Equus, Linnaeus, 1758

Equus View in CoL cf. E. neogeus

( Fig. 9A View Fig 1–E View Fig 2 View Fig ).

Referred material

Left M1 ( PIMUZ A/ V 4236), left p4–m1 ( PIMUZ A/ V 4183) and left m1 ( PIMUZ A/ V 4212), incomplete right metacarpal ( MCIII) with first and second phalanges associated ( PIMUZ A/ V 4248), and incomplete left metatarsal ( MTIII) with articulate tarsal bones ( MHNG GEPI V-3778). Geographic data of the specimens are provided in Additional file 1.

Description and comparisons

PIMUZ A/V 4236 is a relatively complete left M1

( Fig. 9C View Fig 1–C View Fig 2 View Fig ), missing the cementum of the mesial part and with an occlusal length of approximately 30 mm. Te occlusal surface is well preserved and it is possible to observe the pli caballin and a well-defined and triangular protocone that is longer at its distal part than in the mesial part. Te lower left p4–m1 (PIMUZ A/V 4183, Fig. 9A View Fig 1–A View Fig 2 View Fig ) have an occlusal length of 26 mm and 24 mm, respectively. Left m1 (PIMUZ A/V 4212, Fig. 9B View Fig 1–B View Fig 2 View Fig ) has an occlusal length of 23 mm. Te occlusal surface of the specimens is well-preserved, showing a double knot characterized by a rounded metaconid and a relatively angular metastylid, an elongated ectoflexid, and a protoconid and hypoconid with slightly straight labial edges (see Alberdi et al., 2002; Prado & Alberdi, 2017).

Te specimen PIMUZ A/V 4248 is an incomplete right MCIII, missing its proximal part and articulated with first and second phalanges ( Fig. 9E View Fig 1–E View Fig 2 View Fig ). Because the bones are preserved in physical connection, we have not been able to take precise measurements, but its maximal length is approximately 83 mm. An evident epiphyseal line shows an incomplete fusion between the diaphysis and epiphysis, which suggests that PIMUZ A/V 4248 could belong to a subadult individual. MHNG GEPI V-3778 is represented by a left MTIII, missing the left section of the distal epiphysis, for this reason a total length of 240 mm is estimated; all the tarsal bones are still articulated, including the talus and calcaneus

( Fig. 9D View Fig 1–D View Fig 2 View Fig ).

Remarks

Te dental occlusal pattern and characters present in PIMUZ A/V 4183, 4212, and 4236 (e.g., protocone, metaconid, pli caballin), as described above, can likely be associated more with Equus than Hippidion (see Prado & Alberdi, 2017, and references therein). Te first phalanx articulated to the right MCIII (PIMUZ A/V 4248) is slen- der and more elongated than those present in Hippidion ; its maximal length of approximately 83 mm is similar to those reported for Equus (see Prado et al., 2012). Similarly, the rather slender morphology of the left MTIII (MHNG GEPI V-3778) is characteristic of the genus Equus , with values of a maximal length (240 mm) similar to estimates for E. neogeus (see Prado et al., 2012, table 1).

Te taxonomic validity of the South American fossil Equus species have been under debate in the last decades (see Alberdi & Prado, 2004; Prado & Alberdi, 1994, 2017). Hoffstetter (1950) included all different species of Equus from South America in the subgenus Amerhippus , based only in the lack of infundibular marks in the lower incisor surface and consequent loss of surface enamel; nevertheless, the validity and use of Amerhippus have been questioned by Orlando et al. (2008) and Prado & Alberdi (2017). Traditionally five species of Equus were recognized in the fossil record of South America (e.g., Prado & Alberdi, 1994), but Prado & Alberdi (2017) based on both extensive revision of the morphological and morphometric characters of cranial and postcranial elements recognized as valid only three species (1) E. andium (?Middle–Late Pleistocene), (2) E. insulatus (Middle– Late Pleistocene), and (3) E. neogeus (Late Pleistocene) . Recently, Machado & Avilla (2019) questioned the diagnosis and taxonomic validity of the three Equus species suggesting the possibility of only a single species of native Equus in South America, namely E. neogeus . We do not rule out that the specimens described herein from the Pampas region could belong to E. neogeus due to their morphology.

Equidae indet.

( Fig. 9F View Fig 1–J View Fig 2 View Fig ).

Referred material

A lower symphyseal bone with six incisors (MHNG GEPI V-3780), right m1 or m2 (PIMUZ A/V 4203), and indeterminate lower molar (PIMUZ A/V 4244), two isolated fragmented molariforms and an isolated incisor (PIMUZ A/V 4241), thoracic vertebra (PIMUZ A/V 4132), and second phalange of the III digit (PIMUZ A/V 4242). Geographic data of the specimens are provided in Additional file 1.

Description and comparisons

Te lower symphyseal bone with six incisors MHNG GEPI V-3780 ( Fig. 9G View Fig 1–G View Fig 2 View Fig ) shows a dental occlusal surface with advanced wear, likely suggesting an old individual. Tis specimen, together with the other isolated molariforms (PIMUZ A/V 4203, 4241, 4244, Fig. 9I–J View Fig 2 View Fig ), the thoracic vertebra (PIMUZ A/V 4132, Fig. 9H View Fig ), a second phalange of the III digit (PIMUZ A/V 4242, Fig. F1– F View Fig 3 View Fig ), are fragmented or in poor preservation state.

Remarks

Te fragmentary and/or poor preserved condition of the above-mentioned specimens makes it difficult to determine diagnostic characters for more precise identifications.