TY - JOUR
T1 - Developmental identity versus typology
T2 - Lucy has only four sacral segments
AU - Machnicki, Allison L.
AU - Lovejoy, C. Owen
AU - Reno, Philip L.
N1 - Funding Information:
The authors thank Yohannes Haile-Selassie, Curator of Physical Anthropology Cleveland Museum of Natural History, and Judith Cupasko, Curatorial Associate Harvard Museum of Comparative Zoology, for access to comparative specimens in their care and Lyman Jellema and Mark Omura for curatorial assistance. They also thank Yohannes Haile-Salassie for providing the photographs used in Figure and Tim White and Kelsey Kjosness for helpful comments. Deneen Wellik (University of Michigan Medical Center) kindly loaned Hox knockout mouse skeletons for our inspection.
Publisher Copyright:
© 2016 Wiley Periodicals, Inc.
PY - 2016/8/1
Y1 - 2016/8/1
N2 - Objectives: Both interspecific and intraspecific variation in vertebral counts reflect the action of patterning control mechanisms such as Hox. The preserved A.L. 288-1 (“Lucy”) sacrum contains five fused elements. However, the transverse processes of the most caudal element do not contact those of the segment immediately craniad to it, leaving incomplete sacral foramina on both sides. This conforms to the traditional definition of four-segmented sacra, which are very rare in humans and African apes. It was recently suggested that fossilization damage precludes interpretation of this specimen and that additional sacral-like features of its last segment (e.g., the extent of the sacral hiatus) suggest a general Australopithecus pattern of five sacral vertebrae. Methods: We provide updated descriptions of the original Lucy sacrum. We evaluate sacral/coccygeal variation in a large sample of extant hominoids and place it within the context of developmental variation in the mammalian vertebral column. Results: We report that fossilization damage did not shorten the transverse processes of the fifth segment of Lucy's sacrum. In addition, we find that the extent of the sacral hiatus is too variable in apes and hominids to provide meaningful information on segment identity. Most importantly, a combination of sacral and coccygeal features is to be expected in vertebrae at regional boundaries. Discussion: The sacral/caudal boundary appears to be displaced cranially in early hominids relative to extant African apes and humans, a condition consistent with the likely ancestral condition for Miocene hominoids. While not definitive in itself, a four-segmented sacrum accords well with the “long-back” model for the Pan/Homo last common ancestor. Am J Phys Anthropol 160:729–739, 2016.
AB - Objectives: Both interspecific and intraspecific variation in vertebral counts reflect the action of patterning control mechanisms such as Hox. The preserved A.L. 288-1 (“Lucy”) sacrum contains five fused elements. However, the transverse processes of the most caudal element do not contact those of the segment immediately craniad to it, leaving incomplete sacral foramina on both sides. This conforms to the traditional definition of four-segmented sacra, which are very rare in humans and African apes. It was recently suggested that fossilization damage precludes interpretation of this specimen and that additional sacral-like features of its last segment (e.g., the extent of the sacral hiatus) suggest a general Australopithecus pattern of five sacral vertebrae. Methods: We provide updated descriptions of the original Lucy sacrum. We evaluate sacral/coccygeal variation in a large sample of extant hominoids and place it within the context of developmental variation in the mammalian vertebral column. Results: We report that fossilization damage did not shorten the transverse processes of the fifth segment of Lucy's sacrum. In addition, we find that the extent of the sacral hiatus is too variable in apes and hominids to provide meaningful information on segment identity. Most importantly, a combination of sacral and coccygeal features is to be expected in vertebrae at regional boundaries. Discussion: The sacral/caudal boundary appears to be displaced cranially in early hominids relative to extant African apes and humans, a condition consistent with the likely ancestral condition for Miocene hominoids. While not definitive in itself, a four-segmented sacrum accords well with the “long-back” model for the Pan/Homo last common ancestor. Am J Phys Anthropol 160:729–739, 2016.
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U2 - 10.1002/ajpa.22997
DO - 10.1002/ajpa.22997
M3 - Article
C2 - 27101066
AN - SCOPUS:84978766853
SN - 0002-9483
VL - 160
SP - 729
EP - 739
JO - American Journal of Physical Anthropology
JF - American Journal of Physical Anthropology
IS - 4
ER -