Craniodental Allometry, Prenatal Growth Rates, and the Evolutionary Loss of the Third Molars in New World Monkeys

Abstract

A growing body of literature demonstrates that genetic patterning mechanisms underlie the relative proportions of the mammalian postcanine dentition with the third molar being key to understanding variation within the molar row. With this relatively recent insight, there has been renewed interest in mammalian taxa that have lost the third molars. Within platyrrhines, the marmosets and tamarins (Callitrichidae family) are characterized by small body size, claw-like nails, twinning, and reduced molar number. Small body size is hypothesized to have resulted in the third molar being crowded out of the jaws leading to its evolutionary loss in this family. To further explore this hypothesis, we measured the cranium and dentition of 142 individuals spanning all five platyrrhine families. These data reveal that callitrichids have a significantly smaller proportion of mandibular postcanine tooth row length relative to other platyrrhines, refuting the "crowding out" hypothesis. However, postcanine tooth row length is significantly correlated with mandibular length and cranial length (P < 0.01) across all platyrrhines providing evidence for a strong allometric association between postcanine tooth row length and body size more generally. The small body size that characterizes callitrichids results in part from slower prenatal growth rates. Given the allometric relationship between postcanine tooth row length and body size, reported here and in previous studies, we hypothesize that the evolutionary loss of the third molars in callitrichids results from the inhibition of third molar development as a consequence of the slower prenatal growth rates associated with small body size in this family.

Evidence of strong stabilizing effects on the evolution of boreoeutherian (Mammalia) dental proportions

“…The first and second generations of mammalian teeth begin developing in utero and finish erupting well past sexual maturity in many taxa (e.g., humans) and are thereby subject to many stages of life history including gestation, labor and delivery, weaning, and sexual maturity (Smith, 2018;Tucker & Sharpe, 2004). Based on previous work on primate dental proportions Monson et al, 2019), this study predicts a correlation between MMC and/or PMM and life history in boreoeutherian mammals (H3).…”

“…All life history and body size variables were log‐transformed for analyses with the exception of litter size. It has been previously hypothesized that slower prenatal growth rates can lead to reduction or complete lack of development of the third molars in primates (Monson et al, ). The first and second generations of mammalian teeth begin developing in utero and finish erupting well past sexual maturity in many taxa (e.g., humans) and are thereby subject to many stages of life history including gestation, labor and delivery, weaning, and sexual maturity (Smith, ; Tucker & Sharpe, ).…”

“…Dental phenotypes can also vary with life history traits like age at weaning, prenatal growth rates, and gestation length (Monson, Coleman, & Hlusko, 2019;Smith, 1989Smith, , 2018Smith, Crummett, & Brandt, 1994). Other research has reported that some aspects of dental variation are more strongly associated with phylogenetic relatedness in mammals than diet or life history strategies (Gamarra, Delgado, Romero, Galbany, & Pérez-Pérez, 2016;Macholán, 2006;Monson & Hlusko, 2018a, 2018b.…”

Genetic correlations in the dental dimensions ofSaguinus fuscicollis

“…Body size reduction is expected to occur more rapidly than postcanine tooth reduction in dwarfing lineages (Gould, ), and Ford () hypothesized that spatial constraints resulting from mismatched jaw and tooth reduction would lead to third molar agenesis in the callitrichine lineage. However, the postcanine tooth rows of callitrichine primates are not large relative to body size compared to other platyrrhine primates (Monson, Coleman, & Hlusko, ; Plavcan & Gomez, ). This raises the question, how did tooth size reduction keep pace with body and jaw size reduction during callitrichine evolution, and might the genetic architecture of the dentition have played a role?…”

“…Callitrichine body size reduction is furthermore closely tied to slowed prenatal growth rates, and not to variation in postnatal growth rates (Montgomery & Mundy, ). These changes in prenatal growth rates have been hypothesized to underlie third molar agenesis in the callitrichine lineage (Monson et al, ). The strong genetic correlations between tooth types demonstrated in the present study indicate that the genetic and developmental pathways by which callitrichine primates have achieved their diminutive size have influenced the genetic architecture of tooth size in ways that could have impacted the recent evolution of the callitrichine dentition.…”