Revista Mexicana de Ciencias Geológicas

 

The oldest stratigraphic record of the Late Cretaceous shark Ptychodus mortoni Agassiz, from Vallecillo, Nuevo León, northeastern Mexico

 

El registro estratigráfico más antiguo del tiburón del Cretácico Tardío Ptychodus mortoni Agassiz, de Vallecillo, Nuevo León, noreste de México

 

Alberto Blanco–Piñón¹, Luis M. Garibay–Romero^2,3, and Jesús Alvarado–Ortega²

 

1 Centro de Investigaciones en Ciencias de la Tierra, Universidad Autónoma del Estado de Hidalgo, Apdo. Postal 1–288, Admón. 1, 42001 Pachuca, Hidalgo, Mexico. albblanc@yahoo.com

2 Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México, D.F., Mexico.

3 Universidad Autónoma de Guerrero, Unidad Académica de Ciencias de la Tierra, Ex–Hacienda de San Juan Bautista, Taxco el Viejo, Guerrero. Mexico.

 

Manuscript received: February 7, 2006
Corrected manuscript received: November 2, 2006
Manuscript accepted: November 14, 2006

 

ABSTRACT

In this paper we report the oldest geologic world record of Ptychodus mortoni, from the Vallecillo Member (Agua Nueva Formation), at Vallecillo, Nuevo León, northeastern Mexico. The presence of the ammonite Watinoceras coloradoense, allows the placement of the sediments of the Vallecillo member within the lower Turonian (Watinoceras coloradoense zone). With reference to this new data, the presence of Ptychodus mortoni in the Vallecillo member extend the temporal distribution of this species into the earliest Turonian. Also, this study suggests that the utility of this shark species as biostratigraphic control in late Cretaceous sediments should be revised in greater detail.

Key words: Ptychodus mortoni, shark, lower Turonian, Late Cretaceous, Vallecillo, Mexico.

 

RESUMEN

En este artículo se reporta el registro geológico más antiguo de Ptychodus mortoni, proveniente del miembro Vallecillo (Formación Agua Nueva), en Vallecillo, Nuevo León, noreste de México. La presencia del ammonite Watinoceras coloradoense, permite ubicar a los sedimentos del miembro Vallecillo dentro del Turoniano inferior (zona de Watinoceras coloradoense). De acuerdo con estos nuevos datos, la presencia de Ptychodus mortoni en el miembro Vallecillo extiende la distribución temporal de esta especie al Turoniano temprano. Además, este estudio sugiere que la utilización de esta especie de tiburón como control bioestratigráfico en sedimentos del Cretácico debe ser revisada en detalle.

Palabras clave: Ptychodus mortoni, tiburón, Turoniano temprano, Cretácico Tardío, Vallecillo, México.

 

INTRODUCTION

Ptychodus is a highly specialized durophagous shark that lived during the Late Cretaceous. It has a characteristic crushing dentition that allowed the shark to feed on hard–bodied preys, such as mollusks (Cappetta, 1987). Remains of this genus have been reported from the Albian to the Maastrichthian nearly worldwide (Johnson and Lucas, 2003).

Ptychodus mortoni is a ptychodontid species that is known from North America, Europe, and Africa (Cappetta, 1987). Blanco et al.(2001) and Blanco–Piñón et al.(2002) reported a small set of associated teeth of P. mortoni from the Turonian deposits in northeastern Mexico, collected in the Vallecillo quarries, state of Nuevo León (Figure 1). Blanco–Piñón et al. (2005) determined the age of the deposit as early Turonian, but did not presented the details about its stratigraphy. The purpose of this paper is to redescribe the specimen and its stratigraphic occurrence with a special emphasis on the fact that the specimen represents the oldest stratigraphic record for Ptychodus mortoni.

 

SYSTEMATIC PALEONTOLOGY

Class Chondrichthyes Huxley, 1880
Subclass Elasmobranchii Bonaparte, 1838
Cohort Euselachii Hay, 1902
Order incertae sedis
Family Ptychodontidae Jaekel, 1898
Genus Ptychodus Agassiz, 1835

Ptychodus mortoni Agassiz, 1843

Referred specimen. FCT–341, six associated teeth (Figure 2) presumably from a single shark housed in the Colección Paleontológica at the Facultad de Ciencias de la Tierra (FCT), Universidad Autónoma de Nuevo León, Linares, Nuevo León, Mexico.

Occurrence. A limestone bed in the uppermost part of the Vallecillo Member of the Agua Nueva Formation, at Vallecillo, Nuevo León, northeastern Mexico (Figure 1).

Description. The six teeth vary in size and morphology (Figure 2a). In occlusal view, their shape is roughly rectangular with rounded corners, the anterior margin convex and the posterior one concave (Figure 2a). The length and width of these teeth range from 13 to 15 mm and from 19 to 25 mm, respectively. Their crowns are high and conical, measuring from 10 to 12 mm in crown height. There are about twelve ridges radiating from the apex of the crown, which are intensely branched and grade into a marginal area that is characterized by reticulated pattern (Figure 2c). A series of small cavities are present on the root surface, but other root features are not observable.

Remarks. Species of the genus Ptychodus are identified on the basis of crown shape and cusp morphology (Cappetta, 1987). The teeth of P. mortoni are distinguishable from other ptychodontid species by having a high, conical cusp with a series of radial ridges (Cappetta, 1987). This combination of characters is clearly observable in the specimen described here. Teeth of P. mortoni from Vallecillo show some morphological variation, which suggests a heterodont dentition. This condition along with the fact that Vallecillo teeth have a sharp cusp tip differentiates from the dentition of P. cyclodontis Mutter, Iturralde–Vinent y Carmona, 2005, which is the only other ptychodontid species with radial ridges on its tooth cusps.

 

DISCUSSION

Figure 3 summarizes the stratigraphic range and geographic distribution of Ptychodus mortoni. The species is known from upper Turonian through lower Maastrichtian deposits. In Europe and Africa, P. mortoni is uncommon but has been reported from the middle to upper Coniacian of Belgium (Herman, 1977), England (Woodward, 1887, 1889), and Italy (D'Erasmo, 1922). In Africa, P. mortoni is known only from the Itombe Formation (upper Turonian) at the Cuanza Basin, in Angola (Antunes, 1961; Antunes and Cappetta, 2002). In the U.S., P. mortoni has been reported from Coniacian–Campanian deposits in the Gulf region (e.g., Alabama, Mississippi) and the Western Interior (e.g., Texas, New Mexico, Kansas, South Dakota and Wyoming) (Williston, 1900; Applegate, 1970; Cappetta, 1987; Williamson et al., 1989; Williamson and Lucas, 1990; Welton and Farish, 1993; Williamson et al., 1993; Cicimuri; 1998, Brito and Janvier, 2002; Johnson and Lucas, 2003; Everhart and Caggiano, 2004). In Mexico, P. mortoni has been described previously from upper Turonian rocks of Coahuila, northeastern Mexico (González–Barba et al., 2001), the Turonian Agua Nueva Formation in Xilitla, state of San Luis Potosí, central Mexico (Maldonado–Koerdel, 1956), and the lower Maastrichtian Mexcala Formation in the state of Guerrero (Alvarado–Ortega et al., 2004).

At the Vallecillo locality, the rock exposure consists of at least 2 m of a homogeneous sequence of laminated pink marlstone beds (Blanco et al., 2001, Blanco–Piñón et al. 2002). Towards the top of the section, the sequence grades into a monotonous, alternating gray–pinkish limestone and brown shale with a bed thickness varying from 10 to 20 cm. The Ptychodus specimen was collected from the uppermost part of the Vallecillo Member. The presence of the ammonite Watinoceras coloradoense Henderson (Blanco–Piñón et al., 2005) in the Vallecillo Member places this rock to be lower Turonian. This age assignment concomitantly marks the oldest known fossil record for Ptychodus mortoni. According to Gradstein et al. (1995), taxon range for W. coloradoense spans approximately from 93.5 to 92.6 million years.

According to its previously reported temporal distribution, remains of Ptychodus mortoni were considered as index fossils that signify the age range of Coniacian–Maastrichtian (see Johnson et al., 2002; Johnson and Lucas, 2003; Williams and Lucas, 2005). However, the specimen described here unequivocally confirms the presence of P. mortoni during the early Turonian. The extended chronostratigraphic range makes the species less useful as a biostratigraphical marker.

According to Blanco (2003) and Blanco et al. (2005), the Vallecillo Member was deposited in the outer part of a platform under anoxic/dysoxic conditions probably related to the late Cenomanian – early Turonian oceanic anoxic event (OAE–2). Besides Ptychodus mortoni, the fossil fauna of the Vallecillo Member includes unidentified lamniform sharks (Blanco et al., 2005),Nursallia gutturossum and other unidentified pycnodontid as well as undetermined pachycor–midfish. Teleostean fish are represented by Vallecillichthys multivertebratum (Ichthyodectiformes), Goulmimichthys roberti andAraripichthys sp. (Pachyrhizodontiformes sensu Cavin, 2001) (Blanco and Cavin, 2003); Robertichthys riograndensis and Rhynchodercetis regio (Dercetidae) (Blanco–Piñón and Alvarado–Ortega 2005, 2006); as well as Tselfatia formosa (Tselfatiiformes) (Blanco and Cavin, 2003). Reptiles are represented by non–determined turtles (Blanco et al., 2001) and a plesiopedal mosasaur (Jacobs et al., 2005). Invertebrates are represented by the ammonites Watinoceras color adoense (Blanco–Piñón et al., 2005), Mammites, Spathites and Collignoniceras (Blanco et al.2001); as well as bivalves of the genus Inoceramus (Blanco 2003). Among the groups enlisted above, inoceramids are the only shelled macroinvertebrates that could have served as pray taxa for Ptychodus mortoni, rather than ammonites or vertebrates such as the reptiles and the fishes. According to Cappeta (1987), the dentition of pattern of Ptychodus mortoni was capable of crushing shelled mollusk such as small inoceramids. Kauffman (1972) suggested this hybodontid shark species could attack small inoceramids rather than adults based on the presence of coprolite–like structures composed by crushed fragments of immature inoceramids. Until now, no evidence of attack of Ptychodus mortoni sharks on ammonites has been reported.

 

ACKNOWLEDGEMENTS

The authors are deeply grateful to the González family, especially to Roberto González (Vallecillo, Nuevo León, Mexico), who provided us the material described in this manuscript. We also thank L. Espinosa–Arrubarrena for the preliminary revision of this manuscript, and K. Shimada for his comments on the final version of this document. Special thanks goes to V. Griener for processing the photographs used in this paper, and M. Polcyn for providing part of the literature cited in this work. Financial support was provided by PROMEP (Project 72636363).

 

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