As some of you might know, all of my ‘free’ time last month was eaten up by a major project (a book chapter) that had a very tight deadline. This meant no time whatsoever for such stuff as blog-writing, hence the (mostly) recycled babirusa stuff. That project is now (mostly!) complete, but I’m still struggling for time due to other projects and jobs. Meanwhile, I thought I’d recycle this…
The illustration was previously used here, back on ver 1 in 2006, and again here in a ver 2 article on ameghinornithids. Eurotamandua joresi, shown at top as a skeleton and in the middle as a life restoration, is a middle Eocene fossil from Messel, Germany.
Its strong similarity to modern anteaters led Storch (1981) and Storch & Habersetzer (1991) to classify Eurotamandua as a true anteater (viz, as a member of the xenarthran clade Vermilingua), and even as a member of the anteater ‘subfamily’ Myrmecophaginae (within Vermilingua, the ‘family’ Myrmecophagidae has been split by some authors into Myrmecophaginae [for Protamandua, Tamandua, Neotamandua and Myrmecophaga] and Cyclopinae [for Cyclopes and Palaeomyrmidon]). Given that all other xenarthrans (Xenarthra = armadillos, sloths and anteaters, and their extinct relatives) are almost exclusively South American*, the presence of a member of the group in Eocene Europe – at a time when South America was an island continent – has always posed a major biogeographical enigma [adjacent skeletal reconstruction of Eurotamandua from wikipedia].
* They invaded North America during the Miocene, Pliocene and Pleistocene of course, and an Eocene sloth is known from Seymour Island, Antarctica.
The possibility that anteaters might have swam or rafted across the Atlantic seems somewhat remote, leaving vicariance as the only solution. But South America’s terrestrial contact with North America (and hence Europe) and Africa (via Antarctica) became severed in the Cretaceous. Ergo, if Eurotamandua really is an anteater, members of the Eurotamandua lineage would have to go back to the Cretaceous, if vicariance was the explanation for Eurotamandua‘s presence in Europe. Storch & Habersetzer (1991) embraced this hypothesis, and therefore proposed that cyclopine and myrmecophagine anteaters diverged late in the Cretaceous. The existence of anteaters at this time would also mean that members of the armadillo and sloth lineages were also around in the Cretaceous, and other crown-placentals would have to have existed at the same time as well [life restoration of Eurotamandua below from wikipedia].
But – – is Eurotamandua really a modern-type anteater? Moreover, is it really a member of Vermilingua, and is it really, definitely, a xenarthran? Actually, all of these identifications have been challenged. In a conference abstract (the full results of which were never published so far as I know), Shoshani et al. (1997) proposed that Eurotamandua might not be a xenarthran at all, but that it was instead a close relative of pangolins. A few other authors have noted resemblances to the Messel pangolin Eomanis (this is the animal shown at the bottom of the composite image at the top; the holotype specimen is shown below, from here), and others have suggested that Eurotamandua might be a palaeanodont or palaeanodont relative (Rose & Emry 1993, Rose 1999), and these are regarded as stem-pangolins by some (I really must cover palaeanodonts at some stage… have been meaning to do so for ages). If these suggestions are correct [UPDATE: read the comments for more on Eurotamandua being close to pangolins!], Eurotamandua isn’t a xenarthran at all [photo of Eurotamandua holotype below from here].
In Storch’s 1981 description of Eurotamandua, rugose areas on the posterior thoracic and anterior lumbar vertebrae were interpreted as accessory xenarthrous articular surfaces (aka xenarthrales), and adjacent concavities on the sides of the centra were regarded as articular facets for these structures. The presence of these bits and pieces would show that Eurotamandua was xenarthrous, and hence a member of Xenarthra. The detailed photos published by Szalay & Shrenk (1998) showed, however, that the anapophyses of Eurotamandua (these are the projecting structures that support the xenarthrales) are not dorsoventrally deep, nor do they have dorsal and ventral articular processes as they do in xenarthrans; furthermore, the supposed articular areas on the sides of the centra (for reception of the xenarthrales) are artifacts and unlike the two distinct facets present in xenarthrans. Gaudin (1999) also argued that xenarthrales are absent in Eurotamandua and that an affinity with xenarthrans was unlikely. Szalay & Shrenk (1998) looked at a lot of other characters in Eurotamandua, and also concluded that it wasn’t a xenarthran; instead, they proposed that it represented an entirely different, anteater-like lineage that they termed Afedentata (they first used that term in a 1994 abstract: I don’t know what it means). They suggested that ‘afredentatans’ might share an ancestor with xenarthrans and palaeanodonts. All of these groups share a tubular muzzle and a few other features.
Gaudin & Branham (1998) found, however, that when all the morphological character data available was included in a data set with xenarthrans and other xenarthran-like placentals (such as pangolins and palaeanodonts), Eurotamandua still came out as a xenarthran, albeit not as a member of Vermilingua. Instead, they recovered it as a sister-taxon to the Pilosa + Vermilingua clade (the sloth + anteater clade) within Xenarthra. And what does all this mean for the biogeographical problem alluded to above?
Such a phylogenetic position leaves open the mystery of how a group which originated in and purportedly was restricted to the island continent of South America until the emergence of the Panamanian isthmus 2 million years ago managed to land an early member in Western Europe in the middle Eocene some 40-50 million ago (Gaudin & Branham 1998, p. 260).
In other words, we’re still none the wiser, and the ‘out of place anteater’ problem becomes an ‘out of place xenarthan’ one if Gaudin & Branham (1998) are right. Are they right? As mentioned above, Tim Gaudin concluded in a more recent paper (Gaudin 1999) that Eurotamandua is not a xenarthran at all, meaning that this might represent the ‘consensus view’ among specialists [Eomanis holotype shown here].
The identification of Eurotamandua as an anteater hasn’t just been challenged by morphological data: molecular studies also indicate that it’s unlikely to be correct. Delsuc et al. (2001) used molecular dating to show that the earliest divergence in Xenarthra (between armadillos and the members of the sloth + anteater clade) must have occurred round about the time of the Cretaceous-Paleogene boundary, and that sloths and anteaters must have diverged in the Early Eocene. Eurotamandua really doesn’t fit with this, as there is really no obvious way for Vermilingua to have originated in South America in the Early Eocene and to have then gotten to Europe within about 10 million years (unless anteaters are actually expert mariners, or able to teleport). Delsuc et al. (2001) therefore noted that their results better agreed with a non-xenarthran position (a la Szalay & Shrenk (1998) and Rose (1999)) or basal xenarthran position (a la Gaudin & Branham (1998)) for Eurotamandua.
Incidentally, Delsuc et al. (2001) dated the cyclopine-myrmecophagine split to the Late Eocene. While we’re talking about anteater phylogeny, let’s use this opportunity to remind ourselves that the Giant anteater Myrmecophaga tridactyla is, according to the phylogeny, a secondarily terrestrial descendant of arboreal ancestors, given that other myrmecophagines, and cyclopines, are arboreal. Arboreal ancestry might explain some of the peculiar morphological details of the Giant anteater, but that’s a subject for another time.
In conclusion… while we remain uncertain just what Eurotamandua really is, the idea that it’s a member of Vermilingua is pretty much dead, and the idea that it’s a xenarthran is now widely doubted. I personally think that Rose (1999) made a good argument for palaeanodont affinities. If this is right, that ‘out of place xenarthan’ problem disappears.
So, must get round to palaeanodonts some time. And ernanodontids.
For other Tet Zoo articles on xenarthrans see…
- Ten things you didn’t know about sloths
- Five things you didn’t know about armadillos
- What was that skull? (on glyptodonts)
- I, Priodontes, the tatuasu
- The Panamanian Blue Hill Monster (or Cerro Azul Monster)
Refs – –
Delsuc, F., Catzeflis, F. M., Stanhope, M. J. & Douzery, E. J. P. 2001. The evolution of armadillos, anteaters and sloths depicted by nuclear and mitochondrial phylogenies: implications for the status of the enigmatic fossil Eurotamandua. Proceedings of the Royal Society of London B 268, 1605-1615.
Gaudin, T. J. 1999. The morphology of xenarthrous vertebrae (Mammalia: Xenarthra). Fieldiana: Geology 41, 1-38.
– . & Branham, D. G. 1998. The phylogeny of the Myrmecophagidae (Mammalia, Xenarthra, Vermilingua) and the relationship of Eurotamandua to the Vermilingua. Journal of Mammalian Evolution 5, 237-265.
Rose, K. D. 1999. Eurotamandua and Palaeanodonta: convergent or related? Paläontologische Zeitschrift 73, 395-401.
– . & Emry, R. J. 1993. Relationships of Xenarthra, Pholidota, and fossil “edentates”: the morphological evidence. In Szalay, F. S., Novacek, M. J. & McKenna, M. C. (eds) Mammal Phylogeny (placentals). Springer (New York), pp. 81-102.
Shoshani, J., McKenna, M. C., Rose, K. D. & Emry, R. J. 1997. Eurotamandua is a pholidotan not a xenarthran. Journal of Vertebrate Paleontology 17 (supp. 3), 76A.
Storch, G. 1981. Eurotamandua joresi, ein Myrmecophagidae aus dem Eozän der “Grube Messel” bei Darmstadt (Mammalia, Xenarthra). Senckenbergiana lethaea 61, 247-289.
– . & Habersetzer, J. 1991. Rückverlagerte Choanen und akzessorische Bulla tympanica bei rezenten Vermilingua und Eurotamandua aus dem Eozän von Messel (Mammalia: Xenarthra). Zeitschrift für Säugetierkunde 56, 257-271.
Szalay, F. S. & Schrenk, F. 1998. The Middle Eocene Eurotamandua and a Darwinian phylogenetic analysis of “edentates”. Kaupia 7, 97-186.