Traces or perforations caused by living organisms after the animal's death can be found on various dinosaur bone remains. These perforations, known as bioerosion structures, provide information that helps us understand the relationships between living organisms in the past, reconstruct palaeoecosystems and improve our understanding of the fossilization process. Now, a study published in the journal Earth-Science Reviews has identified this type of perforation in bones and, for the first time, also in pieces of dermal armour (osteoderms) from titanosaurs at the Lo Hueco site (Cuenca, Spain), dating from the Late Cretaceous.
The results indicate that the titanosaur carcasses from Lo Hueco were not rapidly buried, as had previously been suggested, but remained exposed long enough to allow specialized insects (mainly necrophages and saprophages) to bore into them. The study revises the palaeoecological reconstruction of the Late Cretaceous at the Lo Hueco site, offering a new interpretation of its sedimentary, ecological and environmental dynamics.
Furthermore, the study shows that a detailed ichnological analysis — that is, of fossilised traces or footprints — of bioerosion structures at sites with abundant preserved skeletal remains could be very useful for gaining a precise understanding of the process by which the remains accumulated and the palaeoenvironmental conditions under which this occurred.
In addition to expanding the fossil record of insect-induced bioerosion in dinosaur fossils, thanks to the exceptional preservation of the Lo Hueco site, the study has helped to consolidate the validity of the ichnogenus Cubiculum (which includes the identified perforations). Specifically, this ichnogenus is characterized by boreholes with a hemispherical or pouch-like shape which, when compared with modern analogues, have been attributed to the bioerosive activity of dermestid beetles.
The study is led by Professor Zain Belaústegui, from the Faculty of Earth Sciences and the Biodiversity Research Institute (IRBio) at the University of Barcelona. The study also involves experts from the National University of Distance Education (UNED), which leads the national research project that has funded this study, as well as forensic entomologists from the University of Alcalá (UAH).
A site rich in titanosaur fossils
At the Lo Hueco site, one of the most important in the European Late Cretaceous, there are not only more or less isolated bone remains, but also relatively complete skeletons of large titanosaur sauropods that lived 70 million years ago.
"Studying the fossil record of bioerosion caused by insects on different types of bone tissue (bones, horns or osteoderms) can be very useful for gaining insight into the taphonomic process undergone by these skeletal remains, whether they are isolated (bones) or articulated (more or less complete skeletons)," explains Zain Belaústegui, from the Department of Earth and Ocean Dynamics at the UB. "In any case, this suggests that these remains were exposed for long enough for these scavenging organisms to bore into these skeletal structures."
A unique palaeoecological window onto the past
Based on ichnological research — the discipline that studies the fossilized traces or footprints left by the activity of organisms in the past — "the question arises as to whether the carcass of a large vertebrate could sustain an entire community of scavengers, necrophages and saprophages for a relatively long period", explains the expert, who adds: "Therefore, if the skeletal remains are fossilized with traces of bioerosion, they may be highly indicative of a specific palaeoenvironmental condition."
Insect borings provide key information on how long the carcasses were exposed before being buried. Experiments with larvae of the modern beetle Dermestes frischii, which is capable of producing similar bioerosion structures to those found in the ichnogenus Cubiculum, show that these structures can form over periods of at least 240 hours, and even much longer under natural conditions. "The more taphonomic information we can obtain, the more palaeoecological and palaeoenvironmental data we will have on the skeletal remains under study," notes the researcher.
The team has developed a much more accurate taphonomic interpretation, as the abundance of insect borings suggests that the carcasses were exposed for a longer period of time. "This would indicate a longer biostratigraphic stage for the two main fossil-bearing levels at Lo Hueco (G1 and G2), ruling out the rapid burial of the titanosaur carcasses that had previously been inferred," explains Belaústegui.
The study also includes a comprehensive review of more than 140 references on insect bioerosion of bone tissue, ranging from the Middle Triassic to the Holocene. However, only one of these references relates to the Iberian Peninsula, highlighting the significance of the new study and the need for further research in this area.
