Megadontia, striae periodicity and patterns of enamel secretion in Plio‐Pleistocene fossil hominins

Abstract

Early hominins formed large and thick-enamelled cheek-teeth within relatively short growth periods as compared with modern humans. To understand better the developmental basis of this process, we measured daily enamel increments, or cross striations, in 17 molars of Plio-Pleistocene hominins representing seven different species, including specimens attributed to early Homo . Our results show considerable variation across species, although all specimens conformed to the known pattern characterised by greater values in outer than inner enamel, and greater cuspal than cervical values. We then compared our results with the megadontia index, which represents tooth size in relation to body mass, for each species to assess the effect of daily growth rates on tooth size. Our results indicate that larger toothed (megadont) taxa display higher rates or faster forming enamel than smaller toothed hominins. By forming enamel quickly, large tooth crowns were able to develop within the constraints of shorter growth periods. Besides daily increments, many animals express long-period markings (striae of Retzius) in their enamel. We report periodicity values (number of cross striations between adjacent striae) in 14 new specimens of Australopithecus afarensis , Paranthropus aethiopicus , Paranthropus boisei , Homo habilis , Homo rudolfensis and Homo erectus , and show that long-period striae express a strong association with male and average male-female body mass. Our results for Plio-Pleistocene hominins show that the biological rhythms that give rise to long-period striae are encompassed within the range of variation known for modern humans, but show a lower mean and modal value of 7 days in australopithecines. In our sample of early Homo , mean and modal periodicity values were 8 days, and therefore similar to modern humans. These new data on daily rates of enamel formation and periodicity provide a better framework to interpret surface manifestations of internal growth markings on fossil hominin tooth crowns. Importantly, our data on early hominin cross striation variation may now contribute towards solving difficult taxonomic diagnoses where much may depend on fragmentary molar remains and enamel structure.

Lamellar Bone is an Incremental Tissue Reconciling Enamel Rhythms, Body Size, and Organismal Life History

“…Specimens reported in Refs. [23] and [77] unless noted otherwise a Body mass estimates from McHenry [75,76] using an automated Montage Explorer capture. In addition, averages of cross-striation widths in micrometers were divided into averages of the distance between striae of Retzius for two regions of the same tooth.…”

“…1, Table 1). Employing data from the most comprehensive study and compilation of Plio-Pleistocene hominin repeat intervals to date [26], we also tested this relationship for early hominins, which are found to have repeat intervals ranging from 6 to 9 and which do indeed significantly correlate with estimated body mass and described by the relationship r = 0.87, p \ 0.01, r 2 = 0.75, y = 5.262 ? 0.0470 * x (Fig.…”

Retrieving chronological age from dental remains of early fossil hominins to reconstruct human growth in the past

“…(e) Summary points about dental development in early hominins The cumulative rates of enamel formation follow a similar trajectory in both Pan and early hominins (irrespective of enamel thickness and crown formation times) that is faster than that in modern humans Lacruz et al 2008). Estimates for gingival emergence times for M1 in several early hominin specimens all fall within the range expected for Pan, and in fact are all earlier than the time proposed for free-born, free-living chimpanzees.…”

“…We now know that these longperiod markings (first described by Anders Retzius (1837) and, therefore, also referred to as Retzius lines) occur in the enamel of other primates including early fossil hominins (figure 1). Of 29 australopiths examined so far, 17 (59%) showed a mean periodicity of 7 days and of seven early Homo specimens examined so far, two had long-period lines 7 days apart, four were 8 days apart and one was 9 days apart (Lacruz et al 2008). Fossil teeth, however, are precious and it is only rarely possible to employ partially destructive techniques to retrieve data about their growth.…”

Two Deciduous Human Molars from the Early Pleistocene Deposits of Barranco León (Orce, Spain)

“…In consultation, Bermú dez de Castro agreed with a tentative human deciduous molar assignment (Arribas and Palmqvist 2002:68). The final assignment of BL5-0 to an early Homo was based on the study of the microstructure of the enamel along the fracture surface, a character that plays a central role in interpreting fossil hominin taxonomy (e.g., Lacruz et al 2008).…”