Homologies of the Anterior Teeth in lndriidae and a Functional Basis for Dental Reduction in Primates PHILIP D. GINGERICH Museum of Paleontology, The University of Michigan, Ann Arbor, Michigan 48109 KEY WORDS Dental reduction a Lemuriform primates . Indriidae . Dental homologies - Dental scraper . Deciduous dentition - Avahi ABSTRACT In a recent paper Schwartz ('74) proposes revised homologies of the deciduous and permanent teeth in living lemuriform primates of the family Indriidae. However, new evidence provided by the deciduous dentition of Avahi suggests that the traditional interpretations are correct, specifically: (1) the lat- eral teeth in the dental scraper of Indriidae are homologous with the incisors of Lemuridae and Lorisidae, not the canines; (2) the dental formula for the lower deciduous teeth of indriids is 2.1.3; (3) the dental formula for the lower perma- nent teeth of indriids is 2.0.2.3;and (4)decrease in number of incisors during pri- mate evolution was usually in the sequence 13, then 12, then 11. It appears that dental reduction during primate evolution occurred at the ends of integrated in- cisor and cheek tooth units to minimize disruption of their functional integrity. Anterior dental reduction in the primate Schwartz ('74) recently reviewed the prob- family Indriidae illustrates a more general lem of tooth homologies in the dental scraper problem of direction of tooth loss in primate of Indriidae and concluded that no real evi- evolution. All living lemuroid and lorisoid pri- dence has ever been presented to support the mates (except the highly specialized Dauben- interpretation that indriids possess four lower tonid share a distinctive procumbent, comb- incisors and no canines. He then gave several like configuration of the anterior lower denti- reasons in support of the contrary interpreta- tion - the dental scraper or "toothcomb" tion that two incisors and two canines make used in ingesting resin, prying bark, and in up the indriid scraper. grooming. This dental scraper is composed of In the course of a study of dental variation six teeth in Lemuridae and Lorisidae, gen- in Indriidae, a specimen ofAvahi retaining de- erally considered to be four incisors bordered ciduous teeth was found that supports the tra- by two canines (Swindler, '76). The dental ditional interpretation of homologies of the scraper of Indriidae consists of only four permanent and deciduous dentition of In- teeth. It is agreed that these teeth are homol- driidae. This new evidence is presented to help ogous with four of the six teeth in the scraper clarify anterior dental reduction in Indriidae. of lemurids and lorisids, but the question re- The whole problem is especially important as mains whether the two teeth lost in going it has a direct bearing on our understanding from the generalized lemurid condition to the of the general problem of dental reduction in more specialized indriid condition were in- primate evolution. cisors or canines. The four teeth remaining in the dental scraper of Indriidae are usually MATERIALS AND RESULTS considered to be incisors (i.e., the canines Dried skulls of adult animals of virtually all were lost; see Vallois, '55; Le Gros Clark, '71; genera of Lemuridae, Lorisidae, and Indriidae Martin, '72; among others), but they have were examined in the collections of the Cleve- also been interpreted as an incisor pair bor- land Museum of Natural History, the British dered by left and right lower canine teeth (i.e., Museum (Natural History) in London, the an incisor pair was lost: e.g., Gregory, '20: Rijksmuseum van Natuurlijke Historie in p. 214). Leiden, and the Laboratoire d'Anatomie Com- AM. J. PHYS. ANTHROP., 47: 387-394. 387 388 PHILIP D. GINGERICH paree in Paris. In addition, I studied the de- certainly a deciduous canine. The lower de- ciduous dentition in various stages of replace- ciduous formula of the Indriidae is thus prob- ment in many of these genera. The deciduous ably 2.1.3 as it is in Lemuridae and Lorisidae, dentitions of representative Indriidae and which also matches the upper deciduous for- Lemuridae are illustrated in plate 1. mula in these three families. Lemuridae, Lorisidae, and Indriidae have As shown in figures 3 and 4, the incisors and the same number of deciduous teeth (six) in canines of the deciduous dental scraper in each mandible. These teeth in Lorisidae and Lemuridae are all replaced by incisors and ca- Lemuridae (figs. 3, 4) have basically the same nines of the permanent scraper, whereas in morphology, and the deciduous formula is Propithecus, Zndri, and Avahi (fig. 21, the de- agreed to be 2.1.3, with the first three teeth ciduous canine “A’ is not replaced by a per- forming a deciduous dental scraper. In the manent tooth. Thus it appears that the per- Indriidae, both lndri and Propithecus (fig. 1) manent lower dental formula of Indriidae is have a rather different configuration, with properly interpreted as 2.0.2.3,which matches only two procumbent anterior teeth in each the permanent upper formula of 2.1.2.3except mandible contributing to the deciduous den- for loss of the canine. tal scraper. In Zndri and Propithecus these are followed by a small tooth, a relatively large DISCUSSION OF HOMOLOGIES IN INDRIIDAE tooth, a small tooth, and another relatively large tooth. By comparison with Lemuridae, Schwartz (’74) based his conclusion that the the deciduous dental formula of Indriidae is lower deciduous dental formula of Indriidae is usually given as 2.1.3 in spite of the morpho- 1.1.4 and the lower permanent formula 1.1.2.3 logical differences of the teeth from those in on three lines of evidence: (1) the morphology Lemuridae. However, Schwartz (‘74) has re- of the lateral tooth in the indriid dental cently interpreted this deciduous formula as scraper (both deciduous and permanent) is 1.1.4 (as did Gregory, ’20). most similar to that of the canine in lemurids The smallest indriid species, Avahi lanker, and lorisids; (2) tooth “A” in indriids some- significantly has an anterior deciduous denti- times occludes behind the upper canine and tion more similar to that of Lemuridae than to thus by definition tooth “A” must be a de- Zndri and Propithecus (fig. 2). Of particular ciduous premolar and not a canine; and (3) importance is the third tooth in the deciduous ontogenetic studies on the mammalian denti- series, a tooth Schwartz (‘74) called tooth “A.” tion have shown that incisor teeth develop Tooth “A” in Zndri and Propithecus (fig. 1) is a from the dental lamina in an anteroposterior very small tooth usually separated from the direction. Each of these lines of evidence is dental scraper by a small diastema. However, discussed in turn. in Avahi this tooth “A” has an elongated, pro- Dental morphology is correlated with tooth cumbent crown that forms a functional unit function, and morphological similarity is not with the other teeth of the deciduous dental always a reliable indicator of homology. The scraper. The crown of tooth “A” in Avahi is left and right lateral teeth in the indriid den- relatively much larger than tooth “A” in Indri tal scraper are most similar morphologically and Propithecus. It does not show the wear to the canines in the lemurid or lorisid dental found in Schwartz’s Age Group I1 Indriidae scraper. The main feature that makes them and its position cannot be attributed to mesial similar is a raised crest running along the lat- drift associated with wear. Because of its sim- eral margin from the tip to the base of the ilarity in position and morphology to the crown. However, a very similar raised lateral lower deciduous canine of Lemuridae, tooth crest (a “margocristid”: Gingerich, ’76) is also “A’ in Avahi is almost certainly a deciduous present on the enlarged procumbent central canine as well. Since the remaining teeth in incisors of early Tertiary plesiadapid, mi- the deciduous dentition of Avahi are very sim- crosyopid, and omomyid primates. Further- ilar to those of Propithecus and Zndri, it is dif- more, a 6-tooth dental scraper or comb vir- ficult to avoid the conclusion that the reduced tually identical to that of a lemur, including tooth “A” in the latter two genera is homol- the raised margocristids on the lateral teeth, ogous with tooth “A” in Avahi. Thus, in spite is known in the early Eocene condylarth of the reduced size of tooth “A’ in Zndri and Thryptacodon (Princeton University no. Propithecus compared to the deciduous canine 208531, which also retains large projecting ca- of Lemuridae, tooth “A” in Indriidae is almost nine teeth - the lemur-like lateral teeth of DENTAL REDUCTION IN PRIMATES 389 the scraper are thus incisors and not canines Schwartz cites on dental development in as in Lemur. The presence of raised margo- mammals, Osborn (’73: p. 554) states that in- cristids on the lateral teeth in the dental cisor, canine, and molar determinants are scraper of indriids is probably closely corre- usually present in the earliest embryos in lated with tooth position and dental function which tooth formation is evident. In other and not a reliable indicator of their homology words, the canine determinant is present with the lower canines of lemurids. whether the incisor developmental sequence The fact that tooth “A” sometimes occludes approaches it or not. Thus it is unlikely that behind the deciduous upper canine (in very Schwartz is correct in assuming that inhib- young individuals of Schwartz’s Age Group I) ition of the posterior incisor series would ef- does not necessarily define its homology, as fect an inhibition of the canine. Following Schwartz (‘74: p. 112) suggests. The conven- Osborn (‘731, the fact that deciduous incisors tional identifications of teeth widely used in develop from front to back would make it ap- mammalogy refer primarily to tooth positions pear that the easiest way to reduce the num- and occlusal relationships in primitive and ber of incisors in a dental series would be to generalized mammals - to prove that a given suppress those that develop last, i.e., reduce tooth in a specialized mammal is homologous the sequence by losing I3 first, then 12,and fin- with the canine tooth in a generalized mam- ally I,.