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Two new glyptodont records (Mammalia: Cingulata) from the late Pleistocene of Tamaulipas and Tlaxcala, Mexico: implications for the taxonomy of the genus Glyptotherium.

Paleontologists base the classification of fossil organisms on morphological characters. However, the limited amount of specimens and missing characters, due to poor preservation, often hinder the delimitation of fossil morphospecies (Wiens, 2003; Wilkins, 2009). This situation has led to an exaggerated number of genera and species based on a limited number of characters from very few individuals. As a result, it is necessary to recover and evaluate the widest possible array of fossil elements to achieve a better understanding of the range of morphological variation between the organisms that represent a given taxon.

Our study focuses on the case of glyptodonts, an extinct group related to armadillos and sloths that originated in South America. Their earliest record in North America comes from layers dated between 3.9 and 3.1 million y ago (Blancan North American Land Mammal Age, or NALMA) in Guanajuato, Mexico (Gillette and Ray, 1981; McDonald, 2002; Carranza-Castaheda and Miller, 2004; Flynn et al., 2005). The North American glyptodonts belong to the genus Glyptotherium Osborn, 1903, which has five recognized species: Glyptotherium texanum Osborn, 1903 (Blancan), Glyptotherium arizonae Gidley, 1926 (Irvingtonian), G. floridanum (Simpson, 1929), G. cylindricum (Brown, 1912), and Glyptotherium mexicanum (Cuataparo and Ramirez, 1875) (Rancholabrean) (Gillette and Ray, 1981).

In Glyptotherium, hundreds of small osteoderms, also known as scutes, make up a typical carapace. Very few complete carapaces or cranial and postcranial elements are recovered, and osteoderms are usually found isolated. As a result, the identification of North American glyptodonts relies mostly on characters of the carapace and osteoderms (Gillette and Ray, 1981). However, these characters are not always reliable because intraspecific variation exists, and the morphology of osteoderms from the same specimen varies according to each region of the carapace (Gillette and Ray, 1981). In addition, some localities from Texas have two morphotypes of the species G. floridanum based on osteoderm morphology. Previous studies interpreted the different osteoderms as sexual dimorphism, following the idea that it would be difficult for two different species of such large size and with similar habits to compete in the same area (Gillette and Ray, 1981). The recent study of infant, juvenile, and adult specimens shows that morphological variation throughout development also exists (D. D. Gillette, pers. comm.). These problems are the reason for the frequent, nonspecific identification of Glyptotherium throughout the range of these glyptodonts (Gillette and Ray, 1981).

In Mexico, the published glyptodont record includes G. floridanum (Rancholabrean NALMA, three localities), G. cylindricum (Rancholabrean NALMA, three localities), G. mexicanum (Rancholabrean NALMA, four localities), and G. texanum (Blancan NALMA, one locality). In addition, several nonspecific collections have been made at six localities (McDonald, 2002; Carranza-Castaheda and Miller, 2004; Mead et al., 2007) (Fig. 1). It is important to highlight that the majority of the collected glyptodont material in Mexico has not been assigned to the species level and, thus, the morphological information for the genus has not been thoroughly discussed or verified.

We report data from two partial carapaces, postcranial elements, and a skull recovered from two localities from the Mexican states of Tamaulipas and Tlaxcala (Fig. 1). We compared the osteology of both specimens to other recovered glyptodonts from Mexico and the latest revision of the genus Glyptotherium (Gillette and Ray, 1981) to assign them to species level. Both specimens were assigned to the same species based on the study of previously unknown linking characters between the species G. floridanum and G. cylindricum. Our study also reviewed distribution of glyptodonts in Mexico during the Pleistocene.

MATERIALS AND METHODS--We identified both specimens of glyptodonts by comparing them with the latest revision of the genus Glyptotherium (Gillette and Ray, 1981) and with previously recovered specimens housed in paleontological collections in Mexico. We deposited the fossils described here in the Museo de Paleontologia Maria del Carmen Perrilliat M., Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Mexico City, under the catalog numbers IGM 9562 and IGM 9563.

Comparative material included glyptodont material housed at the following Mexican collections: Coleccion de Macromamiferos, Museo de Paleontologia, Universidad Autonoma del Estado de Hidalgo, Pachuca, Hidalgo, specimens UAHMP-428, UAHMP-429, UAHMP-431, UAHMP-426, UAHMP-427, UAHMP-430, UAHMP-960, UAHMP-967, UAHMP-358, UAHMP-919A, UAHMP-919B, UAHMP-919D, and UAHMP-919E; Museo de Paleontologia Maria del Carmen Perrilliat M., Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Mexico City, specimens IGM 5955 and osteoderms with locality numbers 2436 and 2568; and the Coleccion de Referencia, Museo de Paleontologia de Guadalajara, "Federico A. Solorzano Barreto," Guadalajara, Jalisco.

We divided the carapace fragments into six regions: cephalic, anterolateral, lateral, posterolateral, caudal, and dorsal. Then we assigned each isolated osteoderm to one of the six regions using as criteria its symmetry, dimensions, texture, and thickness (Gillette and Ray, 1981; Castro-Azuara, 1997). From each scute we took the following linear measurements: anteroposterior total diameter, anteroposterior diameter of the central figure, total transverse diameter, and thickness. We took additional notes for each osteoderm including: the total number of peripheral figures, number and position of piliferous foramina, and relief of the central figure. We obtained the mean measures of the osteoderms from each available region to compare them with those obtained from a sample of 30 osteoderms from the same regions identified in the comparative material. We took all measurements with a caliper and measuring tape and report them in millimeters. For the study of IGM 9563, we obtained 527 CT scans of the skull and 391 of the mandible to reconstruct the occlusal surface and dimensions of the upper and lower teeth. We used the terminology and measurements suggested previously for cranial, postcranial, and carapacial elements (Gillette and Ray, 1981).

Systematic Paleontology

Mammalia Cuvier, 1798

Xenarthra Cope, 1889

Cingulata Illiger, 1811

Gylptodontoidea Gray, 1869

Glyptodontidae Gray, 1869

Glyptodontinae Trouessart, 1898

Glyptotherium Osborn, 1903

Glyptotherium cylindricum (Brown, 1912)

Gillette and Ray, 1981

Syn. Brachyostracon cylindricum Brown 1912

Materials--The specimen IGM 9562 from Tamaulipas is moderately well preserved and includes: atlas, right scapula, right and left humeri, right ulna; right metacarpal II, right ungual phalanx, digits II, IV, and V of the manus; pelvis; right phalanx II, digits III and V, two right ungual phalanges, of the pes; two caudal vertebrae; seven chevrons; nearly complete carapace divided in various fragments and isolated osteoderms (Figs. 2, 3).

The preservation of the specimen IGM 9563 from Tlaxcala is quite poor and includes a partial skull missing both nasals; part of the left maxilla and the right part of the occipital region; nearly complete upper teeth series (missing only [N.sup.5]), teeth cusps highly devastated; mandible, the right ascending ramus is missing; complete lower teeth series, teeth cusps highly worn; complete and well-preserved atlas (Fig. 4); nearly complete cervical tube with multiple fractures; proximal epiphysis fragment of left tibia; fragments of the carapace and isolated scutes.

Locality and Age--Ejido de San Lazaro, Municipio de Villagiun, Tamaulipas, is an area characterized by shallow valleys limited by elongated, undifferentiated Pleistocene volcanic bodies of approximately 20 m in height, running from east to west, and crossed by seasonal streams (Servicio Geologico Mexicano, productos/cartas/cartas50/geologia50/ 1678_G14-C69_GM. html). Quaternary fossiliferous sediments cover the valleys and are composed of light yellow, silty clay with small calcareous concretions 1-2 cm in diameter. Along with IGM 9562, associated fossil material includes the lower jaw, skull, and teeth of Equus conversidens as well as isolated molars of Mammuthus and Bison species; the latter suggesting a Rancholabrean NALMA for the faunal association (Montellano-Ballesteros, 2000; Reynoso and Montellano-Ballesteros, 2004).

The other locality is a ravine located at the limits of the San Mateo Huexoyucan and the San Tadeo communities, Municipio de Panotla, Tlaxcala. The fossil remains were found in lacustrine sediments composed of ash and clay (C. Castafieda-Posadas, 2011). Along with the IGM 9563 elements, fossil remains include Equus, Mammuthus, and Bison species. (Bonilla-Toscano, 2011). The faunal association also suggested a Rancholabrean NALMA (Castillo-Ceron et al., 1997).

Description and Comparison: Skull--Some of the cranial characters of IGM 9563 are unavailable due to the poor preservation. Diagnostic characters include a foramen on the surface of the mandibular fossa as in G. arizonae and G. floridanum and the conical shape of the occipital condyle as in G. texanum and G. arizonae (Gillette and Ray, 1981). The cranial dimensions are larger than those of G. texanum and more similar to the dimensions of G. arizonae, G. floridanum, and G. cylindricum.

Lower Jaw--The mental foramen of IGM 9563 is placed anteriorly to the first lower tooth ([N.sub.1]) as in G. arizonae, G. floridanum (Gillette and Ray, 1981), and in a recently recovered specimen from Venezuela identified as G. cylindricum (Carlini et al., 2008). The inferior margin of the horizontal ramus is rounded as in G. arizonae, but the mandibular symphysis is curved as in G. floridanum (Gillette and Ray, 1981).

Upper and Lower Dentition--In IGM 9563, the [N.sup.1] has a slightly sigmoidal shape, without clearly developed lobes. The [N.sup.2] is submolariform and has an asymmetric middle lobe. The last four upper teeth, with the exception of [N.sup.5] whose shape was inferred by the shape of the alveolus, had rounded and convex anterior borders, characteristics shared with G. cylindricum (Gillette and Ray, 1981). Within the lower dentition, the [N.sub.1] is submolariform and irregularly ovoid, similar to the [N.sub.1] of G. texanum and G. cylindricum (Gillette and Ray, 1981) but, unlike the latter, it is placed behind the symphysial curvature. The [N.sub.2] is submolariform, without the development of the lingual anterior lobe, similar to G. texanum and G. cylindricum (Gillette and Ray, 1981). The posterior lobe of the [N.sub.3] is perpendicular and convex. The anterior lobe is not square-shaped, a feature shared with G. cylindricum. The [N.sub.4] is molariform and its lobes are perpendicular in relation to the longitudinal axis of the tooth, as in G. floridanum (Gillette and Ray, 1981). Teeth [N.sub.5-7] are similar to those of G. cylindricum and G. floridanum, with lobes perpendicular to the axis of the teeth (Gillette and Ray, 1981). The anterolateral outline of the [N.sub.8] is slightly convex and the middle and posterior lobes are smaller than the anterior lobe as in G. floridanum.

Postcranial Elements--The atlas of IGM 9562 and IGM 9563 are very similar to the atlas of G. cylindricum (Gillette and Ray, 1981). The alar processes are laterally directed with regard to the transverse axis of the atlas. A small rectangular process on the dorsal surface of the dorsal arch in IGM 9562 is interpreted here as an abnormal character that may have been caused by pathology. In IGM 9563 the odontoid process in the cervical tube is anteriorly directed and inclined almost 45[degrees] as in G. floridanum (Gillette and Ray, 1981).

Pelvis--In IGM 9562 the position of the transverse processes of the last sacral vertebrae, according to the sagittal axis of the pelvis, is perpendicular as observed in G. floridanum and G. cylindricum (Gillette and Ray, 1981). Both posterior borders of the ischial crests converge as in G. texanum, G. arizonae, and G. floridanum (Gillette and Ray, 1981). The anterior surface of the ilium is concave-convex as in G. texanum, G. arizonae, and G. cylindricum (Gillette and Ray, 1981). Based on these three characters, the pelvis of the Tamaulipas specimen is similar to G. floridanum and G. cylindricum.

Limb Bones--The absence of the supratrochlear foramen in both humeri is a characteristic shared with G. texanum and G. floridanum (Gillette and Ray, 1981); however, the humeri are larger than those of G. texanum and thus more similar to those of G. floridanum.

Caudal Armor--The caudal armor fits previous descriptions of the genus without evident differences between species (Gillette and Ray, 1981).

Osteoderms--Carapacial osteoderms show the classic rosette configuration with a central figure surrounded by smaller peripheral figures separated by shallow grooves (Gillette and Ray, 1981). The dorsal (exterior) surface has a rugose and punctuated texture, with a well-defined, hexagonal outline, especially on those from the lateral, anterolateral, and posterolateral regions of the carapace. The hexagonal shape gradually disappears toward the sagittal line of the carapace and the outline of the scutes becomes more flower-like. The central figure of the osteoderms is flat (occasionally slightly concave) and larger than 50% of the total diameter of the scutes. The size of the central figure in the more-internal regions of the carapace becomes smaller than 50% of the total osteoderm diameter. The maximum number of peripheral figures observed throughout the carapace was ten, with abnormally large piliferous foramina standing out in some regions. The latter could be attributed to microbial pre- or postmortem processes.

Poor preservation of the osteoderms of IGM 9563 impeded the observation of characteristics such as the concavity and the level of the central figure in relation with the peripheral figures. The overall number of peripheral figures, as well as the outline of the osteoderms, is similar in both IGM 9562 and IGM 9563 to G. cylindricum; however, in IGM 9563 the maximum diameters (anteroposterior and transversal) of the scutes are clearly smaller and thinner than in IGM 9562, resembling the osteoderms of the G. floridanum identified as female. In this respect, the specimen from Tamaulipas, IGM 9562, resembles the males of G. floridanum and G. cylindricum.

We refer IGM 9562 to G. cylindricum based on the following diagnostic characters (Gillette and Ray 1981): the direction of the alar processes of the atlas, the absence of the supratrochlear foramen, the transverse processes of the last sacral vertebra, the anterior surface of the characters of the pelvis and the ilium, the overall number of peripheral figures, and the proportion between the diameter of the central figure and the total diameter of the osteoderms.

IGM 9563 is referred to here as G. cylindricum because it exhibits a combination of diagnostic characters of both G. floridanum and G. cylindricum. It shares with G. floridanum the presence of the mandibular fossa, the shape of the inferior border of the ramus, the shape of the [N.sub.4] and [N.sub.8], the angle of the odontoid process of the cervical tube, and the general size and thickness of the osteoderms. IGM 9563 shares with G. cylindricum the shape of [N.sup.1], [N.sub.1], [N.sub.2], and [N.sub.3] and the direction of the alar processes of the atlas. This specimen also shows characteristics that are present in both species such as the position of the mental foramen, the general size of the skull, and the occlusal shape of [N.sub.5-7].

DISCUSSION--During the 19th Century, a large number of glyptodont species and genera were described from South America (Gillette and Ray, 1981). Until the early 20th Century, around 30 genera had been described based on carapacial elements only (Fernicola and Porpino, 2012). The study of North American glyptodonts started in the late 19th Century with a similar tendency, and Glyptodon Cope 1889, Glyptotherium Osborn 1903, Brachyostracon Brown 1912, Boreostracon Simpson 1929, and Xenoglyptodon Meade 1953 were recognized. Years later, an extensive review of the available glyptodont material led to the conclusion that there is only one glyptodont genus in North America and that previously recognized genera were in fact synonymous to Glyptotherium (Gillette and Ray, 1981).

The diagnosis of the glyptodonts of North America includes a total of 41 characters (Gillette and Ray, 1981). In the case of G. floridanum and G. cylindricum, five of these characters slightly differ and seven are identical in both species, whereas the remaining 29 characters are not comparable because they are unknown from either of the two species. Furthermore, the lack of skeletal characters frequently leads to the exclusive use of carapacial characters.

We measured the ratio of central to total diameters and counted the number of peripheral figures of the available glyptodont osteoderms. Our study concluded that these characters do not necessarily reflect differences between species because the mean of the measurements taken for IGM 9562 and IGM 9563 along with the comparative material largely overlap. Within the comparative material, we recognize the occurrence of two different osteoderm morphotypes in Mexico, which are similar to the sexual dimorphism inferred by previous studies for G. floridanum based on the presence of two distinguishable osteoderm morphotypes found in the same localities. These studies attributed larger scutes to males, whereas the smaller scutes with a central figure occupying almost 50% or less of the total diameter were referred to females of G. floridanum (Gillette and Ray, 1981; Mead et al., 2007). The osteoderms of female G. floridanum differ from those of G. cylindricum in their overall smaller size, the concave surface of the central figure, and the higher level of the central figure's marginal borders, which stand above the superficial level of the peripheral figures. However, the characteristics of the osteoderms of G. floridanum identified as males resemble those of G. cylindricum (Gillette and Ray, 1981).

Even though glyptodont material is abundant throughout Mexico, the study of Glyptotherium has many problems. The recovery of new specimens is often made with a limited stratigraphic control of localities (McDonald, 2002), and many specimens housed at paleontological collections have not been studied yet. In addition, the absence of skeletal elements and the morphological uncertainties of carapacial characters have obscured the identification of most glyptodont reports in Mexico.

We refer IGM 9562 to the species G. cylindricum using the characters of the atlas, humeri, sacral vertebra, ilium, the number of peripheral figures, and central to total diameter ratio as criteria. In the case of IGM 2, the mandibular fossa, the shape of the horizontal ramus, the [N.sub.4-8], the cervical tube, and the overall size of the osteoderms suggest similarity with G. floridanum. However, the atlas and the [N.sup.1-4], [N.sup.6-8], [N.sub.1-2] and [N.sub.5-7] resemble those of G. cylindricum.

We propose that there are not enough characters to differentiate G. floridanum from G. cylindricum, with the latter taking taxonomic priority over the former. Our study of IGM 9562 and IGM 9563 offered a previously unseen bridge between the osteological characteristics of G. floridanum and G. cylindricum as a result of the observation of both skeletal and carapacial material. This conclusion is in keeping with the observations of D. D. Gillette (pers. comm.) that Glyptotherium had little or no evolution subsequent to the origin of the genus, a condition he recognized as evolutionary stasis. The diagnostic utility of carapacial elements may be evaluated in further studies by implementing a morphometric methodology (Ramirez-Cruz, 2014). This methodology could provide an overall view of the level of variation within the scutes from the same specimen and between scutes from different specimens. The analysis would contribute to a better understanding of the limits of the information coming from the carapace.

CONCLUSION--The study of the association of skeletal and carapacial elements in both glyptodont specimens denoted the presence of mixed morphological characters that had been identified in neither G. floridanum nor G. cylindricum. The new elements were crucial for identification and represent a bridge between some of the previously recognized osteological characters, showing that the two species possess a greater number of similarities than differences, and thus we consider them as synonyms. We assign the glyptodonts from Tamaulipas and Tlaxcala to the species Glyptotherium cylindricum (Brown, 1912), which has nomenclatural priority over G. floridanum (Simpson, 1929). The recovery of new glyptodont specimens will improve our understanding of the level of variation within the current species as well as the chronological and geographical distribution of the North American glyptodonts.

This paper constitutes a partial fulfilment of the Graduate Program in Biological Sciences of the National Autonomous University of Mexico (UNAM). G. A. Ramirez-Cruz acknowledges the scholarship and financial support provided by the National Council of Science and Technology (CONACyT), and UNAM. We thank M. C. Perrilliat, J. Juarez Woo, and V. M. Bravo Cuevas for granting access to paleontological collections under their supervision; E. Alexanderson Rosas and L. Osorio Cardiel for their help to obtain the CT scans at the Unidad PET/CT de la Facultad de Medicina de la UNAM; M. E. Olson for his advice and revisions during the preparation of this paper; and D. D. Gillette for his priceless advice and comments.


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Submitted 14 December 2013.

Acceptance recommended by Associate Editor, Jennifer K. Frey, 23 April 2014.


Posgrado en Ciencias Biologicas, Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Delegacion Coyoacan, 04510 D. F., Mexico (GARC)

Departamento de Paleontologia, Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Delegacion Coyoacan, 04510 D. F., Mexico (MMB)

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Author:Ramirez-Cruz, Gonzalo A.; Montellano-Ballesteros, Marisol
Publication:Southwestern Naturalist
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Date:Dec 1, 2014
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