"The labyrinthodonts" might sound like the name of one of those weird fantasy monsters from Pan's Labyrinth, but it's actually a widely used (and nowadays disused) scientific term that refers to a large grouping of early tetrapods. Anyone perusing classical literature may notice that this term often comes up a lot more than 'temnospondyl' (in addition to other terms like 'batrachomorph'). But the name is pretty rare outside of the scientific literature, so what's it mean? The oldest reference that I can find of the term 'labyrinthodont' is a publication by von Meyer in 1842. It was quickly picked up by other workers (e.g., Burmeister, 1849) before being formalized as the name of a taxonomic group by Owen (1860). The name derives first from the temnospondyl genus Labyrinthodon, which is no longer a valid name (being a junior synonym of Mastodonsaurus). This in turn derives from the labyrinthine (resembling a labyrinth or maze) structure of the teeth that was recognized quite early on in the general history of paleontology (1830s), let alone the study of temnospondyls. The term has largely fallen out of favour with respect to taxonomy (discussed below), although it still occurs sporadically throughout the literature in the 21st century (e.g., Martin & Merriam, 2011; Pledge, 2013) and remains widely used to describe the tooth structure that gave the group its name.
Where is it? Plicidentine can be found in more or less any of the teeth, ranging from those along the edge of the mouth (marginal teeth) to those on the roof of the mouth (palatal teeth). It is most developed near the base of the tooth and becomes virtually non-existent at the tips.
Furthermore, plicidentine occurs widely among vertebrates. It pre-dates temnospondyls, being known in sarcopterygian (lobe-finned fish) from which tetrapods evolved, including the still living coelacanth (Meunier et al., 2015). It also occurs in a few actinopterygians (ray-finned fish), which comprise almost all living fish species, such as Lepidosteus (a genus of gar; e.g., Wyman, 1843). It also extends past early tetrapods, being found in totally disparate amniote groups such as parareptiles (e.g., MacDougall et al., 2014), eureptiles (e.g., de Ricqlès A & Bolt, 1983), varanoid lizards (includes monitor lizards like the Komodo dragon; e.g., Maxwell et al., 2011a), and ichthyosaurs (e.g., Mazin, 1981; Maxwell et al., 2011c). It remains controversial in a number of amniote groups (e.g., mosasauroids, snakes) because various workers have attempted to identify plicidentine based on external features without thin-sectioning that can fully confirm the presence or absence of these structures (see Maxwell et al., 2011b, for some discussion). Below are some examples of plicidentine in non-temnospondyls. Are there different types of plicidentine? There are indeed. Not only does plicidentine vary qualitatively between taxa and clades, but there are defined categories. Schultze (1970) was the first to define these categories (polyplocodont, eusthenodont, and dendrodont; see below). The last two types are essentially only found in fish - variations of the polyplocodont type include the typical labyrinthodont plicidentine as well as different forms found in other tetrapods. Meunier et al. (2014) defined a fourth type - simplexodont - which is a relatively simple and incipient form of plicidentine found in some fish. There are also qualitative differences between plicidentine in more closely related taxa. For example, among temnospondyls, there is great variation in the complexity and number of the folds. Below are some images from Warren & Davey's (1992) work comparing plicidentine among temnospondyls (from left to right - Metoposauridae, Capitosauridae, and Rhytidosteidae).
So...what are labyrinthodonts again? An eclectic, non-monophyletic group of early tetrapods that includes temnospondyls (hence why I'm talking about it) but also plenty of non-temnospondyls. Cool name, not widely used anymore. Up next week: 'fangs,' 'tusks,' and other uncomfortably large teeth Refs
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