Vera—Eocene archeopithecids from Patagonia


by, respectively, 11 synapomorphies (5, 7, 10, 12, 28, 29, 33, 35, 42, 52, and 66) and 12 synapomorphies (3, 13, 17, 20, 25, 26, 27, 34, 36, 40, 49, and 66). It should be highlighted that Archaeopithecus rogeri is not


grouped into the notopithecid clade, being the most noteworthy difference with respect toVera’s (2016, fig. 2B) cladogram,where A. rogeri is the sister taxon of the clade gathering Transpithecus and Guilielmoscottia. In fact, in one of the most parsimonious topologies (tree 0 of 204; Fig. 8.2), Archaeopithecus rogeri (with 10 autapomorphies) splits from node A as sister taxon of a most- inclusive clade (node B), gathering the notopithecid clade (node C) and node D, which groups some oldfieldthomasids (node E) and node F (interatheriids, hegetotheres, mesotheres, and archae- ohyracids). Node A (Fig. 8.2) links Archaeopithecus rogeri with node B by two synapomorphies: 52 and 62. According to this result, Archaeopithecus rogeri is a basal notoungulate, not directly related to any group (family) of this order; therefore, previous hypotheses linking this taxon to Oldfieldthomasia (Reguero and Prevosti, 2010) or notopithecids (Vera, 2016) are discarded. The strict consensus (Fig. 8.1) yielded relatively good branch support and a good fit with the stratigraphic record of the taxa


included, in agreementwith the topology obtained byVera (2016, fig. 2B). However, the relationships among Eocene taxa are not well established, as shown the polytomies on nodesAand B (Fig. 8.1), in comparison with the better resolution of more-derived notoungulate groups. These particular polytomies are due to the shifting position mainly of the oldfieldthomasiids members, such as Kibenikhoria and Oldfieldthomasia (Fig. 8.2; node E). Despite recognizing only one taxon, the family Archaeopithe-


cidae as a Linnaean hierarchical rank is here maintained until new studies of basal groups are undertaken, and the relationships among Paleogene notoungulate families are identified more clearly.


Body mass estimation and paleoecological inferences.—Size, which is one attribute of niche differentiation in herbivorous mammals (Jarman, 1974; Owen-Smith, 1988), also allows understanding the paleobiology and paleoecology of a fossil organism (Elissamburu, 2012). The most common way to esti- mate body mass in fossil ungulates is applying allometric equa- tions derived from extantmammals, using linear regression based on dental, craniomandibular, and postcranial measurements (Damuth, 1990; Janis, 1990; Scott, 1990; Mendoza et al., 2006; Tsubamoto, 2014). Particularly for ungulates, Janis (1990) rea- lized that the parameterswith less variation and greater correlation with body mass on herbivorous ungulates were the lengths (MDD) of each lowermolar (m1,m2, and m3),M2, and the series m1–3, with the last being the most reliable. Several estimations were carried out for different groups of South American notoun- gulates (Madden, 1997; Elissamburu, 2004; Elissamburu and Vizcaíno, 2004; Croft and Anderson, 2008; Reguero et al., 2010; Vizcaíno et al., 2010; Scarano et al., 2011; Cassini et al., 2012a; Elissamburu, 2012), including a geometric morphometric approach by Cassini et al. (2012b). Based on several allometric equations from different authors, Elissamburu (2012) estimated body size for 50 genera of Notoungulata and proved that lower molar row length (LMRL) and first lower molar length (FLML) offer more stable estimation values, with Janis’ (1990) predictors being best for notoungulate taxawith unknown postcranial bones, such as most of the Eocene groups. Specifically, regarding early


1291


Paleogene Patagonian taxa, Elissamburu (2012; table 2) provided mean body mass values for archaeopithecids (Acropithecus)and notopithecids (Notopithecus). In turn, Scarano et al. (2011) proposed new linear regression models that proved to be most successful for small herbivorous ungulates with body mass under 13 kg; for larger ungulates, the equations postulated by other authors (Damuth, 1990; Janis, 1990) worked better. Following these inferences, body mass was estimated for


Archaeopithecus rogeri and notopithecids (Notopithecus, Antepithecus, Transpithecus, and Guilielmoscottia) using Vera’s (2013b) dataset and applying equations from Janis (1990) and Scarano et al. (2011). In particular for Notopithecus, body mass was also inferred from astragalar parameters (Tsubamoto, 2014) based on one specimen (MPEF-PV 1113, Vera, 2012b). Mean values for each model and taxon, as well as the average total, are listed in Table 3. Taking into account average total values (Table 3), the


estimated body mass of Archaeopithecus rogeri (1.62 kg) falls between the estimated values for the notopithecids Notopithecus adapinus (1.40 kg) and Antepithecus brachystephanus Ame- ghino, 1901 (1.68 kg), and is surpassed by Transpithecus obtentus (1.82 kg) and Guiliemoscottia plicifera (2.38 kg). However, after exploring each model separately and applying second lower molar length (SLML), third lower molar length (TLML), and m1–3 length (LMRL) models, the body mass of the archaeopithecid is higher than that of N. adapinus and A. brachystephanus, but is still surpassed by the predicted mass values for T. obtentus and G. plicifera (Table 3). In summary, the body mass values obtained for Archaeopithecus rogeri fall within the range between 1.43 kg and 2.57 kg, overlapping the values for notopithecids (Table 3) and the hegetotheriids Pachyrukhos and Paedotherium (~2 kg, Cassini et al., 2012a, 2012b; Elissamburu, 2012), but below the body mass estimated for the interatheriinae Protypotherium australe (~8 kg, Scarano et al., 2011; Cassini et al., 2012a; 3–5 kg, Cassini et al., 2012b; ~7 kg, Elissamburu, 2012). Indeed, the body mass estimates of the present study for Notopithecus and Archaeopithecus (Table 3) are very close to those previously reported by Elissamburu (2012, table 2). In contrast, when considering models based on postcranial remains, body mass values are higher than those based on dental models, as it is corroborated for Notopithecus (Table 3; Elissamburu, 2012); in this case, when applying Tsubamoto (2014) models based on astragalar lineal measurements, body masses predicted for Notopithecus do not vary between equations (Table 3), and their values are closer to those estimated using humerus length than those based on other long bones (Elissamburu, 2012). Archaeopithecus rogeri is a brachydont notoungulate


restricted to the Casamayoran SALMA, but characterized by having relatively higher crowns than other contemporaneous brachydont groups, such as notopithecids and oldfieldthoma- siids. Explaining this incipient protohypsodonty (Mones, 1982) is not easy, given that Archaeopithecus is ancestral to any other notoungulate group; phylogenetic effects and an adaptive differential use of feeding source (substrate preference) should be considered, particularly during the middle Eocene when a global cooling occurred and open-vegetation habitats expanded in central Patagonia (Bellosi and Krause, 2014, and references therein).


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