Hemiauchenia gracilis Meachen, 2005
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publication ID |
https://doi.org/ 10.4202/app.2011.0005 |
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persistent identifier |
https://treatment.plazi.org/id/03988852-FFC3-3644-4401-9D47FA917D26 |
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treatment provided by |
Felipe |
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scientific name |
Hemiauchenia gracilis Meachen, 2005 |
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Hemiauchenia gracilis Meachen, 2005
Figs. 3–7, Tables 1–4.
Type material: UF210707, holotype, a right mandibular fragment including p4–m3 ( Meachen 2005).
Type locality and horizon: De Soto Shell Pit 5 in De Soto County, Florida, Caloosahatchee Formation , latest Blancan (UF locality DE011) .
Referred material.—Locality of Barranca del Berrendo (HGO−28): UAHMP−357, distal portion of a metatarsal; UAHMP−1142, partial skull. Locality of Barranca San Agustín (HGO−29): UAHMP−419, distal part of a left scapula; UAHMP−515, distal portion of a left tibia. Locality of El Barrio (HGO−47): UAHMP−1144, fragmentary left mandible preserving p4–m 3 in situ, as well as the alveoli for i1–i3 and c; UAHMP−962, metatarsal fragment; UAHMP−954, two proximal phalanges of the same individual.
Emended diagnosis.—Small−sized llama (mean UTRL = 91.08 mm); moderately hypsodont teeth (mean molar crown height along mesostyle ~ 20 mm); both upper and lower molariforms covered by cementum; well−developed parastyle and mesostyle on M1–M3; upper molars with U−shaped fossettes; lack of p1 and p3; robust posterolophid on m3; and gracile limbs.
http://dx.doi.org/10.4202/app.2011.0005 parietal bone frontal bone sharp (CS) high (OR)
50 mm sagittal crest frontal bone pb 50 mm palatine foramen P3 palatine notch M3 P3 M3
Description
Skull and upper dentition.—Specimen UAHMP−1142 ( Fig. 3A–E) is a partial skull comprising parts of the cranial vault, as well as the palatines, maxillae (preserving both left and right P3–M3), and several unidentifiable fragments. The frontal bones are directed forwards and downwards, transversely narrow and rounded posteriorly, and expanding transversely towards their anterior borders. The cranial suture between the frontals is well delineated. The parietal bones are dorsally convex and separated by a narrow, triangular sagittal crest ( Fig. 3A, B). The palatines are slightly concave ventrally and bear a prominent sagittal crest on their dorsal side. The suture between each maxilla and palatine is V−shaped, pointing anteriorly. The palatine foramina are located in line with the posterior border of P3, while the palatine notch is sharply V−shaped and extends anteriorly to the level of the anterior portion of M2 ( Fig. 3C, D).
The mean UTRL of specimen UAHMP−1142 is ca. 90 mm, indicating a small−sized individual, while the molariforms are in moderate to late−moderate stages of tooth wear, suggestive of a young adult. The cheek teeth show little crenulation and are transversely compressed, with a mean molar crown height (along the mesostyle) of ca. 20 mm. The P3 is small, unworn and resembling a blade, two−rooted, and shows two small lingual ridges located at the base of the tooth. Its occlusal surface is situated below the crown height of P4 or any of other upper molars, indicating that this tooth was non−functional. In occlusal view, the P4 is subquadrangular in outline, with a rounded lingual and a straight labial border, and a single U−shaped fossette. The parastyle is rounded and well developed, while the metastyle is relatively poorly developed. A faint, rounded rib is present on the ectoloph. M1–3 are distinguished by having prominent styles and ribs. The protocones and the metaconules are rounded and subequal in size, and the pre− and postfossetes are U−shaped. A thin layer of cementum is present on the ectoloph of the molars ( Fig. 3D, E).
Mandible and lower dentition.—The mandibular fragment UAHMP−1144 ( Fig. 4A, B View Fig ) is shallow and slender. A relatively short diastema, 49.2 mm long, is located between the alveoli for the canine and p4. The alveolus for the canine is large, subovoid, and deep. The diastemal crest is sharp. The mandibular depth increases anteroposteriorly (depth below anterior p4 = 23.6 mm; depth below posterior m3 = 34.8 mm). A large and well−developed mental foramen is situated below the alveolus for the canine, while a second, small foramen occurs below the midpoint of p4 ( Fig. 4A View Fig ).
While p4 and m1 exhibit a late stage of tooth wear, m2 is at an early late stage of wear, and m3 at a late moderate stage, with the wear on the molar series as a whole corresponding to wear stage three of Breyer (1977). Taken together, this wear stage and the full eruption of m3 are indicative of an adult individual. The length of the p4–m3 dental series is approximately 82 mm. A thin layer of cementum, most evident at the base of dental crown, is present on the cheek teeth. The p1 and p3 are absent. The p4 is triangular in occlusal view and bears two fossettids at the posterior end of the occlusal surface, with the more anterior fossettid being the larger one. The m1 is quadrangular and marked by a hypoconid much broader than the protoconid. The m2 is rectangular, with the protoconid and hypoconid being rounded and subequal in
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size, and is the only tooth showing a weak anteroexternal stylid (“llama buttresses”). A robust and well−differentiated hypoconulid is present on m3 ( Fig. 4B View Fig ).
Postcranial elements.—Specimen UAHMP−419 ( Fig. 5A View Fig ) represents the distal end of a left scapula. The infraspinous fossa is very large near the distal end of the shaft. The scapular spine is straight and overhangs the infraspinous fossa. The scapular neck is broad and thick, and the glenoid cavity large and subcircular in ventral view. The lateral and medial edges of the glenoid cavity are rounded, and a small tuberosity is present adjacent to the lateral edge. The coracoid process is massive and rugose.
Specimen UAHMP−515 ( Fig. 5B View Fig ) represents the distal end of a left tibia, marked by a dorsoventrally compressed diaphysis. The lateral and medial grooves on the distal articular surface are subovoid, strongly concave, limited by malleoli, and separated by a sagittal ridge, which terminates anteriorly in a prominent, blunt tongue. The lateral groove is shallower and wider than the medial one, suggesting a llama−like style of articulation with the proximal trochlea of the astragalus ( Webb 1965). The fibular groove is U−shaped and limited by two small malleoli. The lateral and medial sides of the distal end of the shaft are slightly convex.
Specimen UAHMP−962 ( Fig. 6A View Fig ) consists of the proximal portion of a left metatarsal with a length of 24.6 cm. Approximately 75% of the bone are preserved, suggesting a total length of about 33 cm. The proximal articular surface is trapezoidal, with a central deep concavity and three articular facets. The latter include (i) the large, slightly convex, and subovoid cuboid facet; (ii) the bean−shaped entocuneiform facet, slightly smaller than the cuboid facet; and (iii) the small, subrounded ectomesocuneiform facet, separated from the entocuneiform facet. The slender and transversely compressed diaphysis bears a short, narrow channel, extending on to the proximal quarter of the shaft. While the lateral and medial sides of the bone are flattened, its dorsal side is slightly convex and smooth. By contrast, its ventral side is concave, thus forming a wide, deep groove along the shaft.
Specimen UAHMP−357 ( Fig. 6B View Fig ) consists of the distal ends of the proximally fused metatarsals III and IV, with both bones preserving their respective condyles. The lateral sides of the bones are convex, whereas their medial, dorsal and ventral sides are flattened. The large condyles bear prominent sagittal ridges, which extend dorsoventrally on to the distal portion of the shaft.
Finally, UAHMP−954 consists of two proximal phalanges of a single individual ( Fig. 7 View Fig ). In both specimens, the proximal epiphysis is not completely ossified, suggesting a relatively young individual. The diaphysis is subrounded in cross section, with convex dorsal and flattened ventral sides. The depressions for the collateral ligaments are rough and large. The proximal posterior surface is rough and bears a W−shaped scar for the insertion of the suspensory ligament, which extends on to the proximal quarter of the shaft. Distally, the bones terminate in a dorsoventrally expanded articular surface formed by two trochleae, with the lateral trochlea being relatively larger than the medial one.
Geographic and stratigraphic range.—Blancan III or Early Blancan (approximately 4.1–3.0 Ma) of Guanajuato, central Mexico ( Jiménez−Hidalgo and Carranza−Castañeda 2010) and Idaho, USA ( Ruez 2009); Late Blancan (2.5–1.3 Ma) of Florida ( Meachen 2003, 2005), Arizona, and New Mexico, USA ( White and Morgan 2005); Irvingtonian (1.3–0.15 Ma) of Sonora, Northern Mexico ( Croxen et al. 2007; White et al. 2010); and Rancholabrean (160–9.5 ka; following Bell et al. 2004) of Hidalgo, central Mexico (this study) ( Fig. 8 View Fig ).
Discussion
The fossils from Hidalgo show several features typical of lamines, including a high−domed cranium with a weak sagittal crest, reduced premolars, anteroexternal stylids (“llama buttresses”) on the lower molars, a lack of both P2 and p2, a sharp diastemal crest on the mandible, and fused metatarsals. However, the material from Hidalgo differs in several respects from other Pleistocene North American (NA) and South American (SA) lamines, including Palaeolama (NA and SA) , Camelops (NA) , Lama (SA) , and Vicugna (SA) .
The mandible and postcranial elements of Palaeoloma differ from the Hidalgo specimens in possessing: (i) a P4 with V−shaped crescents; (ii) a mandible consistently deeper below p4 than below m1 and m2; (iii) a p4 with complex infolding; and (iv) short and robust metatarsals. Similarly, Camelops differs from the specimens described here in (i) a very deep and robust mandible; (ii) robust metatarsals; (iii) proximal phalanges with a raised suspensory ligament scar extending almost to the center of the shaft; and (iv) its larger size. Finally, in contrast to the specimens from Hidalgo, a p3 may sometimes be present in the South American genera Lama and Vicugna . In addition, the genus Lama is distinguished by having strong anteroexternal stylids on the lower molars, as opposed to the much weaker stylid restricted to m 2 in the material from Hidalgo ( Webb 1965, 1974; Honey et al. 1998; Meachen 2003).
The specimens from Hidalgo described in the present study share with Hemiauchenia the following suite of diagnostic characters: (i) cement−covered cheek teeth; (ii) a mandible increasing in depth from p4 to m3; (iii) a simple and triangular p4; (iv) a sharply V−shaped palatine notch; (v) long and slender metatarsals; and (vi) proximal phalanges with a Wshaped suspensory ligament scar ( Webb 1974; Honey et al.
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1998; Hulbert and Webb 2001). More specifically, the length of the upper premolar−molar tooth row (P3–M3) of UAHMP−1142 (mean UTRL = 91.08 mm) is indicative of a small−sized form, which may indicate its affinity with relatively small species, such as H. gracilis and H. edensis ( Meachen 2003, 2005; Webb et al. 2008), rather than the medium−sized H. macrocephala or the large H. blancoensis ( Webb 1974; Breyer 1977; Kurtén and Anderson 1980).
The morphology of the upper cheek teeth of UAHMP−1142 resembles that of H. gracilis and H. macrocephala in the presence of a two−rooted P3, molars covered by a thin layer of cementum, U−shaped molar selenes, well−developed styles and ribs, and a small degree of crenulation ( Meachen 2003, 2005). However, H. macrocephala differs from the Hidalgo material in having a subtriangular and relatively large P4 (as compared to the molars) ( Webb 1974), as well as larger and more robust molars, resembling H. blancoensis in this regard ( Table 1). In H. edensis , the cheek teeth lack cementum and the molar selenes are V−shaped ( Meachen 2003, 2005; Jiménez−Hidalgo and Carranza−Castañeda 2010).
Breyer (1977) demonstrated the significance of mandibular morphology in the recognition of different species of Hemiauchenia . The mandible from Hidalgo (UAHMP−1144) resembles that of H. gracilis and H. edensis in being shallow and slender, while being deeper than that of “ Hemiauchenia ” minima, and considerably shallower than that of H. macrocephala and H. blancoensis ( Table 2; Meachen 2003). The length of the diastema in UAHMP−1144 (ca. 49 mm) is relatively longer than in the holotype of H. gracilis from the Blancan of Florida (UF 210707, MDL = 39.2 mm) ( Meachen 2003), but shorter than in the holotype of H. macrocephala from the Irvingtonian of Texas (TMM 18621, MDL = 61 mm), or the specimens referred to H. blancoensis from the Blancan of Nebraska (e.g., UNSM 21382, MDL = 67.3 mm; UNSM 213890, MDL = 70.8 mm) (see Breyer 1977: fig. 2B, D). The length of the lower tooth row in UAHMP−1144) is similar to that of Hemiauchenia gracilis (UF 210707, right mandibular fragment with p4–m3 from De Soto Shell Pit site, Late Blancan of Florida), longer than that of “ H. ” minima and H. edensis (72 mm, including the p3) ( Meachen 2003), and shorter than that of H. blancoensis and H. macrocephala ( Table 2).
UAHMP−1144 resembles H. gracilis in the lack of p1 and p3, the presence of cementum on the lower molars, and a robust posterolophid on m3 ( Meachen 2003, 2005). Unlike UAHMP−1144, the material of H. gracilis from Florida is marked by the presence of a very prominent "llama buttress" on m3 (see Meachen 2003, 2005). This difference may be explained in two ways: (i) it is possible that throughout the evolution of H. gracilis there was an evolutionary trend towards a decrease in the anterolingual development of the stylids, resulting in their near or complete loss in the Rancholabrean forms; or (ii) the posterior margin of m 2 in the material from Hidalgo closely approximates the anterolabial margin of m3, thus possibly preventing the complete development of the latter. A much larger fossil sample is needed to properly address these questions.
The lower cheek teeth of H. edensis differ from those from Hidalgo in a lack of cementum ( Webb et al. 2008), whereas the lower molars of H. blancoensis and H. macrocephala are distinguished by having strong anteroexternal stylids. Furthermore, a two−rooted p3 may sometimes be present in the latter two species ( Webb 1974; Breyer 1977).
The lamines generally resemble each other in terms of their postcranial morphology, with only a few characters, such as the size and proportion of the limb elements, showing some taxonomically significant variation (see Webb 1974; Honey et al. 1998). The estimated length of UAHMP−962 is comparable to that of UF176935 ( Table 3), a complete metatarsal of Hemiauchenia gracilis from the Late Blancan locality of Inglis 1A, Florida ( Meachen 2003, 2005). By contrast, UAHMP−962 is longer than the metatarsal of H. edensis , and shorter than those of H. macrocephala and H. blancoensis ( Meachen 2005) . The length/width ratio of UAHMP−962 is indicative of a long and slender metatarsal, pointing to a gracile individual. The metatarsals of H. edensis are short and slender (GL: SD, 16.0), those of H. macrocephala are long and slender (GL: SD, 12.2), and those of H. blancoensis are long and robust (GL: SD, 14.0), whereas those of H. gracilis are very long and slender (GL: SD, 16.7) ( Meachen 2005: 455). The limb proportion of UAHMP−962 is similar to that of H. macrocephala and H. blancoensis ; nevertheless, the metatarsal from Hidalgo is shorter and has a smaller diameter, indicating a less stout−legged form ( Table 3).
The proximal phalanges from Hidalgo (UAHMP 954a, b) are of comparable size to those of Hemiauchenia gracilis , as represented by UF179638 and UF179639, two proximal phalanges from the locality of Inglis 1A, Late Blancan of Florida ( Meachen 2003). By contrast, the specimens from Hidalgo are shorter than proximal phalanges assigned to H. blancoensis and H. macrocephala ( Table 4). The length/ width ratio of the specimens from Hidalgo is 6.0 (GL:SD), giving the phalanges a gracile appearance. This proportion is comparable to that of H. edensis (6.1), relatively shorter than that of H. gracilis (~ 7.1), and larger than that of H. macrocephala (5.4) ( Meachen 2003, 2005).
The size and slenderness of UAHMP 419 (distal part of scapula; maximum distal width of 79.1 mm [including the coracoid process]) and UAHMP 515 (tibial fragment; maximum distal width of 56.31 mm) ( Fig. 5 View Fig ), are suggestive of small−sized individuals with gracile extremities.
Overall, the comparison of the postcranial material from Hidalgo with selected species of Hemiauchenia does not seem to provide much useful taxonomic information, owing to its relatively homogenous morphology and size. Nevertheless,
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we here consider these elements to belong to the same species, given their association with diagnostic cranial and dental material which can be confidently referred to H. gracilis .
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