Date Published: May 7, 2012
Publisher: Hindawi Publishing Corporation
Author(s): C. Verity Bennett, Anjali Goswami.
Quantitative analysis of morphology allows for identification of subtle evolutionary patterns or convergences in anatomy that can aid ecological reconstructions of extinct taxa. This study explores diversity and convergence in cranial morphology across living and fossil primates using geometric morphometrics. 33 3D landmarks were gathered from 34 genera of euprimates (382 specimens), including the Eocene adapiforms Adapis and Leptadapis and Quaternary lemurs Archaeolemur, Palaeopropithecus, and Megaladapis. Landmark data was treated with Procrustes superimposition to remove all nonshape differences and then subjected to principal components analysis and linear discriminant function analysis. Haplorhines and strepsirrhines were well separated in morphospace along the major components of variation, largely reflecting differences in relative skull length and width and facial depth. Most adapiforms fell within or close to strepsirrhine space, while Quaternary lemurs deviated from extant strepsirrhines, either exploring new regions of morphospace or converging on haplorhines. Fossil taxa significantly increased the area of morphospace occupied by strepsirrhines. However, recent haplorhines showed significantly greater cranial disparity than strepsirrhines, even with the inclusion of the unusual Quaternary lemurs, demonstrating that differences in primate cranial disparity are likely real and not simply an artefact of recent megafaunal extinctions.
Euprimates comprises two principal sister groups: Strepsirrhini, including Lemuriformes and Lorisiformes; and Haplorhini, including Tarsiiformes and Simiiformes (Anthropoidea). Strepsirrhines have a smaller geographic range, occupying parts of Southern Africa, Madagascar, and Southeast Asia, than do haplorhines, which, excluding humans, occupy every continent except Australia and Antarctica. In addition, haplorhine primates are far more speciose (~300 species) than strepsirrhines (~100 species) . Tarsiiformes have previously been grouped with the strepsirrhines as “prosimians”, but most recent molecular and morphological analyses [2–4] have placed them firmly within Haplorhini (but see ).
33 cranial landmarks (Table 1, Figure 1) were collected from 29 extant genera and 5 fossil genera of euprimates (Table 2) using a MicroScribe G2X digitiser with a reported accuracy of 0.23 mm. Due to the incomplete nature of fossil material, bilateral landmarks missing from either the right or left side of the skull were mirrored about the midline in Mathematica 6.0 (Wolfram Research Inc., Champaign, IL) in order to maximise sample sizes. Data was subjected to Procrustes superimposition to remove nonshape information , and then principal components analysis (PCA) was performed to analyse morphological difference between individuals . A linear discriminant function analysis (linear DFA) was performed in a pairwise fashion between the following groups of primates: Anthropoidea, Tarsiiformes, extant Lemuriformes, fossil Lemuriformes, all fossil and extant Lemuriformes, Lorisiformes, and Adapiformes. A linear DFA was also performed, using MorphoJ , between Strepsirrhini and Haplorhini with Adapiformes included first in the former and then the latter, to more robustly test for morphological similarity, given the small adapiform sample size. To reduce sampling effects, data was pooled by species for each analysis. To account for allometric effects on cranial shape, a PCA was also performed on the residuals of a multivariate regression of shape with centroid size. Prior to correction for allometry, an analysis of disparity, measured as variance, was calculated for recent haplorhines, recent strepsirrhines, and recent strepsirrhines plus Quaternary lemurs using the Simple3D program in IMP7 .
Cranial landmarks in this study differ from those of Fleagle et al. . Most notably we used fewer points around the orbit and at the anterior extreme of the skull, as these areas are easily broken in fossil specimens. Despite the different cranial landmarks used here, our results are largely in agreement with those of Fleagle et al.  with respect to the general distribution of extant groups in morphospace. For example, both studies found that haplorhines and strepsirrhines were well separated in morphospace with the major components of variation reflecting relative skull length and width and facial depth.