Gualta cuyana Cerdeño and Vera, 2015
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publication ID |
https://doi.org/ 10.4202/app.00301.2016 |
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persistent identifier |
https://treatment.plazi.org/id/03ADD838-FF8D-FF89-8829-FE43FB4A858D |
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treatment provided by |
Felipe |
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scientific name |
Gualta cuyana Cerdeño and Vera, 2015 |
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Gualta cuyana Cerdeño and Vera, 2015
Figs. 4–6 View Fig View Fig View Fig .
Material.—MCNAM-PV 4696, associated left pyramidal, magnum, unciform, and fragment of Mc III; MCNAM-PV 5078, associated left pyramidal, unciform and fragments of humerus and ulna; MCNAM-PV 4094, MCNAM-PV 4785, incomplete tibiae; MCNAM-PV 4303, MCNAM-PV 4891, MCNAM-PV 4089, two right and one left astragali; MCNAM-PV 4894, MCNAM-PV 4896, right and left astragalus fragments; MCNAM-PV 4412, calcaneum fragment; MCNAM-PV 5035, left navicular. All material from Quebrada Fiera, Mendoza Province, Argentina, Deseadan SALMA, late Oligocene.
Description.— Forelimb: The specimen MCNAM-PV 4696 ( Fig. 4A–F View Fig , Table 3) includes the pyramidal, magnum, unciform and part of the Mc III of a same individual. The pyramidal ( Fig. 4A View Fig 1 –A View Fig 4 View Fig ) is a low, irregular bone. The anterior face is convex and projected laterodistally; the lateral face is wrinkled and presents a proximolateral prominence; below and posterior to it there is a high semicircular facet for the pisiform. The proximal facet is large, almost occupying the whole face; its APD is greater than the TD; it is transversely concave and the medial border is straight ( Fig. 4A View Fig 3 View Fig ). This border forms a crest with the medial facet, which is long in APD and low: it is flattened, but with a slight crest in the middle that barely differentiates two articular areas. Below this facet, there is a shallow, wrinkled depression. On the distal face ( Fig. 4A 4 View Fig ), there is a big subtriangular facet for the unciform ( Fig. 4F View Fig ); it is anteriorly wide and transversely concave, and narrows posteriorly, becoming convex and reaching the lateroposterior facet for the pisiform.
The magnum ( Fig. 4B View Fig ) shows a trapezoidal anterior face, with a notch on the medial border; a strong rounded tuberosity occupies most part of the face, surrounded by a shallow proximomedial depression ( Fig. 4B View Fig 1 View Fig ). The proximal face ( Fig. 4B View Fig 2 View Fig ) bears two facets that rise posteriorly with a crest in between. The medial one articulates with the scaphoid; it is subtriangular, as wide as long (APD), and projects medially in anterior view. The other proximal facet is for the semilunate; it elevates backward over a protuberance of the bone, becoming very convex, whereas the most anterior area directs laterally and contacts the lateral facet for the unciform. Just behind the limit of the proximal facet, the bone is low, but the posterior apophysis is lacking. The unciform facet is subcircular-trapezoidal and reaches the distal border ( Fig. 4B View Fig 3 View Fig ). Behind this facet and below the proximal border, there is a wrinkled concavity. On the medial face of the bone, forming an angle greater than 90° with the proximal facet, there is a flat, quadrangular articulation for the trapezoid, projected backwards in a narrow edge, and well separated from the distal border by a concave, wide area where the Mc II would fit ( Fig. 4B 4 View Fig ). Behind the border of the superior medial facet, the bone has another wrinkled depression. Distally there is a smoothly concave facet, anteriorly wide and tapering backwards.
The unciform ( Fig. 4C View Fig ) is long and low. The proximal face is formed by two large facets placed at a strong angle and forming a distinctive acute crest ( Fig. 4C View Fig 1 View Fig , C 2 View Fig ). The most anterior facet, for the semilunate, is smoothly concave-convex, high and narrow, steeply inclined anterodistally and protruding medially at its proximomedial area. The facet for the pyramidal is very concave anteroposteriorly, but transversely convex at its anterior half, where it projects laterally whereas the distal half inclines medially. The posterior limit of this facet is a posterior, narrow and convex tuberosity, whose most proximal area is smooth and could fit with the pisiform, even though there is not a well-defined facet. The lateral face of the bone is flattened, with the central portion a little depressed; its distal border approximates a very open “V” in contrast to the proximal inverted “V” (due to the crest between the two proximal facets) ( Fig. 4C View Fig 2 View Fig ). On the medial face, below the projection of the proximal facet, there is a strong, low tuberosity that widens the bone at this level ( Fig. 4C View Fig 3 View Fig ). In distal view, there is a great concave subtriangular facet for the Mc IV. It forms a marked angle (~50º) with a second articular surface differentiated into two flat zones, for the articulation with the lateral and proximolateral facets of the magnum; therefore, anatomically, these two facets are medial and not distal, but are distally oriented ( Fig. 4C 4, 4E View Fig ).
The fragment of the Mc III ( Fig. 4D View Fig 1 View Fig , D 2 View Fig ) has a large, trapezoidal proximal facet that fits with the described magnum; it is deeper (APD) than wide. On the medial face, there are two small facets for the Mc II, the anterior one more developed in height. Laterally, there are two large, high and slightly concave facets for the Mc IV; the posterior one is more proximally placed whereas the anterior one is a little separated from the proximal border; both facets are separated by a shallow depression, although there is a short bony “bridge” between them at the proximal area.
A second set of associated bones (MCNAM-PV 5078) is composed of the left pyramidal and unciform highly comparable to those herein described and just slightly smaller ( Table 3); together with them there are small portions of the ulna and humerus, which are too incomplete to make comparisons; only the humerus trochlea reveals a smaller size than the specimens MCNAM-PV 3938 and MCNAM-PV 4376 previously described for Gualta cuyana ( Cerdeño and Vera 2015) .
Based on the short descriptions of these bones for the leontiniid Scarrittia ( Chaffee 1952) and the toxodontid Nesodon ( Scott 1912) , it is not easy to establish clear morphological differences between the two groups. As a whole, our material seems to be very close to Scarrittia when compared with the forefoot AMNH 29582, although some differences exist. Some features of our pyramidal (proximal facet not ovoid, having a straight medial border, and lacking an anteromedial tubercle) separate it from Nesodon ( Scott 1912) . The unciform differs from both compared taxa in the absence of a well-developed facet for Mc V; the subtriangular distal facet for Mc IV differs from the quadrate outline described for Nesodon . The concavity of the anterior medial border of the magnum also differs from both taxa, in which this border is straighter. The angle of the anterolateral border of the unciform is less acute than that of both genera, based on the figure of Nesodon ( Scott 1912: pls. 22, 5) and the photographs of AMNH 29582 of Scarrittia . The width of the proximal epiphysis and the angle between the two proximal facets of Mc III are closer to that observed in Scarrittia . Despite the particularities of the described postcranial bones, it seems confident to refer them to the leontiniid Gualta cuyana instead to the toxodontids recognized at Quebrada Fiera.
As stated in the introductory paragraphs,the studied bones are discarded to belong to the notohippid Mendozahippus fierensis ( Cerdeño and Vera 2010, 2014). Even though preserved fragments are not homologous, the proximal TD of the Mc III assigned to G. cuyana measures 34.6 mm ( Table 3) whereas that of the Mt III of Mendozahippus fierensis is clearly smaller (12.4 mm; Cerdeño and Vera 2010: table 3). Measurements provided by Shockey (1997: table 3) for Eurygenium pacegnum from Salla reflect that pyramidal and unciform are smaller than our specimens. With respect to mesotheriids, the morphology of the magnum and unciform, for instance, is too different to consider their belonging to this group ( Cerdeño et al. 2012; Shockey et al. 2016).
Hind limb.—The correspondence of some tibial fragments with the astragali assigned to leontiniids (see below) allows identification of the specimens MCNAM-PV 4094 and MCNAM-PV 4785 as G. cuyana . These specimens ( Fig. 5A– C View Fig , Table 4) reveal that the tibia is relatively long and slender. The shaft is compressed laterally, trihedral in cross-section, becoming wider and more rounded toward the distal end MCNAM-PV 4094; Fig. 5A View Fig ). The tibial crest appears to be short, but it is incomplete. The distal epiphysis is rectangular in cross section, with a prominent medial malleolus that bears a facet at the laterodistal end. The lateral side of the astragalar facet is more oblique and shallower than the medial one; both subfacets are separated by a low intercondylar ridge. Chaffee (1952) described the tibia of Scarrittia as a massive bone, which is not the case for MCNAM-PV 4094 (the most complete of the studied specimens). This difference cannot be adequately evaluated with the present sample, and a broader study of different long bones collected at Quebrada Fiera will be achieved soon. The studied tibiae are similar in outline to that of the toxodontid Nesodon (field number RB2- SHP-13-1, pending catalogue number at MPM-PV, SHP personal data), but with a distal epiphysis less quadrangular and a less vertical malleolus.
MCNAM-PV 4412 ( Table 1) is a calcaneum fragment comparable to the specimens previously ascribed to Gualta cuyana ( Cerdeño and Vera 2015) . It preserves the tuber and the fibular and astragalar facets; the sustentaculum and the distal area are lacking.
Two of the referred astragali (MCNAM-PV 4891 and MCNAM-PV 4303) are very well preserved ( Fig. 6A View Fig , Table 2). The bone is quadrangular in outline, much larger and robust than MCNAM-PV 4215 assigned to Proadinotherium ( Fig. 2G View Fig ). The trochlea is rather symmetrical and shallow ( Fig. 6A View Fig 1 View Fig , A 3 View Fig ); the distal articulation is little detached, with the astragalar neck reduced to a shallow groove. The medial condyle of the trochlea is more projected proximally in MCNAM-PV 4303 than in MCNAM-PV 4891. On the posterior face ( Fig. 6A View Fig 2 View Fig ), there are two flattened facets for the calcaneum, the lateral one slightly concave. Both facets face posteriorly at a different plane and are separated by a deep groove that opens proximally, without forming an astragalar foramen, and widens distally toward the astragalar neck. The lateral facet is longer than the medial one, and it occupies the whole length of the posterior face, being wider proximally than distally, with a medial notch. The medial facet is more rounded and contacts distally with the navicular facet. The medial face of the bone shows a large articulation for the tibia expanded distomedially over the astragalar head. The latter is mostly occupied by the navicular facet that, however, does not reach the medial border. This articulation is subtrapezoidal, anteroposteriorly convex and rather straight transversely ( Fig. 6A View Fig 3 View Fig ), although MCNAM-PV 4303 shows a very slight concavity. The lateral face ( Fig. 6A View Fig 4 View Fig ) shows the flattened articular surface for the fibula and an elliptic depression below it, placed at a centrodistal position. All these features coincide with leontiniids such as Scarrittia (e.g., AMNH 29582, S. canquelensis from Sierra Canquel, Chubut Province; see also Shockey et al. 2012) and Leontinia (e.g., MACN A 12311, L. gaudryi from La Flecha, Santa Cruz Province), in which the trochlea is even a little deeper than in the studied astragali. In contrast, the compared astragali of early Miocene toxodontids differ in the deeper trochlea, the distal area a little more detached, the medial facet for the calcaneum wider distally and the lateral depression centroproximally placed.
The navicular MCNAM-PV 5035 ( Fig. 6B View Fig , Table 5) is composed of a main corpus with the articular facets and a long, narrow and curved posterolateral apophysis, resulting in a relatively narrow and deep (APD) bone. The proximal face ( Fig. 6B View Fig 1 View Fig ) is occupied by a large anteroposteriorly concave facet for the astragalus; its outline is rather square, but two small portions project posteriorly, leaving a wide notch in between. At the level of this notch, the apophysis develops and is directed posteriorly and laterally. In distal view ( Fig. 6B View Fig 2 View Fig ), there are two main facets, a medial one for the mesocuneiform and a larger, lateral one for the ectocuneiform; the former is convex-concave whereas the latter is smoothly convex anteroposteriorly; both facets are placed in a continuous surface with a crest in between. There is no articulation for the entocuneiform. Also seen in distal view, there is a small lateral facet for the cuboid, which in turn forms an acute angle with another lateral facet for the calcaneum. In lateral view, the apophysis surpasses a little the distal face. The anterior and medial faces form a continuous rugose surface, without forming great tuberosities. This morphology coincides with that of Scarrittia canquelensis ( Chaffee 1952) and differs from toxodontids in the presence of a large posterior apophysis. Similarities are evident when comparing MCNAM-PV 5035 with the photographs of the homologous bone of the specimen AMNH 29585, but the size of the latter is much greater. According to our estimates for AMNH 29585, the navicular of G. cuyana is around 36–38% smaller than that of S. canquelensis ( Table 5).
Concerning notohippid morphology, the astragalus of this group is closer to the homologous bone in the toxodontid Proadinotherium (see comments above) than to the specimens assigned to Gualta .
Remarks.—Within the Quebrada Fiera fauna, the leontiniid Gualta cuyana was described based on a skull and associated vertebrae, and some mandibular fragments, but some limb elements (calcanei and humeri) were also originally assigned to this taxon ( Cerdeño and Vera 2015). The present study has allowed the reinterpretation, both anatomically and taxonomically, of some postcranial bones preliminarily identified as toxodontids ( Hernández Del Pino et al. 2013), which are described and discussed below.
Stratigraphic and geographic range.—Late Oligocene; Mendoza Province, Argentina.
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