(Note: I planned originally make just an image with some text, as I made regulary, but I ended make more images than I expected and all form part of the same context. So, for this time I prefer put all in a journal... the images will be uploaded in my gallery later.)
Between the toxodontids, probably one of the most interesting (at least for me) is Mixotoxodon larensis. Although it lived during the Pleistocene, is far to be known so well as the classic Toxodon platensis or another earlier genera like Nesodon or Adinotherium, and is frequent find only short descriptions, that usually says that is the northern equivalent of Toxodon and that was the only notoungulate that moved outside South America. If you look for images of Mixotoxodon in Google, probably you have this: some good reconstructions, few fossils and a lot of unrelated animals... well, you know that could be worst.
On another hand, in the scientific literature, frequently there is only fragmentary pieces. Not very informative, if you want to draw it.
(From Ascanio Rincón, 2003)
In contrast with the scarce images, it was a very widespread animal, and a series of small but relevant pieces has been discovered in the last years in a variety of places, that make that their geographical range be even larger than previously was imagined:
The Mixotoxodon "empire" (every point represents a fossil location). Note that it reach the south of USA and northern Argentina, and is well represented in northern South American and Central America. These locations probably represents a combination of tropical prairies and forests, probably not the best place for the fossilization. Also, its range was different of Toxodon, that lived in the open plains of southern South America, with the Northeastern Brazil as the northern limit. Probably both animals must had different some ecological requirements, although the evergrowing teeth and the isotopes shows that both feed on abrassive plants, like grasses. But, again, how was its physical appearance?
Reading a paper (Lucas et al, 1997), I find a kind of answer. Although is better and worst (at same time) than I expected. Look at this photo:
Let me introduce you to CMCR-CFM 846 , the only relatively complete skull known for Mixotoxodon, in palatal view. It was recovered during the 1930s in Costa Rica, along a mandible, probably from the same individual, and is in the collections of the Museo Nacional de Costa Rica. But the problem is that this photograph is the only way to imagine how the entire skull was: for unknown reasons, the skull is lost, remaining only a pair of pieces. And sadly, apparently nobody took more photos or made an adequate description of the fossil when it was complete. Therefore, we must conform with this:
But we can count with another Mixotoxodon fossils too. We can begin for this premaxilla (the snout) described from northern Argentina in 2012. Look how slender is:
And the original remains of Mixotoxodon, from Venezuela. In this case, we have almost all the mandible and teeth (from Van Frank, 1957):
As you can see, a pattern is emerging: the skull is a bit triangular and mandibles are relatively slender, without chin, like in other advanced toxodontids. With this idea, I’ve tempted to draw how could be the entire head of the animal. Have in mind that here I traced the form of the original skull from the photo, but this kind of exercise, with a source image of very low resolution, surely is plagued with anatomical distortions and pareidolia, so must be taken just as a very tentative reconstruction. To help to visualizate what I made, I put in white the areas reconstructed by me and in colors the different real specimens. In A) is the skull of the photo with the Argentinean premaxilla; B) is a diagram of the lower teeth and the lower jaw; C) the skull in lateral view (with only the preserved portion of the maxilla) and the mandible; and D) The skull in frontal view.
Compared with Toxodon, the snout is short, and more slender, and the tusk-like incisives are shorter too; the front part of the mandible is cylindrical, different to the wide snout of the former (images taken from here: www.fullblog.com.ar/blogs/arge…, and here:dailyfossil.tumblr.com/post/24…, respectively):
On another hand, these features, and the shape of the mandible remind me a bit to the extraordinary horned toxodontid, Trigodon gaudryi (from Ameghino, 1907):
I want to note here that the classification of toxodontids is difficult, and they have been divided in a number of subfamilies across the ayears, but generally are accepted three of them: Nesodontinae (primitive and small forms), Haplodontheriinae (larger forms, with compressed incisors and cranial ornamentation) and Toxodontinae (advanced and large forms with long skulls and flat incisors). I mentioned it due that Trigodon is a Haplodontheriinae, and it shares with Mixotoxodon the cylindrical mandible and a long symphisis (the union of both mandibles) that extends until the first molar in the case of Mixotoxodon. Meanwhile, with Toxodon shares features of the teeth. Is this mixture of features the origin of the very name of Mixotoxodon, and makes harder establish their relationships, although that's another history...
Along the original specimen, Van Frank mentioned some postcranial remains also recovered in the San Miguel site in Venezuela, that could belong to this animal: An broken second cervical (atlas), a single radius, four ulnae (one of these complete), a patella and an astragalus. These elements, in general terms are very similar in proportions to those of Toxodon, although the astragalus and radius are more slender and have specific anatomical differences. In any case, both these bones as the skull show a larger size than Toxodon. The former measures about 1,5 meters until the shoulders, with a skull of 66 cm; if the reconstruction below is right, Mixotoxodon could measure up to 1,70 to the shoulders, with a skull of about 80 cm, cleary bigger than its southern cousin. A recent weight estimation (2012) suggest that Mixotoxodon could reach 3.8 tonnes, which makes sense with this larger body. Oh, and is also the largest notoungulate that ever lived.
And finally, I've decided make a quick reconstruction of the head of the animal. I hope you like the result:
- Ameghino, Florentino. Les toxodontes à cornes. Anales del Museo Nacional de Buenos Aires, 1907.
- Cisneros, J.C. 2005. New Pleistocene vertebrate fauna from El Salvador. Revista Brasileira de Paleontologia, 8(3):239-255.
- Chimento, Nicolás R., and Federico L. Agnolin. "Mamíferos del Pleistoceno Superior de Santiago del Estero (Argentina) y sus afinidades paleobiogeográficas." Papéis Avulsos de Zoologia (São Paulo) 51.6 (2011): 83-100.
- Elissamburu A., 2012. Estimación de la masa corporal en géneros del Orden Notoungulata. Estudios Geológicos, Vol 68, No 1, doi:10.3989/egeol.40336.133
- Laurito, César Alberto. "Análisis topológico y sistemático del Toxodonte de Bajo de los Barrantes, provincia de Alajuela, Costa Rica." Revista Geológica de América Central 16 (1993).
- Lucas, Spencer G., Guillermo E. Alvarado, and Eduardo Vega. "The pleistocene mammals of Costa Rica." Journal of Vertebrate Paleontology 17.2 (1997): 413-427.
- Lundelius Jr, Ernest L., et al. "The first occurrence of a toxodont (Mammalia, Notoungulata) in the United States." Journal of Vertebrate Paleontology 33.1 (2013): 229-232.
- McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8
- Paula-Couto, C. (1979). Capítulo XXI, Ordem Notoungulata Roth, 1903. In Tratado de Paleomastozoologia. Academia Brasileira de Ciências, 590 p, Rio de Janeiro.
- Rincón, Ascanio D. "Los mamíferos fósiles del Pleistoceno de la cueva del Zumbador (fa. 116), Estado Falcón, Venezuela." Boletín de la Sociedad Venezolana de Espeleología 37 (2003): 18-26.
- Rincón, Ascanio D. "New remains of Mixotoxodon larensis Van Frank 1957 (Mammalia: Notoungulata) from mene de inciarte tar pit, north-western Venezuela." Interciencia 36.12 (2011): 894-899.
- van Frank, R. 1957. A fossil collection from northern Venzuela. 1, Toxodontidae (Mammalia, Notoungulata). American Museum Novitates, 1850:1-38.