Time to do more astrapotheres. In the preceding article, we more or less introduced astrapotheres, had a quick look at their diversity, and ran through some of the basal forms. Here we get to the good stuff on astrapotheriid astrapotheres, on lifestyles, and on that question that keeps us all awake at night: just what the hell are astrapotheres anyway?
Astrapotheriid astrapotheres – the astrapotheres with particularly big canine tusks, a specialised narial region and other characters – appear to consist of two clades: Astrapotheriinae and Uruguaytheriinae. Besides Astrapotherium, Astrapotheriinae includes the Parastrapotherium species and possibly Astrapothericulus iheringi. These taxa differ from other astrapotheriids in having a particularly big third upper molar. The least known of the three, Astrapothericulus, is from the Middle Miocene Pinturas Formation and is the smallest known Miocene astrapothere, though having said that I haven’t been able to find a size estimate in the literature. Best known from a lower jaw (a cranium is known but has yet to be described so far as I can tell), it differs from Astrapotherium in not only being smaller, but also in having more erect lower canine tusks, and in having a deeper, narrower lower jaw symphysis. While most usually regarded as an astrapotheriine close to Astrapotherium and Parastrapotherium, Johnson & Madden (1997) found it to be sister-taxon to the astrapotheriine-uruguaytheriine clade.
Parastrapotherium is known from the Upper Oligocene and Lower Miocene: it was highly similar to Astrapotherium, but larger, and differed from it in having particularly wide-based incisors and in having a third premolar, which means that what I said in the previous article about premolars being absent in all astrapotheres but some of the most basal taxa is incorrect. Again, while you wouldn’t know it from the semi-technical and popular literature, Parastrapotherium is reasonably well known and excellent three-dimensional material has been described and figured (Scott 1937). Extensive wear on the anterior surfaces of the lower jaw tusks – often manifested as transverse grooves – shows that these astrapotheres used their tusks extensively for something, but what we don’t know. More on tusks in a minute. Three species of Parastrapotherium have been named: P. holmbergi, P. herculeum and P. martiale
Rather less well known that the astrapotheriines are the uruguaytheriines, and until the late 1990s the only known taxa were Uruguaytherium and Xenastrapotherium, both named in 1928. The exact age and provenance of the former are controversial; the latter was described from lower jaws discovered in Miocene deposits of Venezuela, but has since been reported from Colombia, Ecuador and Brazil. Three species have been named: X. christi, X. kraglievichi and X. aequatorialis, all of which have longitudinal grooves running the lengths of their canines. These were big animals: X. kraglievichi was about the size of a Black rhino Diceros bicornis. In contrast to astrapotheriines, the lower canine tusks are more horizontally placed in uruguaytheriines and have a long axis which is roughly parallel to the long axis of the entire lower jaw [lower jaw of Granastrapotherium snorki shown in adjacent image, redrawn from Johnson & Madden 1997]. A Brazilian uruguaytheriine, Synastrapotherium amazonense, named in 1976 might be the same thing as Uruguaytherium.
Johnson & Madden (1997) named a particularly interesting uruguaytheriine from Colombia, Granastrapotherium snorki (some material from Peru has also been referred to this genus). The generic name reflects the fact that this was a particularly big species, being 20-30% larger than Astrapotherium magnum. Its tusks can be up to 1 m long and it was estimated by Johnson & Madden (1997) to reach a mass equivalent to that of Hippopotamus: 2.5-3.5 tons. The specific name – meaning ‘snorkeled’ – sounds like it should be a joke but is actually a deliberate reference to the comparatively large proboscis that individuals of this species would have possessed. What’s known of its skull shows that this was a short-faced species, with thicker, more projecting bony sheaths to its upper canine tusks than those of Astrapotherium. Its broad palate was shallow (unlike the more vaulted palate of astrapotheriines) and with near-parallel tooth rows (again, unlike those of astrapotheriines, where the tooth rows converge posteriorly).
Unlike other astrapotheres, G. snorki completely lacked both lower incisors and a third upper molar, and its massive upper canine tusks have triangular cross-sections at their tips but are oval or rounded at the bases. G. snorki‘s tusks also lacked the grooves present on the tusks of some other uruguaytheriines. However, its long tusks exhibit wear marks along their entire length and this, combined with its broad palate and shortened face, suggested to Johnson & Madden (1997) that it had – for an astrapothere – a particularly long trunk. The wear marks suggest that the tusks were used like those of elephants in helping to strip and break foliage, and given that wear marks extend all the way to the tusk tips, it is inferred that the trunk must have had a reach extending this far at least. In keeping with the large size of this species, its limb bones were broader and more robust than those of better known species like Astrapotherium magnum [adjacent image, borrowed from the UCMP La Venta Fauna Gallery, depicts two (yes, two) of the G. snorki specimens at UCMP. However, the strongly curved and dorsally projecting canine is not right - contrast it with the the drawing above - and there appears to be a canine groove, which there shouldn't be in this species. While two separate lower jaw parts have been combined for this photo, they're still both from G. snorki (I checked the specimen numbers). I assume that the tooth has come out of its socket but am still mightily confused. And I know that sexual dimorphism is present in these animals (I discussed it in the previous article): I didn't think it was this marked in canine shape though].
A short rant about unoriginal artwork
My attempted life restoration at top shows G. snorki next to A. magnum, but they’re clearly not to scale as the G. snorki here is substantially more than 20-30% larger than the A. magnum. I also have no doubt that my G. snorki is essentially a work of fiction (even though I tried not to make it so), because the published figures depict the cranium in palatal view only (!). You can see from my drawing that I looked at tapirs and elephants while doing it. While on the subject of life restorations, astrapotheres provide me with another excuse to discuss pet peeve # 112: the fact that artists have often copied the integumentary details and colour schemes used by other artists when depicting prehistoric animals (for a previous reference to this area see the ver 1 post on phorusrhacids here).
Dan Varner very kindly supplied the above reconstruction of A. magnum produced by Robert Bruce Horsfall back in the 1930s. You will see that the animals on the left are clearly hairy-skinned, but note that the one on the far right looks naked-skinned (it isn’t naked-skinned, but it looks it if you only take a cursory glance). This ‘naked-skinned’ astrapothere clearly became the template for all the astrapothere depictions that followed, for ever afterwards. Michael Long depicted an identically posed Astrapotherium for Savage’s 1986 Mammal Evolution: An Illustrated Guide, but now with naked, elephant-like skin (see it here), as did Graham Allen for the 1988 Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. And if you google-image Astrapotherium you’ll get to see more of the same. Of course there’s the picture on wikipedia I used previously. So… sigh… informed artists are clearly clever enough not to fall into this trap, but it’s still annoyingly prevalent within palaeontological reconstructions.
Incidentally, Nemo Ramjet wanted to do some astrapotheres for me, but I wouldn’t let him
What did astrapotheres do?
How did astrapotheres live? Many authors have noted that these strange mammals combine a perplexing combination of features: indeed Scott (1937) referred to Parastrapotherium as an animal with the ‘neck of an elephant and the loins of an Indian rhinoceros’ (p. 383). Their feet are elephantine* (but probably longer-toed than those of elephants) while their hands recall those of rhinos, and as we saw previously their columnar (yet relatively slender and proportionally short) limbs appear suited for supporting weight, as does the full fusion of the sacrum to the pelvis (Weston et al. 2004). The intervertebral articulations in the lumbar region are particularly elaborate, which again suggests specialisation for weight-bearing on land. And I haven’t done calculations of any sort, but the hands and feet of astrapotheres look proportionally small to me, and no more spreading in shape than those of fully terrestrial elephants and rhinos [manus of A. magnum shown in adjacent image, from Riggs 1935]. Riggs (1935) concluded that astrapotheres frequented ‘forest or meadowland’ habitats, probably feeding on plants that grew in moist areas like river banks and so on. Most people know that Riggs was supporting the notion of terrestriality in sauropod dinosaurs while many of his contemporaries were interpreting sauropods as aquatic, so it seems his judgment was pretty good on this sort of thing.
* Scott suggested that some astrapotheres might have held the foot in the bizarre ‘pedolateral’ pose known elsewhere for some sloths. However, this is contradicted by a lack of mobility at the ankle.
Alas, it has more often been thought that astrapotheres were aquatic or amphibious. Scott (1937) suggested an amphibious mode of life because he noted how short and poorly developed the neural spines are (Wall & Heinbaugh 1999 also drew attention to this), and he also pointed to the relatively small transverse processes on the dorsal vertebrae. Weston et al. (2004) noted that the long astrapothere body looked odd, and they also drew attention to the fact that the common discovery of astrapothere remains within stream channels might lend taphonomic support to an amphibious mode of life. They didn’t support the idea of an aquatic or amphibious mode of life however, and left the question of astrapothere lifestyle unresolved.
The small neural spines of astrapotheres are indeed odd, especially in view of the fact that we would expect that strong support via nuchal ligaments would be needed for the reasonably big head. Incidentally, similarly small neural spines are also seen in the similarly enigmatic South American pyrotheres. However, everything about astrapothere necks is really odd: their opisthocoelous, anteroposteriorly shortened cervical vertebrae are incredibly, and disproportionately, broad. I can’t do anything other than wave my arms around at this point, but I can’t help thinking that astrapotheres were doing something unusual in terms of how they were supporting the head and using the neck, and that this may explain why they did away with tall neural spines.
And if the small size of the neural spines is, essentially, the only thing that supports the amphibious hypothesis, then count me out. I see no good reason to think that these animals were any more aquatic than rhinos or mastodonts and, as noted above, a skeleton well suited for weight bearing on land perhaps contradicts the idea of aquatic habits. The extensive wear seen on astrapothere tusks – recall in particular the transverse grooves present in Parastrapotherium – indicates that they were using those tusks to do such things as break bark, root in soil, and tear at branches. You can spend your time in the water and still do all of those things (indeed, you can be amphibious and still feed entirely on land – look at hippos), but they are also consistent with terrestrial life. Of course, the superficial similarity that astrapotheres have with tapirs [Tapirus terrestris shown in adjacent image, from here] has led people to assume that the two lived in similar ways. Maybe they did. Ultimately we need more work on this subject. As others have noted before me, there has – historically – been a general rush to assume amphibious habits in big prehistoric animals, often for really poor reasons.
And… what are astrapotheres?
South America’s endemic placentals have always been of mysterious phylogenetic affinities, and the situation today isn’t much better. If you know anything about mammal phylogeny, you’ll be aware that our views on the deep structure of the placental mammal tree have really been shaken up in recent years, mostly thanks to a slew of new molecular studies. The bad news is that we’re still very much in the dark on astrapotheres and the other weird South American placentals, so I regret that there really isn’t much to say on this subject. Ideally, it would be ‘nice’ if astrapotheres and the other South American placentals were found to be part of Atlantogenata – the placental clade that includes Xenarthra and Afrotheria. Some people are intimating that this is indeed the case, although note that astrapotheres are not currently thought to share a common ancestor with litopterns and notoungulates (Muizon & Cifelli 2000, Horovitz 2004).
Cifelli (1983) noted that the highly distinctive ankle morphology of astrapotheres is similar in some respects to that of dinoceratans, but this is probably due to convergence and hasn’t been supported by other data. The late Miguel Soria (1988) published an article that I haven’t yet seen but which looks essential, titled ‘Estudios sobre los Astrapotheria (Mammalia) del Paleoceno y Eoceno. Parte II: filogenia, origen y relaciones’ (nor have I seen part I). Finally, Horovitz (2004) suggested that astrapotheres were close relatives of the periptychids, a mostly North American, mostly Palaeocene group conventionally regarded as ‘condylarths’ close to groups like the hyopsodontids and phenacodontids (e.g., Archibald 1998). Some work now indicates that hyopsodontids and phenacodontids are atlantogenatans and, furthermore, part of Afrotheria (Asher et al. 2003, Zack et al. 2005). If astrapotheres are allied to periptychids, if periptychids are allied to hyopsodontids and phenacodontids, and if hyopsodontids and phenacodontids really are afrotherian atlantogenatans… does this mean we have an end to the mystery? Well, other studies find hyopsodontids and phenacodontids to be part of Laurasiatheria (Wible et al. 2007), the other great assemblage of placental mammals. I think you get the picture, I’ll stop there.
We’ll be coming back to weird South American placental mammals in the very near future, but for now it’s time to move on. I hope you enjoyed the ride. To the sunset, brave Snorki!
Refs – -
Archibald, J. D. 1998. Archaic ungulates (“Condylarthra”). In Janis, C. M., Scott, K. M. & Jacobs, L. L. (eds) Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Cambridge University Press, pp. 292-331.
Asher, R. J., Novacek, M. J. & Geisler, J. H. 2003. Relationships of endemic African mammals and their fossil relatives based on morphological and molecular evidence. Journal of Mammalian Evolution 10, 131-194.
Cifelli, R. L. 1983. Eutherian tarsals from the Late Paleocene of Brazil. American Museum Novitates 2761, 1-31.
Horovitz, I. 2004. Eutherian mammal systematics and the origins of South American ungulates as based on postcranial osteology. Bulletin of Carnegie Museum of Natural History 36, 63-79.
Johnson, S. C. & Madden, R. H. 1997. Uruguaytheriine astrapotheres of tropical South America. In Kay, R. F., Madden, R. H., Cifelli, R. L. & Flynn, J. J. (eds) Vertebrate Paleontology in the Neotropics: The Miocene fauna of La Venta, Colombia. Smithsonian Institution Press (Washington, D.C.), pp. 355-381.
Muizon, C. de & Cifelli, R. L. 2000. The ‘condylarths’ (archaic Ungulata, Mammalia) from the early Palaeocene of Tiupampa (Bolivia): implications on the origin of the South American ungulates. Geodiversitas 22, 47-150.
Riggs, E. S. 1935. A skeleton of Astrapotherium. Geological Series of Field Museum of Natural History 6, 167-177.
Scott, W. B. 1937. The Astrapotheria. Proceedings of the American Philosophical Society 77, 309-393.
Soria, M. F. 1988. Estudios sobre los Astrapotheria (Mammalia) del Paleoceno y Eoceno. Parte II: filogenia, origen y relaciones. Ameghiniana 25, 47-59.
Wall, W. P. & Heinbaugh, K. L. 1999. Locomotor adaptations in Metamynodon planifrons compared to other amynodontids (Perissodactyla, Rhinocerotoidea). In Santucci, V. L. & McClelland, L. (eds) National Park Service Paleontological Research, Technical Report NPS/NRGRD/GRDTR-99/03, pp. 8-17.
Weston, E. M., Madden, R. H. & Sánchez-Villagra, M. R. 2004. Early Miocene astrapotheres (Mammalia) from northern South America. Special Papers in Palaeontology 71, 81-97.
Wible, J. R., Rougier, G. W., Novacek, M. J. & Asher, R. J. 2007. Cretaceous eutherians and Laurasian origin for placental mammals nears the K/T boundary. Nature 447, 1003-1006.
Zack, S. P., Penkrot, T. A., Bloch, J. I. & Rose, K. D. 2005. Affinities of ‘hyopsodontids’ to elephant shrews and a Holarctic origin of Afrotheria. Nature 434, 497-501.