Fossilization of baleen microstructures in the upper Miocene Pisco Lagerstätte (Peru).

Instead of teeth, modern mysticetes possess horny structures (i.e., baleen) that permit various bulk-filtering feeding strategies. Baleen consist of racks of keratinous plates which bear at their medial edge a fringe of tapered bristles acting as a sieve for the filtration of small-sized prey. Baleen are partially calcified (i.e., they include hydroxylapatite crystallites), and interspecific variations in calcification pattern reflect different feeding strategies and prey types. In the mysticete fossil record, preservation of baleen in situ is extremely rare. This is due to easier decomposition of keratine with respect to associated bones, and to early detachment of the baleen rack from the maxilla due to rapid decay of the epithelial tissue holding the baleen plates. Despite this, in the upper Miocene beds of the Pisco Formation (southern coast of Peru), several dozens of fossilized baleen have been described associated to whale skeletons, thus strengthening the qualification of the Pisco fossil deposits as a Lagerstätte. Here we provide the first microscopic and microanalytical characterization of this exceptional fossil record. In 2014, an articulated partial skeleton of a balaenopteroid (clade of the rorquals, grey whales, and relatives) whale with preserved baleen racks was discovered in a deeply cemented dolomitic nodule at Cerro Colorado (Ica region). Both racks are apparently complete, fully articulated, and mainly composed of regularly spaced planar structures marked by rectilinear, parallel grooves. Whitish, string-like structures having a sub-mm diameter are preserved outside these planar structures. We interpreted the planar structure as the natural casts of the plates (displaying prints of the baleen tubules) and the string-like structures as the free termination of the bristles. SEM observations of samples of the string-like structures revealed that they consist of dolomite rods wrapped in an easily exfoliating material strongly reminding the arrangement of the epithelial cells of bristles in extant mysticete baleen, but with a calcium phosphate composition. EDS spectra suggest an identification of this calcium phosphate as apatite. In transverse section of the bristles, the inner lumen can be observed, completely filled with dolomite, as well as the walls consisting of porous and erodible apatite. The 3D morphology of the fragments of the bristles is surprisingly regular, and no evidence of deformation of the tubules by compaction could be found. The preservation of the tubules pertaining to the medullary part of baleen plates (as natural casts) and of the free portion of the bristles (as apatite cylinders) requires that the decay of the tubules was delayed compared to the decay of the keratin matrix of the plates, thus suggesting a higher calcification of the tubules with respect to the intertubular keratin in the mysticete of Cerro Colorado. Coupled with morphometric comparisons with baleen of extant mysticetes, this observation suggests that the middle-sized (7 m to 9 m in total length) balaenopteroid species of Cerro Colorado mainly fed on small-sized planktonic invertebrates. The exceptional preservation of the walls of baleen bristles as apatite indicates that a process of phosphatic permineralization was active during the formation of the concretion enclosing the specimen, favored by the same alkaline environment allowing dolomite precipitation. A rapid formation of nodules before decay, most likely favored by sinking of the whale carcass in a thick, soft sediment layer, chemically prone to dolomite precipitation, is testified by the lack of evidence of compression of diatomaceous sediment and by the 3D preservation of delicate baleen structures inside the concretions.