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. 2012 Jun 7;279(1736):2188-95.
doi: 10.1098/rspb.2011.2481. Epub 2012 Feb 1.

Skeletal development in the African elephant and ossification timing in placental mammals

Lionel Hautier  1 , Fiona J Stansfield, W R Twink Allen, Robert J Asher
Affiliations

Skeletal development in the African elephant and ossification timing in placental mammals

Lionel Hautier et al. Proc Biol Sci. .

Abstract

We provide here unique data on elephant skeletal ontogeny. We focus on the sequence of cranial and post-cranial ossification events during growth in the African elephant (Loxodonta africana). Previous analyses on ossification sequences in mammals have focused on monotremes, marsupials, boreoeutherian and xenarthran placentals. Here, we add data on ossification sequences in an afrotherian. We use two different methods to quantify sequence heterochrony: the sequence method and event-paring/Parsimov. Compared with other placentals, elephants show late ossifications of the basicranium, manual and pedal phalanges, and early ossifications of the ischium and metacarpals. Moreover, ossification in elephants starts very early and progresses rapidly. Specifically, the elephant exhibits the same percentage of bones showing an ossification centre at the end of the first third of its gestation period as the mouse and hamster have close to birth. Elephants show a number of features of their ossification patterns that differ from those of other placental mammals. The pattern of the initiation of the ossification evident in the African elephant underscores a possible correlation between the timing of ossification onset and gestation time throughout mammals.

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Figures

Figure 1.
Figure 1.
Representative ontogenetic stages of elephants. Lateral view of specimens (left) and three-dimensional reconstruction of computerized tomography (CT) scans of skeleton (right) in (a) a 99 day-old specimen (age reconstructed from CRL as described in the text and Hildebrandt et al. [15]), PMLER 1, CRL = 34.7 mm; (b) a 118 day-old specimen, PMLER 2, CRL = 59.3 mm; and (c) a 176 day-old specimen, PMLER 8, CRL = 171.4 mm. (d) Close-up of the pelvic girdle showing a fourth ossification centre. cd, caudal vertebrae; cv, cervical vertebrae; f, femur; fb, fibula; h, humerus; il, ilium; is, ischium; lb, lumbar vertebrae; mc, metacarpals; mp, manual phalanges; mt, metatarsals; p, pubis; pp, post-pubic bones; r, ribs; rd, radius; sa, sacral vertebrae; sc, scapula; t, tibia; th, thoracic vertebrae; u, ulna. Scale bars, 1 cm.
Figure 2.
Figure 2.
Lateral view of three-dimensional reconstructions of cranial CT scans in (a) a 99 day-old specimen (age reconstructed from CRL as described in the text and Hildebrandt et al. [15]), PMLER 1, CRL = 34.7 mm; (b) a 118 day-old specimen, PMLER 2, CRL = 59.3 mm; and (c) a 176 day-old specimen, PMLER 8, CRL = 171.4 mm. boc, basioccipital; dt, dentary; eoc, exoccipital; fr, frontal; ju, jugal; la, lacrimal; mx, maxillary; na, nasal; pa, parietal; pm, premaxillary; soc, supraoccipital; sq, squamosal; tr, tympanic ring. Scale bar, 1 cm.
Figure 3.
Figure 3.
Ossification sequence of (a) cranial and (b) post-cranial elements in elephants (unfilled squares) relative to the mean rank of placentals (solid triangles) with error bars of ±1 s.d.
Figure 4.
Figure 4.
Box plots representing the cumulative percentage of bones present in the adult showing an ossification centre in the cranial and post-cranial skeleton of the elephant, human, cow, sheep, hamster, mouse, rat, and gerbil relative to gestation length.
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