Colpodon antucoensis, Shockey & Flynn & Croft & Gans & Wyss, 2012

Shockey, Bruce J., Flynn, John J., Croft, Darin A., Gans, Phillip & Wyss, André R., 2012, New leontiniid Notoungulata (Mammalia) from Chile and Argentina: comparative anatomy, character analysis, and phylogenetic hypotheses, American Museum Novitates 2012 (3737), pp. 1-64 : 46-48

COLPODON ANTUCOENsis : BIOGEOGRAPHY AND AGE

The discovery of Colpodon antucoensis , new species, at Laguna del Laja, Chile, extends the geographic range of Colpodon . Species of Colpodon were previously known only from various localities in Chubut and Rio Negro provinces of central Patagonia, Argentina. This geographic range extension is modest (see map, fig. 1). At about 37.5° S and 71.3° W, Laguna del Laja is roughly 360 km northwest of the Colhuehuapian Paso Cordoba locality of Rio Negro, Argentina, which previously had been the northernmost record of Colpodon .

The presence of Colpodon at the Estero Correntoso locality of Laguna del Laja initially suggests a Colhuehuapian age for these lower horizons of the Cura-Mallín Formation (Tcm 1). Indeed, Colpodon has been used as the index fossil of the Colhuehuapian since Ameghino first recognized the beds as the “couches à Colpodon ” ( Ameghino, 1902). However, consideration of radioisotopic ages available for the lower Cura-Mallín and for fossil taxa coeval to the holotype of C. antucoensis , as well as noting that this is a new species of Colpodon distinct from Patagonian forms, suggests that caution is warranted in assigning a faunal age to Tcm 1.

Horizons bracketing the type specimen of C. antucoensis yield 39 Ar/ 40 Ar ages of 19.25 ± 1.22 Ma above and 19.53 ± 0.60 Ma below (fig. 4), thus constraining its age at ~19.5 Ma (corrected to ~19.8 Ma for the revised Fish Canyon Tuff age). The age of the Colhuehuapian SALMA is best constrained by radioisotopic and paleomagnetic studies at its type locality at the Gran Barranca ( Ré et al., 2010a, 2010b). Ré et al.’s preferred paleomagnetic interpretation (using the calibration of radioisotopic ages) from the Colhue-Huapi Member is that the “Lower Fossil Zone,” producing the Colhuehuapian SALMA fauna, is in Chron C6An.1n ( Ré et al., 2010b). This suggests an age of 20.0 Ma–20.2 Ma for the Colhuehuapian SALMA, although its full temporal span remains unclear (tentatively estimated at 19–21 Ma by Flynn and Swisher, 1995). This ~20 Ma age estimate is only slightly older than the age (~19.8 Ma) of C. antucoensis in the Tcm 1 of Laguna del Laja.

Overlying the Colhuehuapian SALMA fauna, in the “Upper Fossil Zone” of the Colhue- Huapi Member at Gran Barranca, is a fauna recently recognized as being “Pinturan” ( Kramarz et al., 2010), an informal biochron considered to be intermediate in age between Colhuehuapian and Santacrucian SALMA faunas (Vucetich et al., 2005; Carlini et al., 2005). Ré’s preferred interpretation of radioisotopic-paleomagnetic results in that section is that the “Upper Fossil Zone” containing the “Pinturan” fauna lies within C6n, suggesting an age range from about 19.7 to 18.7 Ma ( Ré, 2010b). Recognizing that “Pinturan” localities outside of Gran Barranca have yielded younger radiometric ages (~17.5 Ma; Fleagle et al., [1995]), Kramarz et al. (2010) used the younger extreme of the age for Chron 6n as the older extreme of the suggested “Pinturan” age, proposing that the “Pinturan” ranged from 18.75 to 16.5 Ma. Croft et al. (2007) extended the beginning of the Santacrucian SALMA to be as old as ~19 Ma, based on dates associated with the Chucal Fauna of northern Chile (correlated to the Santacrucian). This suggests either that the “Pinturan” is a very short time interval preceding the Santacrucian SALMA (consistent with the results of Ré, 2010b, but not with the younger estimates of Fleagle et al., 1995, and Kramarz et al., 2010), or that it falls within the Santacrucian SALMA.

The few other taxa identified to date from the same interval in the Estero Correntoso section (Tcm 1) at Laguna del Laja suggest a post-Colhuehuapian age for Colpodon antucoensis . For example, a lagostomine chinchillid is known from Tcm 1 of Estero Correntoso and is otherwise known only from Santacrucian or younger faunas ( Flynn et al., 2008). A sloth from Estero Correntoso is referable to Nematherium (cf. N. angulatum or sp. nov.; Flynn et al., 2008: fig. 4d). Nematherium is best known from the Santacrucian, but it also occurs in the “Pinturas” fauna of the Upper Fossil Zone of the Colhue-Huapi Member of Gran Barranca ( Kramarz et al., 2010). The only taxon that occurs at both Estero Correntoso and in a known Colhuehuapian age fauna, aside from Colpodon , is Protypotherium sp. However, the age range of this interatheriid notoungulate is so great (extending from the Colhuehuapian and “Pinturan” up into the Chapadmalalan and possibly younger [ Flynn et al., 2008]) that it has no utility in helping to discriminate time based on faunal similarities.

Thus, the radioisotopic age and taxa coeval with Colpodon antucoensis suggest that this species of Colpodon is possibly a little younger than the faunal interval that once carried its generic name—“couches à Colpodon .” Since Colpodon is the only known post-Deseadan leontiniid of southern South America, its temporal range extension also extends the temporal record of leontiniids in southern South America. Only the Laventan-aged Huilatherium of northern South America ( Colombia) is younger (Villarroel and Colwell Danis, 1997).