BIROn - Birkbeck Institutional Research Online

A deﬁning feature of dentitions in modern sharks and rays is the regulated pattern order that generates multiple replacement teeth. These are arranged in labio-lingual ﬁles of replacement teeth that form in sequential time order both along the jaw and within successively initiated teeth in a deep dental lamina. Two distinct adult dentitions have been described: alternate, in which timing of new teeth alternates between two adjacent ﬁles, each erupting separately, and the other arranged as single ﬁles, where teeth of each ﬁle are timed to erupt together, in some taxa facilitating similarly timed teeth to join to form a cutting blade. Both are dependent on spatiotemporally regulated formation of new teeth. The adult Angel shark Squatina (Squalomorphii) exempliﬁes a single ﬁle dentition, but we obtained new data on the developmental order of teeth in the ﬁles of Squatina embryos, showing alternate timing of tooth initiation. This was based on micro-CT scans revealing that the earliest mineralised teeth at the jaw margin and their replacements in ﬁle pairs (odd and even jaw positions) alternate in their initiation timing. Along with Squatina , new observations from other squalomorphs such as Hexanchus and Chlamydoselachus , together with representatives of the sister group Galeomorphii, have established that the alternate tooth pattern (initiation time and replacement order) characterises the embryonic dentition of extant sharks; however, this can change in adults. These character states were plotted onto a recent phylogeny, demonstrating that the Squalomorphii show considerable plasticity of dental development. We propose a developmental-evolutionary model to allow change from the alternate to a single ﬁle alignment of replacement teeth. This establishes new dental morphologies in adult sharks from inherited alternate order. with these character states plotted on a recent phylogeny (Naylor et al. 2012). The basal position and monophyly of the Synechodontiformes, including Synechodus , follows Klug (2010). All neoselachians, as well as the Hybodontoidea, show alternate tooth replacement in some part of their dentition in the embryo, even if this is not retained in the adult (state 1). Within the Lamniformes (Galeomorphii) some tooth ﬁles are lost to produce the appearance of single ﬁle tooth addition, found in only certain parts of the jaw, reﬂecting irregular tooth ﬁle loss (state 2). Within the Squalomorphii, the single ﬁle tooth replacement pattern is developed from secondary modiﬁcation of an alternate pattern (state 3). Within Chlamydoselachus an alternate pattern may be present in the embryo but not in the adult (except proximally); the majority of the dentition shows a single ﬁle arrangement. In Hexanchus (Hexanchi-dae), single ﬁle addition is present in early development and is retained in the adult, whereas in other adult hexanchids (e.g. Notorhychus ) the alternate pattern is only retained in proximal rudimentary teeth. In Squatina an alternate pattern is present in the embryo, whereas the adult dentition possesses a single ﬁle arrangement but has retained alternate addition in proximal tooth ﬁles. In these three taxa, state 3 (single ﬁle) may be related to a ﬁxed number of tooth ﬁles and independent jaw growth, allowing in Squatina interdigitation of upper with lower jaw teeth.