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Wednesday, 31 August 2016

New paper: at last, a small pterosaur species from the latest Cretaceous

As the Cretaceous fossil record enters its final two stages - the Campanian and Maastrichtian - several unusual things seem to happen in the world of flying reptiles. Firstly, we see the end result of a steady drop off in pterosaur diversity leaving only azhdarchids - those often long-necked, long faced animals that we cover here with some regularity - with a strong, widespread fossil record. It's known that nyctosaurids and (thanks to recent discoveries) perhaps pteranodontids survived until the very end of the Mesozoic in at least two locations, but azhdarchids are globally distributed and dominate the pterosaur fossil record at this time. The overwhelming precedence of azhdarchids in the Late Cretaceous is an anomaly: at no other point in the pterosaur fossil record does one clade feature so prominently.

Secondly, Campanian and Maastrichtian pterosaurs are, without exception, pretty big animals. Many species from this time are renowned for their gigantic size: it's these stages which give us the famous 10 m wingspan, 250 kg colossi like Quetzalcoatlus, Arambourgiania and Hatzegopteryx, as well as a number of other giant azhdarchids which are too poorly known for generic titles. Coinciding with the evolution of the giants is a loss of small pterosaur taxa - those animals less than 2.5 m across the wings which are present, more-or-less, throughout the rest of pterosaur history. This shift in body size is sometimes interpreted as pterosaurs demonstrating 'Cope's Rule', the somewhat controversial proposal that species evolve towards large body size over time (Hone and Benton 2007; Benson et al. 2014). It's argued by some that competition from birds may be the driver behind this trend, as early avians fought small flying reptiles for ecological space and ultimately forced pterosaurs into larger sizes (e.g. Benson et al. 2014). Note that this concept is not without its detractors, including myself - I won't go into my reasons now but I plan to outline them here eventually.

Whether you agree with the bird-pterosaur competitive displacement hypothesis or not, we can't disagree that the end of the Cretaceous is almost entirely devoid of small pterosaur remains. Only a handful of specimens record small pterosaurs in the Campanian and Maastrichtian, and they're all tricky to work with. Aside from being highly fragmentary, some are controversially identified (such as Piksi barbarulna, an alleged small pterosaur from the Two Medicine Formation - see Agnolin and Varricho 2012 for the pro-pterosaur case) and others represent probable juvenile individuals (Godfrey and Currie 2005). Whatever it signifies, the lack of diminutive pterosaur specimens from the close of the Mesozoic is a real phenomenon of our fossil record, and any new specimen of a small, latest Cretaceous flying reptile has to be something to get excited about.

Enter: a new small, latest Cretaceous pterosaur specimen to get excited about

Title slide of my SVPCA 2016 talk, discussing the findings of Martin-Silverstone et al. 2016, out today. If you don't get the reference, you clearly get out too much, have too many friends and aren't watching enough crap TV.
It's this point where a new paper, published today by Liz Martin-Silverstone, myself, Victoria Arbour and Phil Currie comes in. Our new work, which you can check out without restriction at the open access journal Royal Society Open Science, presents a new small pterosaur fossil from the Campanian Northumberland Formation of British Columbia. The specimen number - RBCM.EH.2009.019.0001 - is pretty unwieldy, so I've been calling it the 'Hornby azhdarchoid' or the 'Hornby pterosaur' after it's discovery on Hornby Island, just off the coast of Vancouver. As you can see  below, the Hornby specimen is not pretty. Following our presentation of the fossil at SVPCA 2016, pterosaur guru David Unwin suggested we might have the ugliest pterosaur fossil on record (or at least tied the game). But while not well preserved, we do at least have several bones to play with: most of a humerus, three fused vertebrae (from the notarium, a set of fused shoulder vertebral elements), a few loose dorsal vertebrae and some other odds and ends that defy identification. This makes it the first set of associated bones of a small latest Cretaceous pterosaur, which is at least a step in the right direction for their paltry fossil record. For reasons discussed in the paper (concerning taphonomy, element size and likely identifications) we assume these remains represent one individual.

RBCM.EH.2009.019.0001, a fragmentary azhdarchoid pterosaur from the Campanian Northumberland Formation, British Columbia. It's, er, not the prettiest pterosaur specimen you'll ever see. Combination of figures from Martin-Silverstone et al. 2016.
I don't want to rehash the full gory details of our study here - please read the paper for the technical aspects - but instead want to outline our main points. The first thing to clear is that we've been careful to rule out an avian ID for the specimen. The Northumberland Formation contains several bird fossils and the quality of the specimen means that many obvious pterosaur features are missing. The Royal British Columbia Museum was kind enough to ship the specimen all the way from Vancouver, Canada to Southampton, UK just so Liz and I make a thorough assessment on this issue. Happily, we found the specimen to be very pterosaur like in every aspect (even as fragments, pterosaur bones are quite distinctive) as well as differing from Mesozoic birds in several ways. It particularly contrasts in having a notarium, which seem absent from Mesozoic birds (note that we compared the notarium element compared carefully with Mesozoic bird synsacra to be sure of our identification), as well as having a pterosaur-like, rather than avian, proximal humerus morphology. But we're not bird workers so, to be extra sure, we showed the material to fossil bird experts in Canada and the UK (including people who've identified and published on the Northumberland Formation avians). No-one we spoke to suggested an avian ID and, moreover, we are aware that other people with expertise in both birds and pterosaurs (including our paper editor) have seen the material and prefer a pterosaur ID. Based on our research and the testimonials of others, we're as confident as we can be that the Hornby fragments represent a pterosaur, not a bird.

We've identified the Hornby specimen as an azhdarchoid, and noted several features indicative of, but not conclusive to, an azhdarchid ID. We suspect the specimen is an azhdarchid because of its provenance and its basic anatomical characteristics, but the specimen does not contain the right bits to confirm an azhdarchid identity. Nonetheless, narrowing the specimen down to Azhdarchoidea allows us to estimate its body proportions and confirm that the specimen was indeed a small animal when it died. We estimated its wingspan using two methods factoring both the humerus and vertebrae, and each pointed to a wingspan between 1.4 and 1.6 m. That puts our pterosaur at a comparable size to a good sized-seagull and, while these are respectably-sized modern birds, this is small for a latest Cretaceous pterosaur. Rather than poking giraffes in the face, our little chap would only just be beyond predation risk from an average housecat (below). The only contemporary pterosaur competing with the Hornby azhdarchoid for size is Piksi, a poorly known possible pterosaur from the western US. Our new study lists a number of reasons why the pterosaurian characterisation of Piksi is problematic however: in short, its morphology is all wrong for a flying reptile and we suspect a non-pterosaurian ID is more likely. The Hornby specimen is thus a contender for the smallest latest Cretaceous pterosaur currently known.

A 1.5 m wingspan azhdarchoid next to one (SI) MrTiddlesmetre. From Martin-Silverstone et al. (2016).
This million dollar question, of course, is whether the specimen is a small juvenile or a small adult. The former would be neat, but the latter is potentially significant. The findings of recent, detailed histological examinations of pterosaur fossils are permitting increasingly good understanding of their growth regimes (e.g. de Ricqles et al. 2000; Prondvai et al. 2012), so we made a section of the humerus to understand how old the Hornby animal was when it died. Our section showed a mix of bone textures, some indicating that the specimen was still growing, but other features (secondary osteons, an endosteal lamella, lines of arrested growth and a large structure forming on the internal bone surface) are indicative of relative maturity (de Ricqles et al. 2000; Prondvai et al. 2012). We found the endosteal lamella (a band of bone deposited around the internal bone cavity) of particular interest, as this seems to signify the end of internal bone expansion in azhdarchoids, and is thus a hallmark of near-mature animals (note that this is not true for all pterosaurs - see Prondvai et al. 2012). The fused dorsal vertebrae are a further marker of maturity, as pterosaurs do not develop these features until they're at least subadults. The exact timing of notarium formation seems to differ from taxon to taxon (e.g. Bennett 1993; Kellner 2015), but their development does not seem to start until these animals were near to full size, if not at full size already. Putting these and a few other observations together suggests that the Hornby pterosaur was a latest-stage juvenile or subadult: in other words, it looks like a genuinely small pterosaur, not just a juvenile one. We don't know how much larger it might have got before it reached full size, but its ontogenetic characteristics and what we know of pterosaur growth regimes suggests it was close to maximum size at time of death. Given its estimated 1.5 m wingspan, it had a good chance of remaining smaller than the next smallest, 2.5 m wingspan pterosaur currently known the Campanian or Maastrichtian (McGowen et al. 2002).

What's inside the RBCM.EH.2009.019.0001 humerus? A mix of things, but among them are features indicative of late-stage juvenility/subadulthood. Please see the paper for details of this figure. From Martin-Silverstone et al. 2016.

A small pterosaur amongst the pigeons

There's obviously a limit to what a single fragmentary specimen can tell you about the evolution of a group, but what the Hornby specimen means for pterosaur evolution is interesting and - if we've interpreted it correctly - potentially significant. Most obviously, it suggests that small pterosaurs may have been present in the Campanian stage of the Late Cretaceous after all, at least in one part of the world. Regular readers will be aware that there's growing evidence for Late Cretaceous pterosaur faunas being less uniform than previously realised (e.g. Vremir et al. 2013, 2015), and our new specimen plugs into this picture nicely: it increasingly seems that the end Cretaceous wasn't just a stage for large-to-giant long-necked azhdarchids. What's more, while the specimen only provides one data point against the idea that birds ousted small pterosaurs, the presence of at least two types of bird in the Northumberland Formation seems to indicate small pterosaurs and birds coexisted in at least this palaeoenvironment. We might see this as a continuation of the coexistence pterosaurs and birds demonstrate in Jurassic and Early Cretaceous localities: maybe pterosaurs and birds got along OK after all.

...except when pterosaurs stole their eggs. Our PR art for the new paper, where a group of Hornby azhdarchoids perform guerrilla raids on shore-living Campanian bird nests. Take THAT, birds.
To my mind, one of the most significant things we do in the paper is discuss the 'face value' interpretations of Late Cretaceous pterosaur diversity: should we really be interpreting the lack of small pterosaur fossils as a genuine feature of their history when their fossil record is so patchy? We point out that some types of small pterosaurs - juveniles - had to exist in the Late Cretaceous, and yet their fossils are almost entirely unknown. We argue that this indicates a preservation bias against small bodied pterosaurs of any kind in the Campanian and Maastrichtian. Until we amass a good number of small juvenile pterosaur bones from this time without any small adults we cannot distinguish preservational interference from genuine biological signals. Perhaps the shift of pterosaurs from marine to non-marine habits through the Cretaceous (Butler et al. 2013) accounts for this lack of data. It's well known that terrestrial settings are less conducive to preserving relatively delicate fossils and small examples of even robust terrestrial animals like dinosaurs rarely fossilise in these deposits. We have to wonder what chance small pterosaur skeletons - which were strong in life, but fragile and weak once exposed to decay - have of making it into the fossil record in these settings. The fact the Hornby specimen is in such a sorry state perhaps reflects the rough time small pterosaur fossils experience under 'typical' fossilisation regimes, rather than the far gentler handling of animal remains evident at fossil Lagerstätten.

With all this said, the most important message of the paper has to be this: we need more data on small pterosaurs in the latest Cretaceous. The specimens we have are scrappy, hard to work with and offer limited scope for analysis. Thus, any small Late Cretaceous pterosaur material is significant, and whether they're lying unnoticed in museum collections or pulled straight out of the field, they are noteworthy specimens which need to be put on record. Curators and researchers, please keep your eyes peeled!

And that, in a nutshell, is our new paper: be sure to check it out if you want more details. You can also read Liz's take on the study over at The Conversation and other experts have been chiming in at news sites covering the story. With a bit of luck, this is not the only news you'll be hearing about Late Cretaceous pterosaurs from these quarters this year - more on these projects as they move along. All that's left to do is to thank Liz and Victoria for inviting me to collaborate with them on the new specimen - I learned a huge amount trying to get my head around this challenging material and its histology, and had a blast working with them.

This blogpost, paper and artwork are sponsored by Patreon

Regular readers will know that this blog and its art are sponsored by a suite of awesome Patrons, but this post is proof that this support goes further than mere internet tomfoolery and contributes to papers and outreach, too. Supporting my blog from $1 a month not only helps keep this blog ticking over, but helps me contribute thoughts, words and illustrations to scientific research. 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 talking about the PR art I've done for this research: how was the azhdarchoid reconstructed from that pile of rubble specimen? What's the story with the landscape image and why is there an ammonite in it? How many iterations did we go through to get that composition? Head over to Patreon to get access to this and the rest of my exclusive content!

References

  • Agnolin, F. L., & Varricchio, D. (2012). Systematic reinterpretation of Piksi barbarulna Varricchio, 2002 from the Two Medicine Formation (Upper Cretaceous) of Western USA (Montana) as a pterosaur rather than a bird. Geodiversitas, 34(4), 883-894.
  • Bennett, S. C. (1993). The ontogeny of Pteranodon and other pterosaurs. Paleobiology, 19(01), 92-106.
  • Benson, R. B., Frigot, R. A., Goswami, A., Andres, B., & Butler, R. J. (2014). Competition and constraint drove Cope's rule in the evolution of giant flying reptiles. Nature communications, 5, 3567.
  • Butler, R. J., Benson, R. B., & Barrett, P. M. (2013). Pterosaur diversity: untangling the influence of sampling biases, Lagerstätten, and genuine biodiversity signals. Palaeogeography, Palaeoclimatology, Palaeoecology, 372, 78-87.
  • Godfrey, S. J., & Currie, P. J. (2005). Pterosaurs. Dinosaur Provincial Park: A Spectacular Ancient Ecosystem Revealed, 292-311.
  • Hone, D. W. E., & Benton, M. J. (2007). Cope's Rule in the Pterosauria, and differing perceptions of Cope's Rule at different taxonomic levels. Journal of Evolutionary Biology, 20(3), 1164-1170.
  • Kellner, A. W. (2015). Comments on Triassic pterosaurs with discussion about ontogeny and description of new taxa. Anais da Academia Brasileira de Ciências, 87(2), 669-689.
  • Martin-Silverstone, E., Witton, M. P., Arbour, V. M, & Currie, P. J. (2016). A small azhdarchoid pterosaur from the latest Cretaceous, the age of flying giants. Royal Society Open Access, 3, 160333.
  • McGowen, M. R., Padian, K., De Sosa, M. A., & Harmon, R. J. (2002). Description of Montanazhdarcho minor, an azhdarchid pterosaur from the Two Medicine Formation (Campanian) of Montana. PaleoBios, 22(1), 1-9.
  • Prondvai, E., Stein, K., Ősi, A., & Sander, M. P. (2012). Life history of Rhamphorhynchus inferred from bone histology and the diversity of pterosaurian growth strategies. PLoS One, 7(2), e31392.
  • Vremir, M., Kellner, A. W., Naish, D., & Dyke, G. J. (2013). A new azhdarchid pterosaur from the Late Cretaceous of the Transylvanian Basin, Romania: implications for azhdarchid diversity and distribution. PLoS One, 8(1), e54268.
  • Vremir, M., Witton, M., Naish, D., Dyke, G., Brusatte, S. L., Norell, M., & Totoianu, R. (2015). A Medium-Sized Robust-Necked Azhdarchid Pterosaur (Pterodactyloidea: Azhdarchidae) from the Maastrichtian of Pui (Haţ eg Basin, Transylvania, Romania). American Museum Novitates, (3827), 1-16.

Friday, 12 August 2016

Trunk or no trunk, small or giant ears, long or short neck... what did the giant rhinocerotoid Paraceratherium really look like?

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.
Depictions of the giant indricotherines, relatives of modern rhinoceros that lived across mid- and eastern Asia during the Oligocene, have varied over time. We've known about these animals - which are part of a longer-lived (Eocene-Miocene) indricotherine lineage that includes a number of smaller, almost okapi-or horse-like species - for over 100 years and they have become regular fixtures in museums, books and those rare documentaries which offer glimpses into ancient life outside of the Mesozoic. Any yet, when we think of our favourite indricothere paintings - including those by our most celebrated mammalian palaeoartists such as Knight, Burian, Anton, and Buell - they often differ markedly in their depiction of these 15-20 tonne animals. Most notably, their neck proportions, overall robustness, the development of a proboscis or trunk, and - most recently - the size of the ears are all inconsistent. Why are these animals so differently depicted, and should we rule out some of the anatomies we've seen in palaeoart in the last century? Having faced these questions recently when asked to restore this animal myself (above), I thought I'd share some of what I learned in my research here.

The obligatory note on nomenclature

It 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.
A third contrasting reconstruction was published a few decades later by Gromova (1959), based on a composite mounted skeleton in the Paleontological Institute, Russian Academy of Sciences. This reconstruction, executed by N. Yanshinova, was accompanied by several wonderful muscle and skin reconstructions which palaeoart fans will not want to miss. Both the mount and reconstruction show a gracile, giraffe-like form with a remarkably long neck and, in being based on a relatively complete set of giant indricotherine remains, some have argued it is a superior take on indricotherine anatomy than those produced by Osborn, or Granger and Gregory (Fortelius and Kappelman 1993). The most striking aspect of this reconstruction is its very long neck. We have to stress that this is extrapolated from a few incomplete cervicals associated with postcranial material, and its exact length remains uncertain - a complete set of neck bones remains elusive for Paraceratherium. This is another reconstruction which has been quite influential (helped, no doubt, by its apparent basis for the BBC's Walking with Beasts 'Indricotherium') but, again, it has not escaped criticism. Paul (1997) suggested that multiple aspects of this mount and reconstruction were erroneous, including the length of the neck, the size of the pelvis and depth of the ribcage, the length of the feet, and the ratio of the humerus and femur, as well as the fully erect posture of the limbs.

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).
Paraceratherium skulls (above) meet these criteria well and, all else being equal, we have to say that yes, it looks likely that these giant rhinoceratoids had short proboscides in life, presumably to assist browsing from trees and bushes (Prothero 2013). The view that they had more typically rhinoceros-like faces is hard to defend in light of these cranial features: mammal skulls just don't have those retracted nasal openings, associated deep muscle scarring etc. unless they were doing something unusual and sophisticated with their upper lip and nasal tissues. The reality of giant indricotherines with dangly noses may seem hard to swallow for those of us used to shorter lipped versions, but given the relationships between rhinos and tapirs, the fact that some other fossil rhinocerotoids probably had proboscides as well (e.g. Wall 1980), and the independent development of long, flexible noses in numerous mammal lineages, we can't really see this as unusual. Moreover, we need to remember that modern rhinos are derived animals in their own right and separated from the indricotherine lineage by tens of millions of years. They aren't necessarily always going to be the best models for the life appearance of their fossil ancestors.

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.
Yikes, elephant ears? For those of us familiar with the history of indricotheres in art, where their ears are restored as typically rhinoceros-like, this is a shocking, double-take image. Within the book, Prothero justifies the restoration:

"...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.

Silhouettes of the largest land mammals of all time, Paraceratherium transouralicum and Palaeoloxodon namadicus. Note the relatively gracile build of Paraceratherium - all the better for improving surface area:volume ratio, and thus superior for radiating heat. The numbers at the base of the image refer to estimated shoulder heights and tonnage. From Larramendi (2016).
These facts suggest elephants should not be used as direct thermoregulatory models for a giant rhinoceratoid. Modern rhinos other perissodactyls are much more typical in their thermoregulatory approaches: they have sweat glands and use panting behaviours (Hiley 1977) as well as some special tactics, such as enhanced vascularisation in the skin folds of certain rhino species (Endo et al. 2009). We have to assume that indricotherines at least had these entry level perissodactyl adaptations and, if so, they would have an advantage over elephants in hot climates. Indricotherines also benefit from being more complicated in form than elephants. They have longer limbs and necks, as well as a proportionally smaller head, and this enhances their surface area:volume ratio. Again, makes them better adapted to cope with heat as they have a shape better suited to radiating excess body heat. And of course, there's no reason to assume this could not have been augmented with wrinkled or folded skin or sparse hair. The picture emerging from these points is that big ears are only one strategy that big animals may use to keep cool, and maybe one that will only arise in specific circumstances. The idea that indricotherines would have big ears just because of their size is far from certain.

Basic muscle layout and trajectories (arrowed lines) of a modern horse. Note their superficial attachment and position high on the head - the ear canal itself is about halfway down the back of the skull. The 's' is the scutiform cartilage, which hangs out in front of the ear over the jaw muscles. From Goldfinger (2004).
But isn't all this moot because of Prothero's (2013) observations about the mastoid and paroccipital processeses being expanded, and thus giving big ears something to hang off? I'm suspicious about the significance of this observation. So far as I can determine, neither the mastoid or paroccipital have anything to do with anchoring ear tissues in modern perissodactyls (or perhaps any mammal). This might be because in most mammals - primates being one obvious exception - the ear pinnae are vertically displaced from the ear canal and attach to the head via a series of muscles and cartilages at the top of the skull (above). Only select few of the ear muscles reach the skull directly and these anchor, with very small attachments, to the skull midline, dorsoposterior margin and zygomatic arch. The rest have no osteological connection at all, anchoring instead to cartilage, membranes overlying facial musculature, or even the side of saliva glands. The paraoccipital and mastoid processes do have important roles in the muscular system but these are to do with neck, jaw and tongue muscles, not ears. Thus, unless indrictotheres were doing something different to modern mammals, those particularly big processes behind their ear openings were probably more to do with supporting and moving the head than they were holding big ears, and may have little significance to the big-eared indricotherine hypothesis.

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 Patreon

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References

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