Molar crown formation in the Late Miocene Asian hominoids, Sivapithecus parvada and Sivapithecus indicus

Abstract

During the past decade, studies of enamel development have provided a broad temporal and geographic perspective on evolutionary developmental biology in Miocene hominoids. Here we report some of the first data for molar crown development in one hominoid genus, Sivapithecus. The data are compared to a range of extant and extinct hominoids.Crown formation times (CFTs), daily rates of enamel secretion (DSR), Retzius line number and periodicity, and relative enamel thickness (RET) were calculated in a mandibular first molar of Sivapithecus parvada and a maxillary first molar of Sivapithecus indicus from the Siwalik sequence of Pakistan. A CFT of 2.40 years for the protoconid of S. parvada and 2.25 years for the protocone of S. indicus lie within the range of first molar (M1) formation times for the majority of Miocene hominoids (1.96e2.40 years, excluding Proconsul heseloni), and are similar to an M 1 from Gorilla (2.31 years) and M 1 s from Pan (2.22e2.39 years). This is unlike the longer CFTs in modern humans, which appear to be linked with their extended growth period. In contrast to extant great apes and humans, daily rates of enamel secretion are rapid in the Sivapithecus M1s during the early stages of growth, which seems to be a common pattern for most Miocene apes. The rapid accumulation of cuspal enamel in the Sivapithecus molars produced thicker enamel than either Pan or Gorilla in a comparable period of time. Future studies on larger samples of living and fossil hominoids are needed to clarify trends in crown development, which may be better understood in the context of life history strategies coupled with good data on body mass and brain size.

Intraspecific variation in M1 enamel development in modern humans: implications for human evolution

“…The significant increase in cross-striation spacing from the inner to the outer enamel follows the clear pattern reported for extant and extinct hominoids (e.g., Mahoney et al, 2007). Overall, growth rates were slightly faster in the lower first molar inner enamel, and slightly slower in the outer enamel compared to other modern human samples, although each value still lies within the range of these previous studies (Table 8).…”

“…The range of protoconid formation times recorded for the M 1 s overlaps only at the upper end of the range of M 1 protoconid formation times recorded for Pan (2.01e2.61 yrs), and are above the known values recorded for M 1 in Gorilla (2.31 yrs), as well as the fossil hominoids Proconsul heseloni (1.20 yrs), Lufengpithecus hudienensis (2.11e2.18 yrs), and Sivapithecus parvada (2.40 yrs; Beynon et al, 1998;Schwartz et al, 2003Schwartz et al, , 2006Mahoney et al, 2007;Smith et al, 2007a). The longer formation times in the human sample compared to some nonhuman primates might reflect a delay in other aspects of dental development, such as molar eruption times (e.g., Smith et al, 1994).…”

“…A mean value and standard deviation are then calculated for each region (see description in Materials and methods). 135 P. Mahoney / Journal of Human Evolution 55 (2008) 131e147 secretion rates in the region to estimate imbricational enamel formation times (for a description see Mahoney et al, 2007). The total enamel formation time in each cusp was calculated by summing the time taken to form the appositional and imbricational enamel.…”

“…Because of this, histological methods are sometimes used to examine dental development in fossil species. Data of this kind needs to be interpreted by comparisons with extant species, through which insights can be gained into the evolution of different growth patterns (e.g., Bromage and Dean, 1985;Dean, 1987;Ramirez Rozzi, 1993;Macho et al, 1996;Beynon et al, 1998;Dean et al, 2001;Kelley et al, 2001;Dean and Schrenk, 2003;Schwartz et al, 2003Schwartz et al, , 2007Smith et al, 2003Smith et al, , 2004Mahoney et al, 2007). Yet, while there are some good data about intraspecific variation in the sequence and timing of molar cusp formation in modern humans, mostly obtained using these methods, there is still much more to be learned (Reid et al, 1998a;Reid and Dean, 2006).…”

Two-dimensional patterns of human enamel thickness on deciduous (dm1, dm2) and permanent first (M1) mandibular molars

“…49 Following this a standard histological sectioning procedure was followed, 50 and both mesial (protoconid and metaconid) and distal (hypoconid and entoconid) sections were produced. After embedding the molars in a polyester resin, longitudinal sections between 180 and 200 mm were taken (Buehler 1 Isomet low speed).…”