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The Tepexi de Rodriguez region, located in the meridional part of the State of Puebla, Mexico (Fig. 1) is famous because of its fossiliferous localities from Cretaceous-Tertiary, where Tlayua quarries (Albian) and “Pie de Vaca” outcrop (Pliocene-Pleistocene) stands out. Due to the abundance, variety and conservation of its paleobiota, the Tlayua quarry is one of the most important fossiliferous localities from the Cretaceous of North America. About 3 500 samples of invertebrate fossils have been collected from this locality, which were studied or are being studied in the Institute of Geology of the Universidad Nacional Autónoma de México, as much as in many foreign institutions. The best preserved material belongs to vertebrates represented by fishes and reptiles; the fishes include, more than 50 different holosteans and teleosteans (Applegate & Espinosa-Arrubarrena, 1984; EspinosaArrubarrena & Applegate, 1996).
Fig. 1 Location of Tlayuna quarries (18° 35’ N - 97° 56’ W), in Mexico, in the state of Publa,n in the paleontological sone near Tepexi de Rodriguez.
Due to the characteristics facies and the environmental conditions, Mollusca are not abundant; within them there have been collected gastropods (cassiopids) and cephalopods ammonoids of the genus Mortoniceras and belemnoids of the genus Neohibolites. Among the ichnofossils are found Ophiomorpha nodosa, and insects of the family Tipulidae. Various species of echinoderms are also found crinoids, ophiuroids, asteroids, holoturoids and echinoids (Buitrón & Olivos, 1987; Buitrón, 1991).
Geologic setting
The Albian limestones that crop out in the Barranca de Tlayua have been formally designate with the name of Tlayua Formation which includes a sequence with a thickness of about 300 m, divided in three members; lower, medium and upper (Fig. 2).
Fig. 2 A stratigraphic section of the Tlayua Formation with an enlargement of the Middle member, showing the approximate location of the fossil asteroids described this paper.
The lower member consists in a fine grain limestone sequence (mudstone to wackstone), micritic, with few intraclasts and abundant miliolids. Its stratification changes from massive to thick, with thin biostromes, with thicknesses between 20 cm and 50 cm of rudists like Toucasia polygyra (Alencaster, 1973), and ostreids from of the genus Chondrodonta. Within the limestones appear white and black chert concretions and dark flint glasses, in some areas the fossils are completely silicificated (Espinosa-Arrubarrena & Applegate, 1996). The medium member consists of a 48 to 50 m sequence, of thinly stratified, laminated, micritic limestone (mudstone) of cream to reddish colors. The stratification changes its thickness from few millimeters to 15 cm. Inside of the beds there are dark flint glasses of a thickness from a few millimeters to 15 cm and 70 cm long. In some fracture planes, widened by the water and differential dissolution, the lamination is emphasized remarkably, phenomenon that is seen clearly in the wall of the quarry. The bed surfaces are covered by a very thin layer of hematite that gives to the flagstones a characteristic red color. This material represents carpets of Cyanophyta seaweeds microorganisms from normal to hipersaline sea environments.
The best outcrop of the medium member is exposed at the quarry of Tlayua, the one that was worked in the upper intermediate part of the section, where most of the fossiliferous material have been recollected. The specimens consist of cephalopods (ammonoids, belemnoids), regular echinoids (cidaroids) crinoids, asteroids, ophiuroids, holothuroids, ichnofossils (Ophiomorpha nodosa), insects (Diptera), and leaves of gymnosperms (Podozamites), seaweed carpets, fishes and marine or continental reptiles. The age of this member is established by the ammonoids Hysteroceras, Mortoniceras and cf. Anisoceras (Cantú-Chapa, 1987), and by the belemnoids Neohibolites praeultimus Spaeth, N. aff. minimus Stolley, N. minimus clavaformis Seibertz and Buitrón, which indicates a medium and late-Albian age (Seibertz & Buitrón, 1987). The faunistic content proves a mixed thanatocenosis.
The presence of seaweed carpets in the locality (Seibertz & Buitrón, 1987) indicates in general, shallow depth. The rock appears as laminated “mudstone” seaweeds with intercalations of “pellets”. The presences of galleries or small holes on some of the fossils indicates the activity of Ophiomorpha nodosa and the ashes and feldesphatic sandstone and clay between some beds indicates volcanic activity during the Middle Albian time (Frey et al., 1978) so it is concluded that depth, must have corresponded shallow sea. It is considered that intermittent or low energy currents have existed, as O. nodosa increases the size of its population in superficial calm and protected seas, in the locality the number is scarce. On the other hand, Applegate (1987) comments that in the same level small fishes with the heads aligned in the same direction have been found; this might be understood as occasional currents being responsible of this arrangement.
About the continental environment, the presence of a Pseudocarterus or alike insect genus (Diptera, Tipulidae) indicates that they were predators but they also visited flowers and nectar (Downs & Smith, 1969). The adults of Tipulidae are usually found between the weeds and in the shade of trunks with vegetation near aquatic deposits.
Two thirds of the species of the Hybotinae (Diptera, Hybotinae) are found in the tropics and subtropics of the oriental and Neotropical regions (Chuala, 1983 inWaters, 1989) that is why it is supposed a warm-humid climate in this locality. The Upper member consists in an incomplete sequence of medium to thick dolomite and dolomitic limestone beds, gray colored with light and dark tonality whose best outcrops are on the septentrional margin of Tlayua cliff. The dolomite goes transitionally to a fine-grained micritic limestone, from medium to thin bedding, and it is observed in road to the Agua de Luna ranch. The petrographic study shows that there are two microfacies. In both cases, the dolomite is secondary (MalpicaCruz et al., 1989; Pantoja, et al., 1989). This precarious secondary dolomitization occurs generally in the fresh water-sea water interface (Friedman & Sanders, 1967). The Tlayua Formation is correlated with the Morelos Formation (Fries, 1960) with the Orizaba Limestone from the sierra of Tentso (Padilla, 1973) and with part of the Teposcolula Limestone of the Mixteca Alta (Ferrusquía, 1976).
Paleoenvironments
The Albian of the region consist of an extended platform covered by a sea with low depth and moderate energy, with some elevated areas that generate island and lagoons environments a calcareous substratum which depositional conditions were very similar to the presently developing at the Bahamas. The lower member of the Tlayua Formation was deposited in an inner platform, near a litoral zone of moderate to low energy. The facies of this member is distinguished by the presence of micritic limestone, with some intraclasts and benthic foraminifera. Intercalated in this section there are zones with banks of requienid (packstones), and banks of oysters (packstones) that presents bars or islets that delimits the inner platform from the high energy seas zones. In the nearest facies to the intermediate member is observed a transitional change between micritic and microesparitic limestone, with some fractures filled with recristalized calcite in form of dogs teeth and meniscus shaped that indicates diagenetics processes in the vadose zone with predominance of fresh over the sea water. The medium member was deposited in the intertidal zone of a lagoon environment. The microesparitic limestone with thin laminated beds are mainly formed by micrite and in a lower percentage by several characteristic. The main one is by micritic with foraminifers and in general with a benthonic fauna of lagoon environment. Likewise, transitional changes are shown between micritic and microesparite and another that contains incipient microestilolits. The horizontal laminated stratification was produced by the influence of high dynamic currents (by traction) and changes of salinity temperature, pH and in the rate of sedimentation. The intraclasts and the benthonic foraminifers sustain the proposed environment (Malpica-Cruz et al., 1989).
Based on the microfacies and paleoenvironmental studies, Malpica-Cruz et al. (1989) proposed a conceptual model, with local characteristics, for the medium member. The depositional environment belongs to an intertidal zone in a lagoon environment with a wide inundation-desiccation surface with arrival of exogen fauna (coming from reefs, and open sea) during the high tide and the storms. The fauna was caught, during the low tide in puddles and hollows formed by-digging and desiccation.
Materials and methods
The geologic-paleontological prospection on Tepexi de Rodriguez area was realized in different times since 2010. The asteroids fossils from Tlayua quarry consist of impressions that indicate a rapid burial coupled with very diagenetic formation. The colors of these fossils are from honey to almost ivory-colored calcite. The skin has also been replaced by amorphous deposits of calcite which at times may be fragmentary or composed of very small crystals (0.3 - 0.4 mm).
Seven asteroids were deposited in the Colección Nacional de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de México, Mexico City. The catalog number correspond to IGM-3742-IGM-3748. Also some measures were taken, major radius (R), length from centre of the disc to tip of an arm, and minor radius (r), from centre of disc to interradial edge.
Results
SYSTEMATIC PALEONTOLOGY Class Asteroidea, Blainville, 1830 Order Paxillosida Perrier, 1884 Family Astropectinidae Gray, 1840
Diagnosis
A family of mainly infaunal Paxillosida widely ranking bathymetrically; normally with five arms of various length usually merging with the disc to form rounded or at least blunt-angled interracial arcs; abactinal plates highly paxilliform, small and crowded; an apical (epiproctal) cone evident in some specimens; papulae either present over the whole paxillar area or absent centrally and midradially; both series of marginal plates more or less block-like, conspicuous laterally and usually also dorsally and ventrally, armed with more or less conspicuous spinelets or granules and often with some large spines, especially on the inferomarginals, adambulacral plates broad, the farrow margin most often short and angular or convex but sometimes longer and nearly straight, armed with large furrow and subambulacral spines; oral plates large, usually modified triangular in shape but sometimes constricted and bar-shaped (Clark & Downey, 1992).
Genus Astropecten Gray, 1840 Astropecten sp.
Figs. 3, 4, 5
Description
The specimen with the number IGM-3742 (R = 13.68 mm, r = 4.29 mm) represent the actinal surface with ambulacral grooves are well defined, the adambulacral plates are differentiated from inferomarginal plates. Due to the size this organism was a juvenile of the genus Astropecten (Fig. 3). The sea star with the number IGM-3743 (R = 15.09 mm, r = 4.77 mm) represent the abactinal surface of an asteroid; arms are triangular, wider at the bottom than at the tip (1.70 mm), pointed, although this is blunt, there are rows of superomarginal and abactinal plates, epiproctal cone is observed, which indicates that an organism was a juvenile of the genus Astropecten (Fig. 4). The specimen with the number IGM3744 (R = 15.58 mm, r = 5.19 mm) represent the actinal surface, arms are wider at the bottom than at the tip, blunt. The ambulacral grooves are broad at the bottom of the arm and gets narrower toward the tip, also shows that the oral plates are prominent and have a median groove. Due to the size this organism was a juvenile of the genus Astropecten (Fig. 5).
Genus Plutonaster Sladen 1889 Plutonaster sp. Fig. 6
Diagnosis
The sea star with the number IGM-3745 (R =, r = 12.50 mm) correspond of the actinal surface, ambulacral grooves are observed and they are narrow, the adambulacral plates are differentiated from inferomarginal plates. The actinal plates are observed and they possibly have one spine on each plate. These characteristics indicate that this organism was a juvenile of the genus Plutonaster.
Order Valvatida Perrier, 1884 Family Ophidiasteridae Verrill, 1870
Diagnosis
A family of Valvatida with small disc, arms usually long, slender, cylindrical, or sometimes trigonal in cross-section, four to seven (usually five); body usually covered with granular membrane; abactinal plates tessellate; marginal in two series, generally not conspicuous, equal, opposite, lateral, plates arranged in longitudinal series parallel to the furrow; actinal areas small; adambulacal furrow margin without apophyses, furrow spines usually small, blunt; pedicellariae, when present, alveolar (Clark & Downey, 1992).
Genus Tamaria Gray, 1840
Diagnosis
A genus of Ophidiasteridae with heavy, rigid skeleton; arms tapering or petaloid; abactinal plates in regular longitudinal series; body covering granulose; popular pore areas in4-6 longitudinal rows, none below inferomarginals; alveolar pedicellariae present and usually abundant (Clark & Downey, 1992).
Tamaria sp. Fig. 7
Description
The sea star with the number IGM-3746 (R = 11.77 mm, r = 5.75 mm) represent the abactinal surface; arms are wider at the bottom (5.64 mm) than at the tip (1.70 mm). The superomarginal plates are well defined, also it have paxillae in the arms. The epiproctal cone is observed, which indicates that this organism was a juvenile of the genus Tamaria.
Genus Ophidiaster Agassiz, 1836
Diagnosis
A genus of Ophidiasteridae with small disc, arms moderately long, cylindrical, rarely tapering; abactinal plates in regular longitudinal and transverse series; skin uniformly granulose; papular areas in eight longitudinal series, one series each side below inferomarginal plates; pedicellariae, when present, sugartongs form in well-developed alveoli.
Ophidiaster sp. Fig. 8
Description
The star with the number IGM-3747 has five long, slender and cylindrical arms and abactinal plates in regular longitudinal series, these series are interspersed with the popular areas which indicates that this organism was of the genus Ophidiaster.
Orden Spinulosida Perrier, 1884 Family Echinasteridae Verrill, 1870
Genus Echinaster Müller & Troschel, 1840
Diagnosis
Small disc, cylindrical arms, abactinal skeleton reticular; armament ranging from moderately fine with coarse spinelets or small spines, usally single on each plate, sometimes only on the primaries; abactinal and intermarginal meshes occupied by a mixture of papulae and epidermal glands; papulae rares or absent below the inferomarginals (Clark & Downey, 1992).
Echinaster sp. Fig. 9
Discussion
The Tlayua quarry Cretaceous (Albian) has an age of 110 Ma., and has a thanatocenosis which is formed by plants, ichnofossil of invertebrates, molluscs (gastropods, ammonoids), insects, echinoderms (crinoids, asteroids, ophiuroids, echinoids, holothurians) fishes, reptiles (Buitrón & Malpica, 1987; Cantú-Chapa, 1987; Seibertz & Buitrón-Sanchez, 1987; Applegate, 1988; Feldmann et al., 1998; Buitrón-Sanchez, et al, 2009). This is the first record of Cretaceus asteroids of Mexico. Although the conservation of the specimens of asteroids is not very good, morphological aspects that allowed generic identification were observed. Some measures could not be taken, because in some specimens is not visible disk center and others have the tips of the arms.
