THE CO-OCCURRENCE OF A LATE JURASSIC ICHTHYOSAURIAN REPTILE AND AMMONITES IN DURANGO, MEXICO. PRELIMINARY REPORT ON TAPHONOMY, SYSTEMATICS AND BIO-CHRONOLOGY

The fossils studied are Upper Jurassic ammonites together with reptilian, disarticulate vertebrae and teeth, as well as a 2,8 m long but incomplete carcass showing the skull and axial skeleton without limbs. These fossils were found in 1981 by R. Hernández and O. Carranza near Cuencamé (Durango), from transitional sediments between La Casita and La Caja Formations.

The skull is slightly greater than 85 cm. The palatal view shows displaced, broken bones, being identified remains of the pre-maxilla, maxilla and dentary with some tooth, the palatine and pterygoid, and probably the basisphenoid and quadrate. The incomplete axial skeleton preserves 58 to 60 amphicoelous vertebrae, showing moderate relative displacement (subtle overlapping) post-burial, as well as axial torsion (relative rotation of neural spines) due to sedimentary load. No caudal segments of the vertebral column exist. Some two-headed ribs are recognized, broken close to the attaching end. The preliminary study indicates a “fish reptile”, Ichthyosauria, most probably belonging to the family Ichthyosauridae or Ophtalmosauridae. The identification at the genus level is not conclusive at present.

Ammonite identification is rather difficult because of preservation (and preparation). Related to vertebrae were identified Glochiceras (Coryceras) sp. cf. carinatum and Taramelliceratinae (Taramelliceras and probably Mazapilites). Between skull bones, phragmocones of Haploceras sp. can show uncoupling forced by reworking of inner moulds, while small fragments show stumped ribs that could indicate the occurrence of Procraspedites sp. gr. praecursor. Other fragments illustrate the record glochiceratids and haploceratids. Updated ammonite bio-chronostratigraphy indicates the Eudoxus Chron (Late but no latest Kimmeridgian) as the most probable age of the Ichthyosaurid reptile. However, an early Tithonian age (early Triplicatus/Mucronatum Chron) should be considered if the occurrence of Mazapilites is confirmed.

Available data do not allow the conclusive distinction between palaeobiologic and taphonomic factors as the main ones responsible for the co-occurrence of ammonites with the reptile reported, but taphonomic overprint cannot be disregarded, especially if the occurrence of Mazapilites can be demonstrated (i.e. taphonomic condensation).