Evidences of interactions (feed and dispersion) between Cycadales and dinosaurs in Jurassic ecosystems

MARTINEZ, L.C.A.; ARTABE, A.E.; CANUDO, I.; GARRIDO, A.C.; SALGADO, L.

Mendoza

Congreso; 4° International Palaeontological Congress; 2014

IPA

Gut contents preserved within a dinosaur articulated skeleton from the Los Molles Formation are described herein. The dinosaur preserves the skull articulated with a series of vertebrae and the appendicular skeleton. It is considered to be close to the Thyreophora (Ornithischia). A mass of permineralized seeds (the gut content) were recovered close to the ribs. The seeds are ovoid, and almost radial in symmetry. There are at least two types of seeds, distinguished by their size: the largest (30.8 mm long, ca. 16.3 mm wide), and the smaller (13.6 mm long, 7 mm wide). The largest seeds preserved three layers: an outer coat (similar to the sarcotesta), the sclerotesta (with a well-defined coronula at the micropylar zone), and the inner layer (possibly corresponding to the nucellus). The largest seeds can accurately be assigned to the Cycadales. Seeds of mesozoic gymnosperms (i.e. Bennettitales, Caytoniales, Ginkgoales, and Pentoxylales) have a similar morphology, but they are small to minute. Therefore, the largest seeds display a coronula, a character that is diagnostic for the Stangeriaceae and Zamiaceae. The presence of seeds preserved as gut content within an articulated dinosaur skeleton, reinforces the hypothesis about interactions (endozoochory) between cycads and dinosaurs, especially in the dispersion of seeds. The seeds of cycads have a brightly, coloured, and juicy seed sarcotesta. In addition, the sarcotesta has chemicals that facilitate dormancy and its persistence inhibited seed germination. The ingestion of cycad seeds would be limited to dinosaurs that lacked grinding teeth (as is the case of these Ornithischia), because a strong mastication would release the toxins characteristics of this group of plants. It is also inferred that the sclerotesta would have prevented the digestion of the seed content by gastric fluids, allowing its passage through the digestive system up to be excreted as seed kernels. The extinction of herbivores (frugivores) dinosaur taxa would have a negative effect in cycads dispersion, leading to a lesser in the genetic variability of the group. Finally, the extinction of many genera of cycads in the Late Cretaceous/ Cenozoic would be related mainly by the climatic change and absence of dispersion. These factors would have prevented migration of cycads to refuges or tropical areas during the earth cooling periods.