhinoceratoids, and thus forms our primary interest here.
Giant rhino, bulky giraffe or giant workhorse?One reason we see such variation in indricotherine appearance is that researchers have produced vastly different interpretations of its anatomy in the last 100 years. But unlike, say, dinosaurs, where older reconstructions have been (for the most part) abandoned in favour of newer, more accurate interpretations, educators and researchers continue to publish skeletal reconstructions published in the 20s and 30s despite our improved knowledge of indricotherine anatomy, documented criticisms of these older works, and the availability of more modern, theoretically better-informed reconstructions.
Many readers may be aware that the first reconstruction of Paraceratherium, published by Osborn (1923a), showed a form not too far off a giant rhinoceros - a heavyset, short-necked animal with a deep torso and short legs. Osborn published a revised version almost immediately after his first effort, which had a much longer neck and longer legs thanks to data provided by additional fossil material (Osborn 1923b). A shorter-necked version was then produced by Granger and Gregory (1935, 1936), who scaled the remains of numerous, differently-sized individuals from a range of collections to create their robust, gigantic take on indricotherine anatomy. Although this reconstruction has been quite influential, Fortelius and Kappelman (1993) have been critical of the scaling methods used by Granger and Gregory, calling their interpretation 'a highly speculative creation indeed'.
|Paraceratherium has been variably reconstructed over the years, with particular disagreement over how long the neck was compared to the body. So far as I can tell, a consensus on the life appearance of these animals has yet to be reached.|
And so we turn to another indricotherine skeletal reconstruction, produced by Paul (1997). This restoration incorporated data from the same specimens used in the efforts above and came out somewhat 'averaged' between the more heavyset restorations of the early 20th century and the gracile interpretation of the 1950s. It looks, in overall form, more like a giant workhorse than it does a giant rhino or bulky giraffe. Paul (1997) provides some discussion of the reconstruction process - this is worth a read if you're interested in the life appearance of Paraceratherium and its relatives. Paul's interpretation has, to my knowledge, escaped criticism to date and, to the contrary, Larramendi (2016) described this reconstruction as 'accurate', although did not elaborate on why it should be considered superior to older efforts.
The million dollar question here is obvious: which one of these different takes on Paraceratherium is 'right'? To be honest, I'm not sure. The situation is compounded by the fact that a lot of indricotherine literature is obscure, that the specimens fragmentary and that many of them await description. I was hoping that Donald Prothero's recent (2013) book Rhinoceros Giants, which is solely dedicated to Paraceratherium, would provide some insight on this matter, but it's not a great help here - it provides no real evaluation on the different reconstructions and does not even mention Paul's 1997 effort. My work above is primarily based on Paul's (1997) skeletal but this is largely because of principle rather than real insight. Paul's work is the most modern and, of course, he's made a career out of reliably reconstructing extinct animals. The brief endorsement from Larramendi (2016) helps here too, of course, but a longer discussion of the relative merits and detriments of each interpretation would be useful. Opinions from others with more insight into this matter are welcome in the comments below.
A tapir-like proboscis... on a rhino?Turning our attention to the face, did Paraceratherium and its relatives have relatively short-lipped faces like those of rhinos, or long, mobile proboscides like their more distant relatives, the tapirs? Despite mammal lips and nasal tissues being highly fleshly and thus only rarely entering the fossil record, this is a surprisingly easy question to answer. Whether rhino, tapir or anything else, a suite of osteological characters seem to correlate well with the presence of proboscides. Briefly summarised, these are: narrow snouts; retraction of the nasal openings towards the orbits; the presence of large muscle scars, bony knobs and other muscle attachment markers around the nasal opening (particularly in the dorsal region); retraction of the nasal bone (the 'roof' of of the nasal opening); deepening of the premaxillary bone (the bone making the jaw tip); anterior migration of the orbit; a large intraorbital canal (a foramen situated in the cheek region, just in front of the eye - it houses the nerves and blood vessels for our anterior face muscles); and strengthening of the posterior skull regions related to supporting the weight of the head on the neck (Wall 1980). Note that the criteria for elephant-like trunks are similar, but slightly different.
|Paraceratherium transouralicum (formerly Baluchitherium grangeri) skull in dorsal, lateral and ventral views. Note features around the skull anterior linked to proboscis development (see text). From Osborn (1923b).|
And big, elephant-like ears, right?Finally, let's tackle the component that everyone now mentions about indricotheres since seeing the Carl Buell's cover art for Donald Prothero's Rhinoceros Giants:
|Indiana University Press.|
"...indricotheres were larger in body mass than any living elephant and almost certainly had problems regulating their body heat at such large size. Elephants must do all they can to increase the surface area of their bodies to release as much excess heat as possible, which is why they have huge fan-like ears full of blood vessels that are essentially giant radiators. Given the huge size of indricotheres, it seems likely that they too should have had elephant-like ears, or at least very large ears of some shape, much larger than they are usually drawn."
Prothero, 2013, p. 90.
The text continues to suggest that this appearance is not without anatomical support, the prominence of the mastoid and paroccipital processes (projections of bone situated behind the ear opening, adjacent to the posterior surface of the skull) being similar to the condition in certain elephants and mastodonts, and therefore indicative of large, flappy ears (Prothero 2013).
I have mixed feelings about this reconstruction. I like it for two reasons. The first is that it's nice to see indricotheres being distanced from their depiction as giant, long-necked rhinoceroses - again, it's not unreasonable to think they may have looked quite different in to modern rhinocerotids in many aspects. I also like these ears for being an All Yesterdays-style speculation on soft-tissue adaptations in extinct species. If we can use this as an excuse to give fat stores to desert sauropods or fuzzy hides to Arctic ceratopsids, then we can give large ears to giant rhinoceratoids.
On the other hand, I'm not convinced that they're as likely as Rhinoceros Giants suggests. It's clear from our modern fauna that ear size does not correlate with body mass in terrestrial mammals. By this logic many rhinos and giraffes should have proportionally large ears too, which they evidently do not. We also have to consider that even larger animals than indricotheres, dinosaurs, almost certainly got by without giant ears to help lose heat. And yes, while dinosaurs may have used different metabolic strategies to mammals, one inescapable consequence of giant size is a constant high body temperature. At least some investigations into the proportions of large dinosaurs suggest that development of their features - such as sauropod necks - were not driven by thermoregulatory pressures (Henderson 2013).
We should also consider the unusual nature of elephant thermoregulation: they are not typical mammals when it comes to controlling body heat. For one, they're atypically compact compared to other large mammals because they have extremely short necks, giant, round heads, and big, rotund torsos. This is a suboptimal bauplan for thermoregulation because it minimises surface area with respect to volume, and thus reduces the available area for elephants to dump excess heat. Moreover, unlike most mammals, they lack sweat glands (Wright and Luck 1984), do not pant, and they live in climates which are so warm that for much of the day they cannot shed heat through simple convection, big ears or not (Weissenböck et al. 2012). Elephants can, of course, regulate their temperature, but they need to employ different strategies to the rest of us mammals. These include maintaining moist skin with mud bathing and trunk spraying (Wright and Luck 1984), maintaining a sparse set of body hair to aid thermal escape (Myhrvold et al. 2012), using heterothermy (Weissenböck et al. 2012), the development of 'thermal windows' in their skin (Weissenböck et al. 2010), having loose and highly wrinkled skin to boost surface area and - of course - fanning their blood-vessel rich ears to help lose heat, when ambient temperatures are low enough for this to make a difference.
So...Putting all this together, it seems that there might be less need for uncertainty about indricothere appearance than our various artworks suggest. We should be saying 'yes' to some sort of proboscis, and 'probably not' to big ears (or, at least, 'there's no reason for them'). The elephant (or, giant rhino, if you prefer) in the room is the proportion issue, and it would be good to see folks who really know rhinocerotoid anatomy pore over those various reconstructions to ascertain which (if any) are the best representation of indricotherine form.
Next time: either the Next Big (but also kinda small) Thing in pterosaur research, or another trip to the Triassic.
Big rhinos need big support - thank goodness for PatreonThe paintings and words featured here are sponsored by another group of (metaphorically) giant mammals, my Patreon backers. Supporting my blog from $1 a month helps me produce researched and detailed articles with paintings to accompany them, and in return you get access to bonus blog content: additional commentary, in-progress sneak-previews of paintings, high-resolution artwork, and even free prints. For this post, we'll be taking a further look at the anatomy of the Paracertherium in my painting, above. Why do they have little manes and stripy faces? Are those child rhinos at the back a bit fuzzy? And why do the main animals look like they're fighting? Head over, and sign up to Patreon to get access to this and the rest of my exclusive content!
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