versión On-line ISSN 2007-2902
Rev. mex. cienc. geol vol.28 no.2 México ago. 2011
A ceratopsian horncore from the Olmos Formation (early Maastrichtian) near Múzquiz, Mexico
Un cuerno de ceraptósido de la Formación Olmos (Maastrichtiano Temprano) cerca de Múzquiz, México
Hector Gerardo PorrasMúzquiz1 and Thomas M. Lehman2,*
1 Museo de Múzquiz A. C., Zaragoza 209, Múzquiz, Coahuila, 26340, Mexico.
2 Department of Geosciences, Texas Tech University, Lubbock, 794091053, Texas, USA. *Email: email@example.com
Manuscript received: February 21, 2011.
Correted manuscript recived: April 11, 2011.
Manuscript received: April 26, 2011.
An isolated supraorbital horncore collected from the Olmos Formation near Múzquiz is among the longest ever found, and records the presence in this area of a very large ceratopsid. The specimen probably pertains to a chasmosaurine, but differs significantly from the horncores in Coahuilaceratops known from the nearby Cerro del Pueblo Formation, and cannot be attributed with confidence to any other known ceratopsid.
Key words: ceratopsian, Cretaceous, Olmos Formation, Mexico.
Un cuerno supraorbital aislado, colectado en la Formación Olmos cerca de Múzquiz, es uno de los más largos que se ha encontrado, y registra la presencia de un ceratópsido de gran talla en esta área. El espécimen probablemente pertenece a un chasmosaurino, pero difiere significativamente de los cuernos de Coahuilaceratops, reportado de la Formación Cerro del Pueblo, por lo que no puede ser atribuido con certeza a cualquier otro ceratópsido conocido.
Palabras clave: ceratópsido, Cretácico, Formación Olmos, México.
In 1984 the Historic Museum of Múzquiz was established, and residents of the region at that time were asked to donate objects of historical interest to the museum. In addition to cultural and archeological materials, many fossils were donated; however, most of these had been recovered years ago and without proper documentation. Beginning in 2005, a group of local amateur paleontologists (Museo de Múzquiz, A. C.) began collecting additional specimens, documenting and preserving these in the museum. In 2008, the museum collections were enrolled in the Public Registry of Archeological Zones and Monuments (INAH).
Among the objects donated to the museum in 1984 was the suprorbital horncore of a ceratopsian dinosaur (MUZ 309; Figure 1). The label associated with the specimen indicates only "Olmos Formation, Superior Cretaceous, Múzquiz County locality." Catalogued with the specimen were a number of additional fragmentary dinosaur bones, including the distal end of a ceratopsian humerus (MUZ 310) and other indeterminate fragments. Although there are twelve dinosaurbearing localities known in the Múzquiz area, the exact collection site for the horncore has not been precisely established.
Dinosaur bones had been previously reported from the Olmos Formation. In 1968, a fragmentary skeleton of a ceratopsian dinosaur was reported from "Palaú in the coal zone of the Olmos Formation" (OjedaRivera et al., 1968). The specimen consisted of parts of both hind and forelimbs, and was tentatively identified as Chasmosaurus by Wann Langston, Jr. (OjedaRivera et al., 1968; their fig. 8). However, the present whereabouts of the specimen and its original collection site are unknown. Meyer et al. (2005) described footprints of theropod dinosaurs and mentioned the occurrence of hadrosaur and ankylosaur bones in the Olmos Formation. Kirkland et al. (2006) described a partial skeleton of the hadrosaur Kritosaurus sp., and noted the occurrence here also of tyrannosaurid dinosaur teeth. The Múzquiz horncore is of interest therefore in properly documenting the presence here of ceratopsian dinosaurs. Although this is an isolated fragmentary specimen, it is useful to describe and illustrate it for purposes of comparison with ceratopsians from nearby and correlative deposits.
The fossil flora of the Olmos Formation is much better known than its fossil vertebrate fauna, and includes a wide variety of ferns, conifers, and angiosperms (Weber, 1978; CervallosFerriz and RicaldeMoreno, 1995; EstradaRuiz et al., 2007, 2008, 2010). Some of the fossil wood types found in the Olmos Formation have also been found in the Aguja and Javelina formations in Big Bend National Park, about 250 km northwest of Múzquiz in Texas (Wheeler and Lehman, 2009). Marine invertebrates indicate that the Olmos Formation is Late Campanian to Early Maastrichtian in age (reviewed by Kirkland et al., 2006). The flora suggests correlation of the terrestrial facies of the Olmos Formation with the lower part of the Javelina Formation or uppermost part of the Aguja Formation in Texas, and accords best with an early Maastrichtian age.
The Múzquiz horncore is broken into four segments, but these can be joined to show its entire form (Figure 1). The medial side is well preserved and exhibits the longitudinal vascular impressions typical of ceratopsid horncores. The lateral side was no doubt exposed when the specimen was discovered, and much of the cortical bone has been weathered off of that side. The base of the horncore has a spongiosafilled central axis, and is lacking the deep cornual sinus found in many other large ceratopsids (e.g., Farke, 2006). Its preserved length, as restored, is 95.2 cm (straightline distance).
Although the rim of the orbit is not preserved, several features allow for identification of the medial, lateral, anterior, and posterior surfaces. In most or all ceratopsids, the posterior base of the horncore smoothly continues the curvature of the rear flange of the postorbital where it approaches the squamosal suture. In contrast, the anterior base of the horncore typically ends in a sharp angle with a rugose surface where it is sutured to the palpebral, producing an antorbital 'buttress' (e.g., Lehman, 1989). This permits identification of the anterior and posterior surfaces of the Múzquiz specimen (Figure 1).
The lateral side of the horncore in most ceratopsids is flattened above the dorsal rim of the orbit, whereas the medial side flares away from the horncore to form the roof of the frontal sinus (or 'postfrontal fontanelle' of authors, e.g., Lehman, 1989). This permits identification of the inner and outer sides of the horncore in MUZ 309 (Figure 1). If the inner and outer, forward and rear sides of the Múzquiz specimen are correctly identified as described above, then this is a left supraorbital horncore and it is posteriorly curved, rather than anteriorly curved as is more typical for large ceratopsids.
The Múzquiz horncore is very long, a condition typical for supraorbital horncores of chasmosaurine ceratopsids but also known in basal centrosaurines such as Albertaceratops (Ryan, 2007). This horncore is, however, longer than in any ceratopsian other than the largest known specimen of Pentaceratops (Oklahoma Museum of Natural History, OMNH 10165; Lehman, 1998). The horn appears to lack a cornual sinus, a condition among chasmosaurines only found in Chasmosaurus (other recently named taxa such as 'Agujaceratops', 'Mojoceratops', 'Utahceratops', and 'Vagaceratops' are similar in this respect, and perhaps congeneric with Chasmosaurus; e.g., Paul, 2010). A rudimentary cornual sinus is found in Anchiceratops and in Pentaceratops (Farke, 2006). Because the base of the Múzquiz horncore is broken, it would have been somewhat longer than preserved, and so it is possible that a sinus could have been present in the unpreserved part. However, the degree of expansion shown in the preserved base suggests that very little is missing, and we consider it unlikely that a cornual sinus was present. If the orientation suggested above is correct, the horn is posteriorly curved, a condition known only in Chasmosaurus (and closely related taxa listed above).
The Múzquiz horncore is also quite slender for its length (Figure 2; basal anteroposterior width = 20.8 cm, transverse width = 12.2 cm). The ratio of the basal anteroposterior width to the length of the horncore (0.22) is lower than in any chasmosaurine for which measurements are available, other than the largest known specimen of Chasmosaurus mariscalensis (TMM 430981). Large chasmosaurines, such as Triceratops and Torosaurus typically have horncores that are relatively much wider at the base. The base of the horncore is extended posteriorly in large chasmosaurines (Triceratops, Torosaurus, and Pentaceratops) such that the basal transverse to anteroposterior width ratio is low (< 0.5). The Múzquiz horncore retains a relatively high basal width ratio (0.59; Figure 3).
The size and form of the Múzquiz horncore, as well as the proximity of its collection site, suggest that it might pertain to Coahuilaceratops found in the Cerro del Pueblo Formation near Saltillo (Loewen et al., 2010). However, only fragments of the supraorbital horncores were recovered with the type specimen (CPC 276), and the authors restored the horncores to a length of only about 60 cm (Loewen et al., 2010; their fig. 7.4), substantially smaller than in the Múzquiz specimen. Moreover, one of the horncores in the type specimen has a cornual sinus, and a basal width ratio (0.77) much greater than in the Múzquiz specimen. The authors in describing Coahuilaceratops were also uncertain as to whether the horncores were curved anteriorly or posteriorly. The authors restored the horncores with anterior curvature (Loewen et al., 2010; their fig. 7.4).
Although the supraorbital horncores in Coahuilaceratops are smaller, have a greater basal width ratio, and possess a cornual sinus, these differences could be within the realm of individual ontogenetic variation observed in other chasmosaurines (e.g., Lehman, 1989, 1998). If the anteriorly curved orientation shown in the restoration of CPC 276 is mistaken, or if the posteriorly curved orientation suggested here for MUZ 309 is incorrect, then the Múzquiz specimen could pertain to Coahuilaceratops; albeit probably from a much larger individual. However, the type skull of Coahuilaceratops is also from a very large mature animal, and magnetostratigraphy of the Cerro del Pueblo Formation suggests that these strata are probably older than the Olmos Formation (e.g., Eberth et al., 2004). Therefore, it seems doubtful that the Múzquiz specimen belongs to Coahuilaceratops, and it may instead represent a distinct taxon.
The Múzquiz horncore is very long and slender, and probably pertains to a chasmosaurine ceratopsid. Its great length, low basal width ratio, apparent posterior curvature, and lack of a cornual sinus, suggest that it cannot be attributed to Coahuilaceratops magnacuerna known from the nearby Cerro del Pueblo Formation in Saltillo. It is also clear that the Múzquiz specimen pertains to a species unlike either Chasmosaurus mariscalensis (Lehman, 1989) or Torosaurus cf. utahensis (Hunt and Lehman, 2008) the only two ceratopsians thus far known from the nearby Aguja and Javelina formations in Texas. In all likelihood the horncore belongs to an unknown taxon. This could reflect either marked provinciality or environmental segregation of horned dinosaurs at that time (e.g., Lehman, 1997) or possibly that the Olmos Formation was deposited during a time not represented by the Cerro del Pueblo Formation or the Aguja Javelina succession in Texas.
We thank Dr. Cipriano Portales, president of Múzquiz municipality, for his support of this research, Drs. Mark Loewen and Andrew Farke for helpful comments that improved the content of this paper, and Dr. Wann Langston, Jr. for providing information about the history of collecting in the Olmos Formation.
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