Evolutionary Transformations Of The Mammals Dention

Mitt. Mus. Nat.kd. Berl., Zool. Reihe 77 (2001) 2, 167-174

10.10.2001

Evolutionary transformations of the mammalian dentition
P. M. Butler'
With 6 figures

Abstract
Functional aspects of the evolution of the mammalian dental organisation and molar tooth form are reviewed. The sequential
changes are interpreted as adaptive modifications; many of them have taken place independently inmore than one phyletic
group, or a similar functional result has been produced by convergence. The resulting homoplasy is probably a general characteristic of evolutionary change, particularly evident in the teeth because they are especially well represented in the fossil
record.

Key words: dentition, molar function, occlusion, tooth cusp, homoplasy, tribosphenic molars, hypocone.Introduction

Heterodonty and diphyodonty

Teeth form a predominant part of identifiable
mammalian fossil remains, and they have been
extensively used in taxonomy since the days of
Cuvier and Owen. In the latter part of the
nineteenth century the application of the concept of evolution by palaeontologists such as
Riitimeyer, Cope and Osborn led to the Tritubercular Theory (see Osborn 1907, Gregory1934). The subject was put on a firmer factual
basis by improved knowledge of Mesozoic
mammals, beginning with Simpson's classic revision (1928, 1929), and expanded since 1950 by
numerous important discoveries. At the same
time the functional morphology of teeth has
become better understood, with the description
of wear facets (e.g., Mills 1964, Crompton 1971,
Crompton & Kielan-Jaworowska1978) and
chewing movements (e.g. Crompton 1995).
Though much remains uncertain, it is now possible to provide a broad picture of the evolution of mammalian teeth as a sequence of adaptive modifications. Here I will briefly survey
some of the modifications that have occurred,
mainly in the Mesozoic. I will consider first the
organisation of the dentition, particularly the
cheek dentition,and then changes in the form
and function of the molars.

Mammals stand far apart from other vertebrates in the organisation of the dentition. This
may be illustrated by comparison of a primitive reptile, such as an Early Permian pelycosaur, with a Tertiary mammal. On the one
hand there are numerous teeth of similar, simple shape, that are replaced frequently
throughout life; on the otherthere is a definite
number of heterodont teeth, differentiated in
form and function, that are retained throughout the life of the individual, except for a single replacement of the juvenile dentition. The
transformation was accompanied by far-reaching changes of the mouth and jaws. Primitively
the teeth were used merely to hold and position food in the mouth preparatory to swallowing; in mammalsthe food is retained in the
mouth where it is reduced and mixed with saliva by chewing. This involves not only complex
cheek teeth, but also a secondary palate and
specialised jaw muscles. The mammalian diagnostic characters of reduced post-dentary bones
and the dentary-squamosal jaw joint may be
interpreted as adaptive consequences of chewing. Changes in the direction of mammals can
be seenin more than one group of Triassic
theriodonts.

School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey TW20 OEX, U.K.
Received April 2001, accepted May 2001

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Butler. P. M .. Transformations of t he mammalian dentition

The Basal Triassic cynodont Thrinn.uocion is
at an interesting intermediate stage. An enlarged canine separates an incisor region froma
post-canine region, where the teeth have additional cusps. Osborn & Crompton (1973) investigated development of the post-canine dentition by arranging fourteen jaws to form a
growth series. They found that the teeth were
replaced in an alternating sequence as in primitive reptiles, but the number of replacements
was reduced to five or less. The first tooth to
develop at any given locus...