Mesozoic Terrestrial Ecosystems 2006 ARCHOSAURIAN SIZE BIAS IN JURASSIC AND CRETACEOUS FRESHWATER ECOSYSTEMS by ANGELA D. BUSCALIONI 1 and MARIAN FREGENAL-MARTINEZ 2 1 Unidad de Paleontología, Biología. Universidad Autónoma de Madrid, 28049 Madrid, SPAIN (angela.delgado@uam.es); 2 Departamento de Estratigrafía, Geología, Universidad Complutense de Madrid, 28035 Madrid, SPAIN INTRODUCTION PALAEOBIOLOGISTS seek to distinguish taphonomic biases from ecological patterns. There are two alternative preliminary hypotheses: that there is overwhelming taphonomic bias (“the guilty fossil record”) or that the fossil record is “innocent” until proven biased. Here, we provide an “innocent” account of Jurassic to Early Cretaceous freshwater assemblages and offer a possible geobiological context to account for the following two observations: firstly that Jurassic to Early Cretaceous freshwater associations are composed almost exclusively of small vertebrates, are dominated by aquatic and amphibious organisms, and include relatively few terrestrial taxa; secondly, that large bodied dinosaurs occur in different communities from the small vertebrates. In relation to possible size bias, Jurassic and Early Cretaceous vertebrate associations are better compared on the basis of their archosaurian assemblages, since other amphibious to terrestrial animals such as frogs, salamanders, lizards and albanerpetontids underwent miniaturisation during their Mesozoic evolutionary history. The Bathonian Kirtlington fauna (Forest Marble Formation) from the UK (Evans and Milner 1994) was deposited in a freshwater-lagoonal environment and contains small- bodied archosaurs including crocodiles (atoposaurids and Nannosuchus) , the pterosaur Rhamphorhynchus, and dinosaurs (small ornithischians and small to medium-sized theropods). Assemblages of large-bodied taxa are known from the contemporaneous marine and estuarine palaeoenvironments of the Sharps Hill Formation. The Purbeck Limestone Group of Dorset includes several archosaur-bearing horizons in both the Lulworth and Durlston Formations (Salisbury 2002). The Cherty Freshwater member of the Lulworth Formation contains a biota of small to medium-sized animals, including small theropod dinosaurs (e.g. the dromaeosaurid Nuthetes destructor: Milner 2002) and small crocodilians (Theriosuchus pusillus, Nannosuchus and Bernissartia). A contemporaneous large-bodied fauna, dominated by dinosaur footprints and large crocodilians such as Gonioipholis crassidens, Goniopholis simus and Pholidosaurus purbeckensis (Batten 2002), has been recovered from the Intermarine Member of the Durlston Formation where depositional environments and fossil content (e.g. sharks) reflect more saline conditions. Crocodilian and dinosaurian assemblages from the Late Jurassic and Early Cretaceous lacustrine deposits of the Iberian Peninsula are composed of dwarf species. In the lagoonal palaeoenvironment of Guimarota (Purbeck facies, northern Portugal) the dwarf crocodilian assemblage includes Theriosuchus, Lisboasaurus estesi and Lusitanicus mitacostratus (Schwarz and Fechner 2004). Dinosaur teeth from Guimarota also provide evidence for the abundance and species-richness of small taxa (Archaeopteryx, Compsognathus, an indeterminate dromaeosaurid, an indeterminate troodontid, Richardoestesia, Paronychodon and Phyllodon) against the comparative rarity of teeth from large-bodied taxa represented by tyrannosaurids, allosaurids, sauropods and iguanodontids (Zinke 1998). Dwarfism at the locality of Galve (Iberian Ranges, Central Spain) is indicated by the small-bodied neosuchians Bernissartia and Theriosuschus sp.. The rich archosaurian fossil record from Galve, and its palaeoenvironmental characterisation, permits rough estimates of relative abundances for the dinosaurian assemblage in each environment. Large-bodied sauropods and stegosaurians are the dominant elements in the Villar del Arzobispo Formation (Upper Jurassic), which is interpreted as a coastal to estuarine deposit. However, the overlying early Barremian lacustrine (freshwater to lagoonal) deposits of the Camarillas Formation contain a concentration of small bodied taxa, with the theropods represented by small isolated teeth from baryonychids, Coelurosauria indet, dromaeosaurids, Paronychodon and Richardo- estesia (Canudo and Ruiz Omeñaca 2003). 9