Palaeoecology of the fishes from the Early Cretaceous lake of Las Hoyas Cuenca, Spain, with a hypothesis of sexual dimorphism for the Chanidae Rubiesichthys

The fishes from the Early Cretaceous (early Barremian) locality of Las Hoyas, in the Spanish province of Cuenca, are reviewed from a palaeoecological point of view. Las Hoyas was a permanent lake surrounded by actively interacting wetlands, with dry and humid cycles; no marine influence has been detected. A good number of the most generalized fishes from the lake, as well as small piscivore forms, would probably get into the wetlands in different extents throughout time, according to the availability of additional ecological space and food sources. Other fish taxa, such as the pycnodonts and the largest piscivores, must have had more physically restrained niches, probably somehow related with the humidity cycles in the palaeoenvironment. Juvenile fishes are largely represented in this locality, including events of mass mortality. The adults show a very interesting phenomenon of size reduction, probably related with environmental stress.The ecomorphologic overview of the fishes from the permanent waters of the Las Hoyas lake confirms that they must have occupied different niches, which, according to the lacustrine ecological spaces, can be grouped into:1) nekton related with the pleuston/plankton, such as the plankter Pleuropholis (Pholidophoriformes) or the surface insect-larvae eater Gordichthys (Chanidae), plus juvenile stages of many taxa, and possibly some other primitive teleosts that were plankton filters;2) nekton in strict sense, including the piscivore Caturus, Amiopsis, and Vidalamia (Amiidae); the water-bug eater Notagogus (Macrosemiidae); the shrimp-eater Lepidotes (Semionotidae), and ram feeder Rubiesichthys (Chanidae), plus a large amount of primitive teleosts, which constituted the major component of the Las Hoyas fish biomass;3) nekton related with the benthos, such as the predatory coelacanth cf. Holophagus and the pycnodonts Stenamara and Turbomesodon, adapted to a durophagous diet.A hypothesis of sexual dimorphism for the chanid teleost Rubiesichthys gregalis is proposed. Both morphotypes are discriminated by their relative body height only, and do not differ in any other morphometric, meristic, or anatomic character. They both occur in the same levels of the same localities, in about a 50-50% ratio for adult individuals, and there is no difference between the morphotypes and specimens from both known populations, El Montsec and Las Hoyas. Consequently, the two morphotypes of Rubiesichthys are interpreted as sexual dimorphs of a single s