Fossil baleen under the microscope: seeking the key for fine-scale preservation of soft tissues.

Instead of teeth, modern baleen whales bear comb-like, hair-fringed baleen plates that enable bulk-filter feeding on vast amounts of small prey. Baleen is a horny material, being only moderately mineralized by crystallites of hydroxyapatite interspersed within an easily degradable keratinous matrix. Thus, baleen rarely fossilizes, making our knowledge of its evolutionary history highly speculative. Fossil baleen structures have been reported only from a few Neogene localities worldwide, and especially from the late Miocene strata of the Pisco Formation of Peru. We undertook a scanning electron microscopy, microanalytical (EDS, EPMA) and mineralogical (XRD) study of fossil baleen structures from the Pisco Fm. Three specimens have been investigated to date (Gioncada et al., 2016; Marx et al., 2017; this work), all showing evidence of fine-scale preservation of baleen tissues. The first two consist of articulated skeletons, referred to as Balaenopteroidea indet. (rorquals and relatives), from the sites of Cerro Colorado and Cerro La Bruja. Both specimens exhibit baleen racks enclosed in dolomite concretions. Fossilization types include molding of plates or tubules and very fine-scale phosphatization of the latter (comprising their free terminations, i.e., baleen bristles). We argue that both whales foundered in a soft, organic-rich sediment representing an environment chemically prone to rapid nucleation of dolomite, thus allowing the preservation of delicate soft structures as molds. Bio-mediated precipitation of dolomite due to sulphate-reducing bacteria is testified by the observation of ghosts of iron sulphide framboids within the matrix enclosing the baleen. Found at the site of Cerro Ballena, the third studied specimen consists of an articulated skeleton of a diminutive mysticete identified as belonging to the cetotheriid species Piscobalaena nana. This finding represents the first fossil baleen apparatus of an entirely extinct mysticete morphotype, showing an exceptional degree of preservation thanks to phosphatization of the baleen plates, in the absence of a dolomite concretion. The lack of an insulating concretion possibly allowed the development of quite large (1-10 μmsized) crystals of fluorapatite. This ongoing research indicates that (1) the early formation of dolomite around whale carcasses is a prominent, but not necessary, process inducing exceptional baleen preservation, and (2) calcification of soft tissues such as baleen is favoured by the presence of Ca-phosphate crystallites.