Thoracolumbar vertebral number: The first skeletal synapomorphy for afrotherian mammals

Sánchez‐Villagra, Marcelo R.; Narita, Yuichi; Kuratani, Shigeru

doi:10.1017/s1477200006002258

Cited by 92 publications

(99 citation statements)

References 40 publications

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“…Pinnipeds, comprising the walruses, sea lions, and seals, are largely aquatic and do not possess any modifications of the ancestral carnivoran pattern, instead they retain the highly conserved number of 20 thoracolumbar vertebrae typical of other carnivores (41). The same conservatism is also observed in basal placodont reptiles, whose vertebral numbers are similar to the ancestral condition.…”

Section: Discussionmentioning

confidence: 94%

Homeotic effects, somitogenesis and the evolution of vertebral numbers in recent and fossil amniotes

Müller

Scheyer

Head

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

179

226

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The development of distinct regions in the amniote vertebral column results from somite formation and Hox gene expression, with the adult morphology displaying remarkable variation among lineages. Mammalian regionalization is reportedly very conservative or even constrained, but there has been no study investigating vertebral count variation across Amniota as a whole, undermining attempts to understand the phylogenetic, ecological, and developmental factors affecting vertebral column variation. Here, we show that the mammalian (synapsid) and reptilian lineages show early in their evolutionary histories clear divergences in axial developmental plasticity, in terms of both regionalization and meristic change, with basal synapsids sharing the conserved axial configuration of crown mammals, and basal reptiles demonstrating the plasticity of extant taxa. We conducted a comprehensive survey of presacral vertebral counts across 436 recent and extinct amniote taxa. Vertebral counts were mapped onto a generalized amniote phylogeny as well as individual ingroup trees, and ancestral states were reconstructed by using squared-change parsimony. We also calculated the relationship between presacral and cervical numbers to infer the relative influence of homeotic effects and meristic changes and found no correlation between somitogenesis and Hox-mediated regionalization. Although conservatism in presacral numbers characterized early synapsid lineages, in some cases reptiles and synapsids exhibit the same developmental innovations in response to similar selective pressures. Conversely, increases in body mass are not coupled with meristic or homeotic changes, but mostly occur in concert with postembryonic somatic growth. Our study highlights the importance of fossils in large-scale investigations of evolutionary developmental processes.omitogenesis, somatic growth, and Hox gene expression are primary factors driving the formation of the vertebrate body axis (1, 2). Somitogenesis occurs via the rhythmic budding of embryonic somites from the anterior part of the presomitic mesoderm, until a point when the latter has shrunk to such a degree that no further somites can be formed (3). Periodic somite formation is controlled by a molecular oscillator or "segmentation clock" (4), whereas the speed of the clock is highly variable across different vertebrate lineages. For example, the segmentation clock in snakes ticks much faster than in a mouse, resulting in a higher number of relatively smaller somites (3). Because the ossified vertebrae are derived from the embryonic somites through resegmentation (1), vertebral numbers can provide insights into the speed of the segmentation clock and the pattern of somitogenesis. In addition, the relative size of vertebrae relative to the number of vertebrae provides information about the rate of somatic growth (5, 6).Besides the marked variation in the number and sizes of vertebrae, the vertebrate body axis is also highly regionalized. In tetrapods, this regionalization is expressed in the formatio...

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Section: Discussionmentioning

confidence: 94%

Homeotic effects, somitogenesis and the evolution of vertebral numbers in recent and fossil amniotes

Müller

Scheyer

Head

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

179

226

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“…All but a few of ∼5,000 living mammal species possess exactly seven neck vertebrae; other presacral counts also show lower interspecific variation in mammals than in nonmammals (2)(3)(4)(5). Only three genera show departures from the cervical constant: manatees (Trichechus) and tree sloths (Choloepus and Bradypus).…”

mentioning

confidence: 99%

Skeletal development in sloths and the evolution of mammalian vertebral patterning

Hautier

Weisbecker

Sánchez‐Villagra

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

127

149

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Mammals show a very low level of variation in vertebral count, particularly in the neck. Phenotypes exhibited at various stages during the development of the axial skeleton may play a key role in testing mechanisms recently proposed to explain this conservatism. Here, we provide osteogenetic data that identify developmental criteria with which to recognize cervical vs. noncervical vertebrae in mammals. Except for sloths, all mammals show the late ossification of the caudal-most centra in the neck after other centra and neural arches. In sloths with 8–10 ribless neck vertebrae, the caudal-most neck centra ossify early, matching the pattern observed in cranial thoracic vertebrae of other mammals. Accordingly, we interpret the ribless neck vertebrae of three-toed sloths caudal to V7 as thoracic based on our developmental criterion. Applied to the unusual vertebral phenotype of long-necked sloths, these data support the interpretation that elements of the axial skeleton with origins from distinct mesodermal tissues have repatterned over the course of evolution.

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“…Thus, previous knowledge regarding the number of vertebrae is needed to determine this origin. According to Flower and other authors (Flower 1985, Sánchez-Villagra & Narita 2007, Endo et al 2009, Cruz et al 2014, the numbers of thoracic, lumbar and sacral vertebrae are variable in the giant anteater; however, these authors did not report the presence of bilateral or unilateral floating ribs. Flower (1985) and Cruz et al (2014) report Myrmecophaga tridactyla with 16 thoracic vertebrae have two lumbar vertebrae and that specimens with 15 thoracic vertebrae have three lumbar vertebrae, which was confirmed by the results of this study, which noted these findings in most specimens (75.0%).…”

Section: Discussionmentioning

confidence: 93%

“…Some papers have reported the number of vertebrae in this species, although the number of specimens used was restricted, and the only method used was counting; additionally, these reports contained no references to the number of sacral vertebrae and/or ribs (Flower 1985, Sánchez--Villagra & Narita 2007, Endo et al 2009, Cruz et al 2014.…”

Section: Introductionmentioning

confidence: 99%

Morphological evaluation of the thoracic, lumbar and sacral column of the giant anteater (Myrmecophaga tridactyla Linnaeus, 1758)

Borges

Cruz²,

Fares³

et al. 2017

Pesq. Vet. Bras.

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This study aimed to describe the number of thoracic, lumbar and sacral vertebrae in tridactyla through radiographic examinations associated with gross anatomy determination. For this purpose, 12 adult specimens of M. tridactyla were analyzed, assigned to the Screening Center of Wild Animals (CETAS), IBAMA-GO, and approved by the Ethics Committee on the Use of Animals (Process CEUA-UFG nr 018/2014). In the radiographic examinations the following numbers of thoracic (T) and lumbar (L) vertebrae were observed: 16Tx2L (n=7), 15Tx2L (n=3), and 15Tx3L (n=2). In contrast, the numbers of vertebrae identified by anatomical dissection were as follows: 16Tx2L (n=4), 15Tx2L (n=3), and 15Tx3L (n=5). This difference occurred in cases of lumbarization of thoracic vertebrae, as seen in three specimens, and was explained by changes in regional innervations identified by anatomical dissection and the presence of floating ribs (right unilateral=1, left unilateral=1 and bilateral=1), which were not identified by radiographic exams. Regarding the sacral vertebrae there was no variation depending on the methods used, which allowed the identification of 4 (n=1) or 5 (n=11) vertebrae. Thus, we concluded that there is variation in the number of thoracic, lumbar and sacral vertebrae, in addition to lumbarization, which must be considered based on the presence of floating ribs, in this species.

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Thoracolumbar vertebral number: The first skeletal synapomorphy for afrotherian mammals

Cited by 92 publications

References 40 publications

Homeotic effects, somitogenesis and the evolution of vertebral numbers in recent and fossil amniotes

Homeotic effects, somitogenesis and the evolution of vertebral numbers in recent and fossil amniotes

Skeletal development in sloths and the evolution of mammalian vertebral patterning

Morphological evaluation of the thoracic, lumbar and sacral column of the giant anteater (Myrmecophaga tridactyla Linnaeus, 1758)

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