DFG Project FR 1314/9-1
08. 2003 - 08.2005
Partenaires: DFG, Museo del desierto (lien?)
During our research work in October 2000 (DFG project FR 1314/4-1) we identified in the Kimmeridgian (Upper Jurassic) La Caja (La Casita) Formation of north-eastern Mexico remnants of crocodilians, ichthyosaurs and especially of large pliosaurs. The state of preservation of a partial vertebral column of a gigantic pliosaur from Aramberri, Nuevo León, housed in the lecture hall of the Universidad Autonoma de Nuevo León, Facultad de Ciencias de la Tierra in Linares (UANL), launched a field campaign in Aramberri in October 2001 (DFG FR 1314/6-1) which yielded several skull fragments and vertebral remains. The material clearly belongs to the individual housed in Linares and is still under preparation. It indicates a pliosaur reaching at least 15 m in length. Bone structure and state of co-ossification also confirm that this “Monster of Aramberri” was a juvenile. An unhealed bite mark in a skull bone proves that the animal died as a result of the attack of another much larger pliosaur. During the 2001 campaign the original sediment layer bearing the pliosaur was identified and was subsequently exposed during the field 2002 campaign (FR 1314/7-1). In addition to the head of a femur with a diameter of 450 mm, our exploration yielded further bone fragments from the debris down slope and confirmed the pliosaur's position of embedding. The bone-bearing concretion measures approximately 8.5 m in length, appears to be more that 3 m in width and forms a layer dipping 40 to 60°. The exposed section shows a dense packing of large bones. The excavation, preparation and scientific investigation of these concretions and all bone fragments from the debris are of crucial importance to reconstruct the “Monster of Aramberri”. Revelation of its taxonomic status is one of the objectives of the project. Another objective is to describe and identify the remnants of four additional pliosaurs from the La Casita Formation and to discuss the palaeobiological, evolutionary and palaeobiogeographical context of the pliosaurs that inhabited the Mexican Sea during Late Jurassic times.
Upper Jurassic marine reptiles are well known from Europe and were recently discovered also in South America, raising the question of their dispersion way. The existence of a Hispanic corridor linking the two realms was suggested as a main route during the Middle Jurassic, an hypothesis that is reinforced by the re-description of Cuban marine reptiles. Little was known about marine reptiles of north-eastern Mexico, mainly due to fragmentary material which was randomly collected by private collectors and researchers with objectives other than marine reptiles. The new metriorhynchid crocodile Geosaurus vignaudi described from Puebla State (Mexico), corroborates the Hispanic corridor hypothesis, and the existence of “intermediate” forms to be found in the area.
The most complete material concerning marine reptiles from the La Caja Fm. in the region of Aramberri belongs to pliosaurs. In the context of palaeobiogeography, the collection of all remains of the giant pliosaur from Aramberri is essential to understand its systematic affinities. In three excavation campaigns, about 2.5 tons of bone bearing rock have been collected, including fragments of the cervical and the cranial part of the thoracic vertebral column, elements of the shoulder and of the pelvic girdle as well as cranial fragments with bite marks. A spin-off excavation in the marls surrounding the bone-bearing concretion yielded fossil fishes, invertebrates and plant debris which will elucidate the faunal regime near Aramberri which at that time was located near the brim of the shelf.
Besides the gigantic pliosaur proceding from Aramberri, remnants of three more pliosaurs are available for study. They all procede from the La Caja/La Casita Fm. and are of late Jurassic (Kimmeridgian-Tithonian) ages:
A fragment of a pliosaur rostrum from the Kimmeridgian of the Sierra El Montelongo Pedregoso 20 km west of Aramberri. This specimen shows striking similarities with the genus Kronosaurus known from the Lower Cretaceous of Australia and Colombia. If this holds true, the specimen would represent the oldest known kronosaurid pliosaur in the world and help to clarify the affinities within the Pliosauridae.
The terminus of a rostrum which was discovered by phosphorite prospectors in the mountains arround Zaragoza, some 15 km south of Aramberri.
A sequence of 4 vertebrae from the transition neck – thorax adjacent to elements of the shoulder girdle form the region of La Ascension, approximately 80 km north of Aramberri.
A partial skeleton including skull parts with teeth from the Tithonian section of the La Casita Fm. of southern Coahuila with gastric contents.
All four specimens come from the Upper Jurassic portion of the La Caja/La Casita Fm. like the "Monster of Aramberri". So far, not a single marine reptile has yet been documented from this stratigraphic unit with the exception of Geosaurus vignaudi Frey et al. 2002.
The Upper Jurassic pliosaurs from Mexico are crucial to understand the early faunistic development of the Interior Seaway that starts to open towards the north and the Proto-Atlantic and Gulf of Mexico in the south and east as well as the evolutionary history of pliosaurs world wide.
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