Incremental growth and diagenesis of skeletal parts of the lamnoid shark Otodus obliquus from the early Eocene (Ypresian) of Morocco

Abstract

In contrast to ubiquitous extinct shark teeth, elasmobranch skeletons are composed principally of cartilage, a vertebrate tissue that rarely preserves in the fossil record. In a notable exception, shark vertebral centra are secondarily ossified during ontogeny and therefore more prone to fossilization. When they are found in conjunction with taxonomically diagnostic teeth, these centra preserve a record of incremental growth that potentially can be used for macroevolutionary and ontogenetic studies. Consisting of bone (hydroxyapatite mineral phase), however, fossilized centra are prone to diagenesis. The extent of diagenesis is reported here for associated vertebral centra of the extinct lamnoid shark, Otodus obliquus, from the early Eocene (Ypresian) of Morocco, using gross x-radiographic, and FT-IR techniques. These centra demonstrate significant diagenesis, with the existing secondary mineral consisting of francolite (carbonate fluorapatite). Nevertheless, the physical incremental growth bands consisting of alternating dark–light couplets are preserved in the centra. Carbonate δ 18O values across the growth axis of three centra indicate that a proxy signal is archived and interpreted to represent winter/summer seasonal growth for each dark–light couplet band. Given these results, the 19 physical growth bands counted for each of three centra of the same individual represent annuli, i.e., annular growth couplets, and represents a maximum age for this individual when it died. Despite claims to the contrary, pervasive in the literature, once diagenesis is understood, then oxygen isotope analyses of bone carbonate provide important data that can be used to interpret the paleobiology of incremental growth in fossilized vertebrate skeletal tissues. Nevertheless, because diagenesis is present and probably pervasive in fossilized bone, all such incremental studies should use both physical (e.g., x-radiographic and FT-IR) and stable isotopic techniques in order to better understand the paleobiology of extinct vertebrates.