A NEW LOCALITY OF DINOSAUR BONES IN VOLCANICLASTIC SEDIMENTS IN MICHOACáN, MéXICO

The occurrence of dinosaur bones in volcaniclastic settings is unusual. In México, the only locality of this type known previous to this report, was found in Early Jurassic sedimentary rocks of Huizachal Canyon (Tamaulipas), in which dinosaurs, other reptiles and mammals were found within pyroclastic flows. Here, we report the occurrence of Santonian dinosaur (Hadrosauridae; Ornithsichia) bones from Barranca Los Bonetes (Michoacán), preserved in a medial-distal volcanic setting.

Barranca Los Bonetes exposes a thick (~ 750 m) succession of fine-grained (fine to medium sandstones) and coarser (matrix-supported and clast-supported conglomerates) volcaniclastic sedimentary rocks. Six lithofacies are preserved: structureless sandstones, sandstones with tangential cross-stratification, planar-bedded sandstones, matrix-supported conglomerates, clast-supported conglomerates and pedogenic calcretes. Structureless sandstones dominate; by comparison, tangential cross-stratification and planar bedded sandstones are less well-represented. The sandstones are mainly feldspathic litharenites dominated by potassium feldspars and plagioclases. Lithic fragments are primarily volcanic with a lesser sedimentary component. Crystalline quartz is comparatively scarce.

Deposition was evidently episodic. Bioturbation is common at the tops of many units and a coleopteran or hemipteran insect burrow was identified from the distinctive meniscus pattern.

Episodes of non-deposition are also represented by pedogenic calcretes, in which root traces filled with secondary carbonates are preserved. In thin section we found organic remains representing either algae or higher plant material.

The nature and sequence of lithofacies suggests that the deposit was made by sediment gravity flows in the medial-distal part of a volcanic setting; lavas and/or pyroclastic features are absent. Disarticulated dinosaur long bones, vertebrae, and ribs found within the structureless sandstone facies are interpreted as an allochthonous assemblage, presumably caught up and reworked by high-intensity episodic volcaniclastic sediment gravity flows.

Barranca Los Bonetes is the southernmost area bearing dinosaur bones, and preserves the oldest hadrosaur bones in México.