Tuesday, 18 September 2012

Small Llamas from the Late Pleistocene of Central Mexico.

 Camelids (Camels and Llamas) originated in North America about 45 million years ago, from where they spread to Eurasia about 6 million years ago and South America about 1.8 million years ago. They became extinct in North America at the end of the Pleistocene, probably as a result of the arival of humans, but persisted in South America and Eurasia, from where they have been introduced to Africa and more recently Australia.

The small Llama Hemiauchenia gracilis is known from a number of  Late Pliocene to Middle Pleistocene sites across North America, from Idaho south to Mexico and east to Florida. In a paper published in the journal Acta Palaeontologica Polonica on 29 June 2012, Victor Bravo−Cuevas of the Museo de Paleontología at the Área Académica de Biología at the Universidad Autónoma del Estado de Hidalgo, Eduardo Jiménez−Hidalgo of the Laboratorio de Paleobiología at the Instituto de Recursos at the Universidad del Mar, and Gloria Cuevas−Ruiz and Miguel A. Cabral−Perdomo, also of the Área Académica de Biología at the Universidad Autónoma del Estado de Hidalgo describe new material assigned to Hemiauchenia gracilis from three Late Pleistocene sites in Hildago State in Central Mexico.

Map showing the sites where the new material referred to Hemiauchenia gracilis was discovered. Bravo-Cuevas et al. (2012).

The new material came from three separate sites in Hildago State, all assigned to the Rancholbrean North American Faunal Stage on the basis of other fossils present, making them between 240 000 and 11 000 years old; this is the last faunal stage of the Pleistocene in North America.

The first site, Barranca del Berrendo, yielded a partial skull and the distal portion of metatarsels III and IV (foot bones).

Partial skull of Hemiauchenia gracilis from Barranca del Berrendo. (A) Cranial vault in dorsal view. (B) Cranial vault in lateral view. (C) Maxillary fragment in dorsal view. (D) Maxillary fragment in ventral view. (E) Maxillary fragment in lateral view. The arrows in E
indicate mesowear patters: high occlusal relief (OR) and sharp cusp shape (CS). Bravo-Cuevas et al. (2012).

Distal portions of metatarsals III and IV of Hemiauchenia gracilis in anterior view. Bravo-Cuevas et al. (2012).

The second site, Barranca San Agustín produced the distal parts of a left scapula (shoulder blade) and left tibia (shinbone).

Material referred to Hemiauchenia gracilis from Barranca San Agustín. (A) Distal end of left scapula in lateral view. (B) Distal end of left tibia (UAHMO−515) in anterior (B1) and distal (B2) views, showing the articular surface. Bravo-Cuevas et al. (2012).

The third site, El Barrio, produced a fragmentary left mandible (jawbone), a metatarsal fragment and two proximal phalanges (knuckle bones) of the same individual.

Mandible and lower dentition of the camelid Hemiauchenia gracilis, in lateral (A) and occlusal (B) views. Bravo-Cuevas et al. (2012).

Left metatarsal of the camelid Hemiauchenia gracilis in anterior (A1) and proximal (A2) views,
showing the articular surface.
Bravo-Cuevas et al. (2012).

Proximal phalanges of the camelid Hemiauchenia gracilis (A) right in anterior (A1) and posterior (A2) views and (B) left in anterior (B1) and posterior (B2) views. Bravo-Cuevas et al. (2012).

These are the latest known specimens of Hemiauchenia gracilis and are at the southern end of the species known range. This suggests that the species became restricted in the southern part of its range as the Pleistocene progressed, with fossils from the earliest part of its range, the Early Blancan Faunal Stage (equivalent to the Late Pliocene, or between 4.3 and 2.6 million years ago) found from Idaho to Mexico, then by the Late Blancan (Early Pleistocene, or 2 588 000 to 1 808 000 years ago) being found no further north than Arizona, New Mexico and Florida, then in the Irvingtonian (Middle Pleistocene, 1,800,000 to 240,000 years ago) being restricted to Mexico, with the new Rancholabrean fossils found only in Central Mexico.

Map showing the distribution of known sites producing Hemiauchenia gracilis material, with approximate dates  represented by symbols. Bravo-Cuevas et al. (2012).

Like many (most) scientific theories this diminishing distribution over time is only testable in that it could potentially be disproved. A single fossil in an area where the species was thought to have become locally extinct would prove the theory wrong, but it is never possible to say with complete confidence that it was absent from an area, since absence of evidence is not evidence of absence, so the theory can never actually be proved.

See also New species of Ground Opossum from eastern Bolivia, New Caviomorph Rodents from the Early Oligocene Tinguiririca Fauna of the Andean Main Range of central ChileA new Ground Sloth from the Late Miocene of ArgentinaThe origin of domestic dogs, and Mammals on Sciency Thoughts YouTube.

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