versión impresa ISSN 0002-7014
Ameghiniana vol.47 no.3 Buenos Aires set. 2010
Taxonomic reinterpretation of a Notoungulata Typotheria from the early Oligocene of Cañadón Blanco (Chubut, Argentina)
Esperanza Cerdeño1, Marcelo Reguero2 and Bárbara Vera1
1Departamento de Paleontología, Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), Centro Científico Tecnológico-CONICET-Mendoza. Avenida Ruiz Leal s/n, 5500 Mendoza, Argentina. firstname.lastname@example.org. email@example.com
2Departamento Científico de Paleontología de Vertebrados, Museo de La Plata, 1900 La Plata, Argentina. firstname.lastname@example.org
The Cañadón Blanco locality, in Chubut Province (figure 1), is a classical Paleogene site in Patagonia with mammals mostly collected by Dr. S. Roth. As for other classical collections, precise geographic and geologic data were never recorded, and consequently some problems arise. Reguero (1999), studying the Deseadan notoungulates, made a full revision of many of these materials and synthesized the problems with different localities. Concerning Cañadón Blanco, he explained that Roth provided scarce data on the localities where he recovered fossils. Following the inventory of the Museo de La Plata, Roth collected between 1896 and 1898 at several Patagonian localities, including a trip across the Genoa valley to the Paso de Indios, collecting at different sites, Cañadón Blanco among others. Reguero (1999: 43) deduced that Cañadón Blanco corresponds to any basin NW of Laguna Palacios in the Paso de los Indios Department. Roth (1904) described several taxa from the Lower Tertiary deposits of Cañadón Blanco, without any further reference to the fossiliferous beds. Concerning age, Cañadón Blanco seems to correspond to the "Astraponotéen plus supérieur" of Ameghino from Gran Barranca, which is currently correlated to the Tinguirirican age (Reguero, 1999; Reguero et al., 2003).
The specimen presented in this work is a very interesting incomplete mandible of a notoungulate presenting a set of characters that a priori could indicate different families of Typotheria, which is why it deserves a particular study. Reguero (1999: 203) originally considered it to represent a new taxon of the Family Hegetotheriidae, but a different taxonomic interpretation is presented in this paper.
Material and method
The studied specimen, MLP 12-1530, is housed at the Museo de La Plata (MLP). It is an incomplete mandible with the symphysis and the anterior portions of both horizontal rami; it preserves right i1-i2, left i1-i3, and right and left p2-p4 (originally labeled as p3-p4-m1 and already interpreted as p2-p4 by Reguero, 1999).
A morphometric comparison was carried out, using hegetotheriid and archaeohyracid specimens of Oligocene age from Argentina, Chile, and Bolivia.
Abbreviations. In text and table: ant., anterior; H, height; i, incisor; L, length; p, premolar; post., posterior; W, width. Institutions: MCNAM-PV, Museo de Ciencias Naturales y Antropológicas "J. C. Moyano" of Mendoza, Argentina -Vertebrate Paleontology collection; MLP, Museo de La Plata, La Plata, Argentina; SGOPV, Museo Nacional de Historia Natural of Santiago de Chile- Vertebrate Paleontology collection.
The mandibular rami are high (figure 2.1). The symphysis reaches the level of the posterior part of p2, and forms a strong angle with the rami (figure 2.1-2). It is narrow and the i3 is aligned with the jugal series.
Figura 2. Mandibular fragment of cf. Protarchaeohyrax intermedium, MLP 12-1530, from Cañadón Blanco, Chubut, Argentina/ fragmento mandibular de cf. Protarchaeohyrax intermedium, MLP 12-1530, de Cañadón Blanco, Chubut, Argentina. 1, Left lateral view / vista lateral izquierda. 2, occlusal view / vista oclusal. 3, detail of incisors (left i1-i3, right i1-i2) / detalle de los incisivos (i1-i3 izquierdos, i1-i2 derechos). 4, detail of premolars (right p2-p4, anterior to right) / detalle de los premolares (p2-p4 derechos, cara anterior a la derecha). 5, right lateral view showing the curved root of p4 / vista lateral derecha mostrando la raíz curvada de p4. Scale bars = 10 mm / escala gráfica = 10 mm.
The incisors are procumbent, narrow and labio-lingually flattened, with a smooth median lingual groove; it becomes more conspicuous from i1 to i3, clearly differentiating two lingual lobes on the latter (figure 2.3). The labial face is rather straight in i1-i2 and more convex in i3.
Root remains of p1 are preserved. Premolar p2 appears as a long and narrow bilobed tooth, with a well developed labial groove. The anterior lobe is much longer than the posterior one and presents two lingual sulci, shallower than the labial one (figure 2.4). In p3 and p4, the anterior lobe is relatively shorter than that of p2 (slightly shorter in p4 than in p3), though it is still longer and labially more rounded than the posterior lobe. The lingual wall is quite straight, but a smooth concavity is present. There is a little trigonid-talonid fossettid in both premolars, placed slightly posterior to the labial groove (figure 2.4). The broken area of the horizontal rami exposes the p4 roots, which are long, closed, and posteriorly curved (figure 2.5).
Dimensions of the specimen: MLP 12-1530 are presented in table 1.
Table 1. Mandibular and dental dimensions (mm) (left and right sides) of cf. Protarchaeohyrax intermedium, MLP 12-1530, from Cañadón Blanco, Chubut, Argentina. Values in brackets are approximate / dimensiones mandibulares y dentarias (mm) (lados izquierdo y derecho) de cf. Protarchaeohyrax intermedium, MLP 12 -1530, de Cañadón Blanco, Chubut, Argentina. Los valores entre paréntesis son aproximados.
Based on the specimen MLP 12-1530, Reguero (1999) established the genus and species Archaeopachyrucos ulianai as the most primitive known hegetotheriid, separating it from other Oligocene hegetotheriids such as Prosotherium Ameghino, 1897, Medistylus Stirton, 1952, and Propachyrucos Ameghino, 1897. However, the diagnosis is not comparative enough to understand the differences among these taxa, except its smaller size, and one of the mentioned characters suggests referral to the Family Archaeohyracidae. As explained below, the revision of this specimen suggests a new taxonomic interpretation, and therefore the taxon Archaeopachyrucos ulianai, being unpublished (Reguero, 1999), becomes a nomen nudum.
The first contradiction in assigning this specimen to the Family Hegetotheriidae is the absence of an enlargement of the i1-i2, which is a diagnostic character of this family. In fact, in MLP 12-1530, the i1 is slightly smaller than i2, which is smaller than i3. In Prosotherium and Prohegetotherium sculptum Ameghino, 1897, i1 is larger than i2 (Reguero, 1999). In the holotype of Sallatherium altiplanense Reguero and Cerdeño, 2005, from Salla (Bolivia), i1 and i2 are more similar in size, but i3 is reduced and separated. In Prohegetotherium schiaffinoi (Kraglievich, 1932) from this same locality (E.C. unpubl. data), the lower incisors present a slight decrease in size from i1 to i3.
In contrast to the incisor condition, the cheek teeth are apparently closer in occlusal morphology to hegetotheriids in having a flattened lingual wall. However, they also present other features that differ from this family and resemble the Archaeohyracidae, such as the presence of a fossettid in p2-p3, a similarity that was already stated by Reguero (1999) in the diagnosis of his new taxon. The presence of fossettes/fossetids in hegetotheriids is debatable. Reguero and Cerdeño (2005) described a juvenile specimen of the hegetothere Sallatherium altiplanense with fossettes, but this specimen was later recognized by Billet et al. (2009) as an archaeohyracid, Archaeohyrax suniensis. In turn the latter authors assigne to Sallatherium a young adult maxillary with M1-M3 showing small fossettes. In this case, we do not agree with this determination, and consider that it can also correspond to an old specimen of A. suniensis, whose ontogenetic variation of tooth morphology was well established by Billet et al. (2009). The rounded anterior lobe of p3 may recall hegetotheriids, but it is not so different from that observed on some archaeohyracids (see below). In addition, the p2 differs from the homologous tooth in hegetotheriids (Prosotherium, Prohegetotherium, Sallatherium) by being longer and more anteriorly pointed.
Considering the characters just discussed, the studied specimen can be considered an Archaeo-hyracidae and not a Hegetotheriidae. Confusion among basal members of different notoungulate families is not rare and Roth (1904) commented, when describing the archaeohyraciids from Cañadón Blanco, that Archaeohyrax is intermediate between Notopithecidae (presently Notopithecinae-Interatheriidae) and Hegetotheriidae. Keeping in mind that MLP 12-1530 is an archaeohyracid, the following discussion focuses on a comparison with the members of this family recognized in the same locality of Cañadón Blanco and others (Simpson, 1967; Reguero et al., 2003). The knowledge of the Chilean Tinguirirican fauna (Flynn et al., 2003 and references therein) has greatly helped to understand and correlate different Patagonian taxa. Archaeohyracids constitute the most abundant and diverse element in the faunal assemblage of Tinguiririca. Croft et al. (2003) and Reguero et al. (2003) have determined new taxa of this group and have increased the knowledge of other taxa previously known from scarce Argentinean remains, greatly clarifying the systematics of Archaeohyracidae. New interesting Oligocene archaeohyracid material of Deseadan age comes from the localities of Salla (Bolivia; Billet et al., 2009) and Quebrada Fiera (Mendoza, Argentina; Cerdeño et al., 2009).
The small size of our specimen suggests comparisons with the small achaeohyracids described by Reguero et al. (2003) included in the genus Protarchaeohyrax: P. gracilis (Roth, 1904), P. minor Reguero et al., 2003, and P. intermedium Reguero et al., 2003. All of these come from Patagonian Argentinean localities (mainly Cañadón Blanco) and Tinguiririca (Chile). Unfortunately, comparison is limited to the lower dentition, which lacks some diagnostic characters.
Premolar size of MLP 12-1530 (table 1) is generally slightly smaller than that of P. gracilis, both in length and width, excepting the specimen from Tinguiririca SGOPV 2954 whose length is even less (not the width) (Reguero et al., 2003: Table 1). The two mandibles ascribed to P. intermedium have p3 and p4 rather comparable, especially SGOPV 5007. No lower dentition is ascribed to P. minor.
For P. gracilis, Roth (1904, as Archaeohyrax) indicated the presence of shallow internal sulci on the lower molars. The description and figures of P. gracilis (Reguero et al., 2003), particularly the mandible SGOPV 2954 from Tinguiririca, show morphological similarities with our specimen: extension of the symphysis to p2; elongated trigonid of p2; presence of fossettids in p3-p4 after moderate wear; and rather flattened lingual wall. With respect to the latter character, those authors indicated for p2 that "lingually only a short, shallow groove (obliterated by slight wear) occurs". This differs from our specimen, because MLP 12-1530 has a second shallow groove, more anteriorly positioned (figure 2.4). This condition appears instead in P. intermedium, SGOPV 3065 (op. cit., fig. 5), a species that presents flatter lingual walls than P. gracilis in posterior premolars and molars, and narrower teeth (Reguero et al., 2003). A fossettid is also present in p3-m3 (in SGOPV 2954 of P. gracilis, the fossettid is not present in p3). These differences of P. intermedium with respect to P. gracilis are instead coincident with MLP 12-1530, although P. intermedium has a shallow mandibular ramus (5.7 mm below p3, following Reguero et al., 2003) where-as MLP 12-1530 has a remarkably higher one (table 1), also a little higher than P. gracilis.
An elongate p2 with two lingual sulci is also observed in other taxa, such as Archaeotypotherium tinguiriricaense, but this is a significantly larger species, with more procumbent symphysis, among other differences (Croft et al., 2003).
The presence of a lingual sulcus on lower incisors is reported as a diagnostic character for the notoungulate families Mesotheriidae and Interatheriidae (Cifelli, 1993; Reguero, 1999), although with a different pattern of sulcus, but it is also present in some Archaeohyracidae. Among the archaeohyracid material studied by Reguero et al. (2003) and Croft et al. (2003), very few specimens have preserved lower incisors. No specimen with lower incisors has been attributed to the species of Protarchaeohyrax (Reguero et al., 2003). Among the other archaeohyracid genera, SGOPV 3043 of Archaeotypotherium tinguiriricaense shows a faint, short lingual sulcus on i3-c (pers. obs. of the cast), not on i2 as it appears in the figured scheme (Croft et al., 2003: fig. 10). Nevertheless, these teeth differ from those of MLP 12-1530 because, as described by these authors, i3 and c possess a high anterior portion and a low posterior one, similar to the configuration of the trigonid and talonid in generalized mammal lower molars. The short sulcus is present on the anterior part of these teeth. A small lingual sulcus is also observed in i3 of the specimen MCNAM-PV 3844 from Quebrada Fiera (Mendoza), assigned to Archaeohyrax (Cerdeño et al., 2009).
In summary, size and morphology of the premolars of MLP 12-1530 are closer to P. intermedium than to P. gracilis. Nevertheless the difference in the ramus height is remarkable with respect to P. intermedium, which prevents an accurate specific identification. On the other hand, a third species of Protarchaeohyrax, P. minor, was identified upon a unique specimen (MLP 52-XI-4-168a) from Cañadón Blanco (Reguero et al., 2003) with no lower dentition related to it. Consequently, if MLP 12-1530 is identified as P. intermedium, three very similar archaeohyracids would be present at Cañadón Blanco, which seems to be little justified at present.
Protarchaeohyrax minor is based on three upper teeth of dubious position (P4-M2 or P3-M1?) whose general size is not so different from P. intermedium, the development of the parastyle and the little difference in the dimensions of the three teeth being what separate this specimen from P. intermedium (Reguero et al., 2003). In fact, the parastyle of the first tooth (P4?) seems more labially projected than on the other specimens compared from different localities and taxa, even considering that tooth outline and proportions vary a lot with wear in archaeohyracids (Billet et al., 2009; pers. obs. on Quebrada Fiera material). As no lower teeth have been associated to P. minor, it cannot be asserted that MLP 12-1530 could or could not be related to MLP 52-XI-4-168a. What is well established here is the relation of MLP 12-1530 with Protarchaeohyrax, closer to P. intermedium than to P. gracilis. A definite taxonomic answer is not possible without getting further material from Cañadón Blanco. A conservative position is to consider that MLP 12-1530 represents the lower dentition of P. minor, but keeping in mind the possibility of both specimens representing an intraspecific variation of P. intermedium. In any case, the studied mandible comes to increase similarities between the archaeohyracids from Cañadón Blanco and Tinguiririca, that is between Patagonia and Central Chile, during the early Oligocene.
The revision of specimen MLP 12-1530 from the Oligocene of Cañadón Blanco (Chubut Province) allows a new taxonomic interpretation, recognizing it as an Archaeohyracidae and not a Hegetotheriidae. Archaeopachyrucos ulianai, being an unpublished taxon (Reguero, 1999), is a nomen nudum.
MLP 12-1530 is identified as Protarchaeohyrax, closer to P. intermedium than to P. gracilis from Tinguiririca (Chile), but with a higher mandibular ramus, which prevents an accurate specific determination. Considering that a third species is present at Cañadón Blanco, P. minor, only known by three upper teeth, we propose that MLP 12-1530 represents its lower dentition, but keeping in mind that similarities with P. intermedium could reflect intraspecific variation of this species. Better material is necessary to verify this proposal, but MLP 12-1530 reinforces the similarities between Patagonia and Central Chile during early Oligocene (Tinguirirican age).
The authors thanks to D. Croft for revising the draft version of the paper, to G. Farías, from the graphic design Service (MAGRAF) of the Centro Científico Tecnológico-CONICET-Mendoza, for preparing the figures, to the referees (G. Billet and D. Croft) and the editorial staff for the critical reviews, and to the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) for the financial support.
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Recibido: 4 de febrero de 2009.
Aceptado: 5 de octubre de 2009.