Heterodont

"On the Homologies and Notation of the Teeth of Mammalia," by W[illiam] H[enry] Flower, F.R.S. […] The classification and special homologies of the teeth of the heterodont mammals was next discussed.

The adaptation of the form of the teeth on both sides to this position, and the accurate adjustment of their contiguous surfaces, shows that it is a natural conformation. They are, moreover, of very nearly even height throughout the series, and in their configuration present a remarkable and gradual transition from the first incisor to the last molar, easily traced in both jaws, and more even and regular than in any other known heterodont mammal.

The few common characters by which the Edentata are associated are too well known to need repetition. The principal one is the abseance of any trace of the typical heterodont and diphyodont dentition, found in a more or less modified form in all other placental mammals.

[I]n many heterodont forms the teeth at the anterior end of the series of premolars and molars are small teeth, standing to the teeth behind them as the first terms of a series more or less regularly progressing in size.

The process of evolution of the specialised heterodont dentition of the Mammalia, or of the Theriodontia, from the simple homodont and polyphyodont dentition of the lower Reptilia would, I think, necessarily cause a reduction in number of the successional sets of teeth, due to an enlargement of one set and a consequent abstraction of growth, energy and material from the underlying sets.

Finally the structure of the palate in ?Acrodus (and to a lesser extent, Hybodus and Asteracanthus) shows certain general resemblances to that of modern heterodont sharks, particularly in the lower postorbital ramus, absence of an orbital process, well-developed ethmoidal articulations and absence of a basal angle.

Mammalian teeth are characteristically heterodont, i.e. teeth are dissimilar in shape and size.

The first cetaceans were heterodont and had multicusped cheek teeth. […] The earliest odontocetes were heterodont, and the earliest mysticetes retained teeth[…], though vascular grooves on their palates suggest baleen too[…].

The heterodont arrangement is characteristic of the Venus clams and cockles.

In this sense all hinges are Heterodont, and the distinction drawn by [Melchior] Neumayr between Heterodont and Desmodont teeth, so far as their situation is concerned, resolves itself into a comparison between teeth originally laterals and both in one valve, with paired laminæ in the opposite valve, on the one side; and teeth originally cardinals, or, if laterals, then laterals of which one was in one valve and one in the other, with the clasping laminæ also alternated.

I wish, however, at once to say, that a satisfactory solution will only be arrived at by actual observation of the development of the hinge of heterodont bivalves.

The palaeoheterodonts are an extant, Ordovician–Recent, group characterised by heterodont to schizodont dentition. They are freshwater to marine. Palaeoheterodonts are exemplified by the heterodont modiomorphioid Modiolopsis, the freshwater unionoid Unio, and the schizodont trigonoids Trigonia, Myophorella and Neotrigonia.

[A]mongst heterodonts there are several Rodents which have no deciduous teeth, e.g., the rat; […] Among Marsupials, which are true heterodonts, there is only one milk molar on each side in each jaw; […]

On the other hand, in the Mammalian orders with two sets of teeth, these organs are said to acquire fixed individual characters, to receive special denominations, and can be determined from species to species, being equivalent to the Heterodonts. The author [William Henry Flower] then showed that […] among the Heterodonts many were partially, and probably some completely, Monophyodonts.

We have Homodonts, or animals with teeth quite homogeneous in shape, and Heterodonts, those having teeth of dissimilar shapes, the large majority of mammals being Heterodonts.

In the aberrant Orycteropus (Aard-Vark), with ten adult teeth, [Oldfield] Thomas finds seven milk teeth behind the maxillary suture (thus taking us into the molar region of the typical heterodonts).

In the past, this would have been enough to allow one to suggest a close relationship between hybodonts and heterodonts. However, Campagno (1977) has recently attempted to show that hybodonts belong to a more derived position within the euselachians, specifically being allied with the galeoid oryctoloboids and lamnoids. If this is the case, the absence of the orbital process in heterodonts might be considered a highly derived condition and the overall close similarity of the palates of the two groups a convergence due perhaps to a common pattern of fore-and-aft jaw movements.

Ordovician actinodontoids are probably ancestral to the heterodonts, unionaceans, and trigoniaceans.

Finally, Carditoidea is basal not only to all other sampled heterodonts but also to Anomalodesmata. This is unexpected; although they appear to be relatively primitive heterodonts, they are mophologically similar to other heterodonts exclusive of anomalodesmatans. More molecular data for other primitive heterodonts are needed to test this result, especially Crassatelloidea and Lucinoidea.

The second suborder (Macrosformes) corresponds with the Heteroderms of M. André Marie Constant Duméril. These are divided into seven families: […] 4. the Heterodonts, (Dendrophis, Coronella, &c.); […]