Taphonomy of the Late Cretaceous dinosaur-bearing beds of the Laño Quarry (Iberian Peninsula)

Abstract

The Laño quarry, located in the northern part of the Iberian Peninsula (Basque–Cantabrian Region), has yielded a diverse continental vertebrate assemblage from the Late Cretaceous (late Campanian to early Maastrichtian). The fossil assemblage consists of almost 40 species, including bony fish, amphibians, lizards, snakes, turtles, crocodilians, dinosaurs, pterosaurs and mammals. Laño is the most productive European vertebrate locality hitherto known from the Late Cretaceous. The sedimentology of the Laño section shows that the three associated fossiliferous beds (known as L1A, L1B and L2) were deposited in an alluvial system composed primarily of fluvial sands and silts. The sedimentary structures are consistent with channel areas within an extensive braided river. The L1A level was excavated in detail and mapped following a square-metre grid system. The main features of the multispecies vertebrate assemblage are as follows: wide range of body size (including microfossils); large proportion of adult individuals; rare articulated skeletal parts; size-sorting and dominance of small elements; high spatial density; long bones showing some preferential orientation; variable dip; variable degree of breakage and large proportion of splintering; wide range of weathering and abrasion; no evidence of predatory activity or chemical alteration. On the basis of these features, we suggest that the Laño association lies between the taphonomic modes for attritional vertebrate assemblages in fluvial channels, though it tends more toward the channel-fill than the channel lag mode. The L1A sample of Laño is a heterogeneous assemblage of elements ranging from isolated bones and teeth to articulated skeletons with allochthonous and para-autochthonous characteristics. It consists of a mixture of vertebrates from different palaeocommunities, and includes aquatic (or semi-aquatic) elements and remains transported from a variety of habitats on the alluvial plain. The dinosaur bones are commonly broken and show a greater degree of abrasion than those of freshwater vertebrates, indicating that they may be allochthonous. However, actinopterygians, amphibians, pleurodiran turtles and eusuchian crocodilians are interpreted as being para-autochthonous. Palaeobatrachid frogs, bothremydid turtles and alligatoroid crocodilians are the most relatively abundant vertebrates of the Laño fossil assemblage. The occurrence of pelomedusid turtles and crocodilians is indicative of an intertropical, warm climate.