Puesta al día de los dinosaurios y otros vertebrados de Lo Hueco en las VI Jornadas de Salas de los Infantes

Continuamos con las crónicas de las VI Jornadas Internacionales sobre Paleontología de Dinosaurios y su Entorno de Salas de los Infantes (Burgos). Una de las conferencias invitadas del congreso corrió a cargo de Francisco Ortega, que habló sobre los avances en el conocimiento de las faunas finicretácicas del yacimiento de Lo Hueco (Fuentes, Cuenca).

El resumen es el siguiente:

The record of the Upper Cretaceous continental tetrapods from the Iberian Peninsula is mainly restricted from Campanian to Maastrichtian, and only a few, exclusively Cenomanian, Spanish and Portuguese sites are informative about the early Upper Cretaceous faunas. It is generally assumed that the European Upper Cretaceous faunas are subject to a complex biogeography derived from processes of insularity and relationships with the rest of Laurasia and, particularly, with Gondwana. It is also assumed a pattern of faunal change from the Campanian and lower Maastrichtian, characterized by the abundance of titanosaurs, to faunas with higher presence of hadrosaurids at the end of the Maastrichtian. However, the knowledge of these faunas is far from well established, and proposed general hypotheses are quite unstable, not only in the Iberian Peninsula, but also in the rest of Europe. In recent years the information available has increased considerably, both for the reanalysis of collections, for field work on traditional areas or by finding of new sites. Among the latter, it is particularly singular the fossil site of Lo Hueco (Fuentes, Cuenca Province).

Lo Hueco was discovered in 2007 during the construction of a section of the railway of the high-speed train from Madrid to Levante. The fossil site belongs to the upper part of the “Margas, Arcillas y Yesos de Villalba de la Sierra” Formation. Sediments at “Lo Hueco” was deposited within a coastal-continental environment, corresponding to a near-coast continental muddy flood plain crossed by distributary sandy channels, exposed intermittently to brackish or marine and freshwater flooding as well as to partial or total desiccation events. The site contains an extraordinary richness and diversity of fossils that constitutes a singular accumulation in which there is an unusual abundance of individuals of continental tetrapods, particularly titanosaur sauropods, crocodiles and turtles. But also, in the case of the titanosaurs, the site has provided multiple partial skeletons in anatomical connection or with a low dispersion of its elements. In this sense, the Konzentrat-Lagerstätten of “Lo Hueco” is singular not only for the Spanish record but also for the whole upper Campanian-lower Maastrichtian European palaeontological record.

Palynological data suggests a late Campanian-early Maastrichtian tropical palaeovegetation with a relatively high richness of angiosperms compared to other nearby and synchronous sites. Particularly, the palynological record of “Lo Hueco” are characterized by the high number of species attributed to Typhaceae, representing an early example of wetland vegetation under stressful condition with such modern plant community characteristics. Invertebrates are relatively scarce and not particularly well preserved in Lo Hueco, but developed analyses indicate that most of the molluscs belong to a freshwater fauna dominated by unionid and heterodontid bivalves. The occurrence of some melanopsid gastropods also suggests the sporadic influence of moderately brackish-water episodes, what would be consistent with the overall environmental hypothesis.

Vertebrates are represented at Lo Hueco by actinopterygians (mainly lepisosteids) and teleosteans fishes, amphibians, pleurodiran and pancryptodiran turtles, squamate lizards, eusuchian crocodiles, euornithopods (rhabdodontids), theropods (mainly dromaeosaurids) and sauropod titanosaurs. Of these, by far, turtles, squamates, crocodiles and dinosaurs are the most abundant and best-known vertebrates.

It has been identified three species of turtles belonging to two main lineages. The scarcer form is an undetermined turtle attributed to Pancryptodira, but most of the turtles are member of Pleurodira. The Lo Hueco turtles have helped to reinterpret the relationships of several European members of Foxemydina Bothremydidae, allowing to define the Iberoarmorican genus Iberoccitanemys.

The material assigned to lepidosauromorphs has been ascribed to Scincomorpha indet., to several forms of Iguania, and to some undetermined Squamata. Some vertebrae were also attributed to a medium sized non-marine “pythonomorph” Varanoidea.

The site has yielded abundant crocodyliform remains, including well-preserved skulls with associated jaws. Except a ziphodont tooth, all specimens identified so far belong to two eusuchian forms that appear to be closely related to each other and with Allodaposuchus, a crocodyliform from the Upper Cretaceous of Romania. Lo Hueco species present combinations of characters that differentiate them from other Allodaposuchus-like forms described in the Upper Cretaceous Iberoarmorican realm.

Dinosaur remains are markedly biased for saurischians, especially sauropods, against ornithischians. The latter are underrepresented compared to other known faunas of the rest of the European Upper Cretaceous. In fact, the review of the available material does not allow to unambiguously identifying any ankylosaur representative, and all the ornithopod remains correspond to a single identifiable euornithopod that can be attributed to genus Rhabdodon. The theropod remains are composed of teeth and abundant postcranial remains. Most of the latter can be assigned to Dromaeosauridae and, at least, represent a primitive still undescribed new form. Diversity of derived theropod comprises four morphotypes of teeth that have been identified as Dromaeosauridae indet., Velociraptorinae indet., cf. Richardoestesia and cf. Paronychodon. The record of non-dromaeosaurid Theropoda at Lo Hueco is scarce, but some still undetermined appendicular remains and a poorly preserved tooth can be recognized.

Finally, as previously discussed, the most abundant dinosaur record in Lo Hueco consists on sauropod remains that, in all cases, can be assigned to titanosaurs. The site has provided thousands of isolated bones and more than twenty sets representing partial skeletons of several individuals. Among these, are relatively frequent sets representing articulated caudal or dorsal vertebrae, associated or not to the pelvic girdle. Appendicular elements generally appear disarticulates but, in some cases, analysis of the spatial distribution of fossils in the site allows to establish relationship between isolated bones and some partial axial skeletons. Moreover, it has also been recognized some cranial remains, both isolated and associated with partial skeletons. At the moment, the analysis of variability of diff erent sets of samples (cranial bones, teeth, caudal vertebrae, appendicular elements) indicates the presence of at least two species of titanosaurs, which seem to present a relatively high morphological variability. Th ese species would represent a robust titanosaur, showing similarities with others from the Iberoarmorican realm, and a slender titanosaur that has exclusive features. The analysis of a braincase throws similarities with Ampelosaurus, a titanosaur previously known in the French Upper Cretaceous, while is noteworthy the absence, at the moment, of elements referable to Lirainosaurus, the only genus of titanosaur previously cited in the Iberian Peninsula.