In contrast, the exoskeleton consists of dermal bones (sensu [7]), which are homologous with bony armor and are often coated with enameloid or dentine tissues in basal vertebrates (Figure1, Table1; [17]). Phylogenetic framework was adopted from [59]. In this context, the positional identities of vertebrae along the anteroposterior axis of the vertebral column (such as occipital, cervical, thoracic, lumbar, and sacral in mammals) coincide precisely with the expression domains of Hox genes in the prevertebral anlagen, and under this Hox-code-mediated specification the number of segments can vary during evolution (for variable numbers of vertebrae, see [147]). Snchez-Villagra MR, Maier W: Homologies of the mammalian shoulder girdle: a response to Matsuoka et al. Instead, in 2015, scientists announced the discovery of bone cells in a 380-million-year-old fossilized sharksuggesting sharks may have evolved from ancestors with bones, had bones themselves at one time, and then lost them in favor of lighter cartilage. Similar situations, in which the homology between structure and gene expression is tightly conserved, include the expression of homeobox genes and primordial segments in the developing vertebrate brain, differentiation of somite-derivatives, and dorsoventral specification of the neural tube (reviewed by [148]). Hirasawa T, Nagashima H, Kuratani S: The endoskeletal origin of the turtle carapace. Jellison WL: A suggested homolog of the Os penis or baculum of mammals. Kuratani S, Murakami Y, Nobusada Y, Kusakabe R, Hirano S: Developmental fate of the mandibular mesoderm in the lamprey, Photograph by Science Photo Library, Alamy. The key to discriminating between these two causal relationships behind evolution is provided abductively through historical and experimental analyses of the correlation between phenotype and the developmental program behind it (for example, skeletal elements can be considered as a phenotype of a skeletal system). The same may be the case in the development of the dermal skull roof; the morphological patterns and homologies may reside in the local environment of the embryos, such that they become specified during a later phase of development. Syst Biol 2005, 54:530547. Accordingly, Hay (1898) [22] distinguished the gastralia from other dermal bones and classified the gastralia as fascia bone. Such a concept had been inherited in the distinction between epithecal and thecal ossifications, as used by Vlker (1913) [24] and Zangerl (1939) [25], which indicate outer and inner dermal layers of ossification, respectively. An endoskeleton is an internal skeleton composed of hard, mineralized tissue that also enables movement by attachment to muscles. This delay suggests the presence of another developmental constraint, which is more or less uncoupled from those needed to maintain the phylotype. In the mouse, the stapes (the homologue of this cartilage bone) is patterned during embryogenesis in the dorsal part of the second pharyngeal arch and is specified through the upregulation of Hoxa2 [131] in the ectomesenchyme. Based on fossil evidence, the gastralia are thought to have evolved from exoskeletal bony scales and thus are exoskeletal elements [21]. Edited by Hanken J, Hall BK. PubMed This supports a theory that the scales of ancient fish may have evolved into teeth as we know them today. Jarvik E: Basic Structure and Evolution of Vertebrates, Volume 2. The developmental origins of these cranial components have been, and remain, the focus of much debate. An exoskeleton is an external skeleton much like the name suggests. We have already seen, in frog development, how morphologically homologous cranial elements arise from cell populations or pharyngeal arches not identical to those in other vertebrate groups. Kuratani S, Matsuo I, Aizawa S: Developmental patterning and evolution of the mammalian viscerocranium: Genetic insights into comparative morphology. Google Scholar. Patterson C: Cartilage bones, dermal bones and membrane bones, or the exoskeleton versus the endoskeleton. TH and SK conceived and wrote the review. Much like the internal endoskeleton of vertebrates, the exoskeleton is responsible for supporting the structure of an animals body. (B) Schematized prototype of the arthrodire dermal skull roof as suggested by Heintz (1932) [115]. Jiang XB, Iseki S, Maxson RE, Sucov HM, Morriss-Kay GM: Tissue origins and interactions in the mammalian skull vault. PubMed Central Noden DM: Control of avian cephalic neural crest cytodifferentiation. Noon; 1736. In the evo-devo context, the phylotypic stage of vertebrate development is recognized as the stage at which so-called tool-kit genes (typically the Hox code) are expressed most conspicuously during development, thus providing the mechanistic bases to explain the significance of this conserved embryonic pattern [160]. This is not necessary with an endoskeleton. What type of animal are leeches & earthworms? Epperlein HH, Khattak S, Knapp D, Tanaka EM, Malashichev YB: Neural crest does not contribute to the neck and shoulder in the Axolotl ( Both types of mollusks have hard outer shells for exoskeletons. J Anat 2009, 214:441464. -Chimaeren. Jollie M: Segment theory and the homologizing of cranial bones. London: Elsevier Academic Press; 2005. Nat Genet 2013, 45:701706. This stone has a mysterious past beyond British coronations, Ultimate Italy: 14 ways to see the country in a new light, 6 unforgettable Italy hotels, from Lake Como to Rome, A taste of Rioja, from crispy croquettas to piquillo peppers, Trek through this stunning European wilderness, Land of the lemurs: the race to save Madagascar's sacred forests. Maes C, Kobayashi T, Selig MK, Torrekens S, Roth SI, Mackem S, Carmeliet G, Kronenberg HM: Osteoblast precursors, but not mature osteoblasts, move into developing and fractured bones along with invading blood vessels. J Embryol Exp Morph 1993, 75:165188. Nat Commun 2011, 2:248. Transgenic and chimeric approaches have revealed that the cranium of the frog violates generally accepted rules of developmentthat is, the developmental origins of the visceral arch and craniofacial skeletons are not found in a canonical set of crest cell streams that are divided into mandibular, hyoid, and branchial arch streams; instead, morphologically homologous dermal elements are derived from inconsistent cell lineages in frog embryos (Figure6D-F) [111,138]. Is histological development as complete a test of homology as morphological development? (Huxley, 1864 [1]: 296). Therefore, in a developmental sense, the endoskeletal neurocranium is a composite structure, derived from both the mesoderm and cephalic neural crest. Accordingly, the comparative morphological understanding cannot easily be formulated into a simple developmental scheme [8]; in particular, developmental understanding of the neural crestmesodermal boundaries in the dermatocranial roof is conspicuously unsure compared with that for the cranial base. Why animals developed four types of skeletons From a Hercules beetle tough natural armor to a sea anemones soft fluid-filled body, nature boasts a variety of skeletons. Dev Biol 1999, 208:441455. Recent studies have shown that osteoblast cells derived from the perichondrium also support endochondral ossification [44]. Evol Biol 1982, 15:287368. Diarthrognathus Book WebAn exoskeleton is a hard external skeleton that protects the outer surface of an organism and enables movement through muscles attached on the inside. Proc Natl Acad Sci U S A 1999, 96:51115116. Furthermore, a recent study tracing the lineages of transgenic cells revealed that trunk neural crest cells do not generate a skeletogenic tissue (that is, ectomesenchyme) [76] although they have skeletogenic potential in the developing head [122]. Starck D: Vergleichende Anatomie der Wirbeltiere, Bd. The dermatocranium (excluding the supraoccipital bone) was primarily derived from the mesoderm ancestrally, and new crest-derived elements were intercalated secondarily to accommodate adaptation to the expansion of the cranial vault in different ways in each animal lineage, thus obliterating homologies of bones. J Morphol 1999, 240:143153. Inconsistency of this type occurs in various phenomena of organogenesis, in which homologous structures are patterned by the actions of non-homologous regulatory genes in each animal lineage [153,154]. (B) Basal jawed vertebrate Compagopiscis (redrawn from [14]). Animals that live or partially live outside of the water need some type of skeleton in order to move and support themselves. Hanken J, Hall BK: The Skull, Volume 13. J Syst Palaeontol 2011, 9:2531. genes and the evolution of vertebrate axial morphology. For example, the orbitosphenoid, a cranial skeletal element, of the Amphisbaenia (Reptilia: Squamata) develops intramembranously, although it clearly is homologous with the cartilaginously preformed orbitosphenoid of other tetrapods [16]. These spiders are often mistaken for other non-venomous spiders like the wolf spider. White EI: A little on lungfishes. These enameloid- and dentine-coated bones occur widely among stem-osteichthyans, and odontogenic components are present in chondrichthyans also. The ancestral developmental pattern and cell-lineage origins of the dermatocranial elements were established in various fossil taxa, which are reflected in some modern taxa, and are secondarily modified in others, possibly because of the loss or fusion of ancestral elements or the addition of new elements. Who buys lion bones? The exoskeleton can be flexible as its not as solid as the endoskeleton. It is true that the morphological homology of skeletal elements cannot be reduced directly to the developmental program, or homology of genes, involved in the generation of homologous structures. Science 2013, 341:160164. J Exp Zool B (MDE) 2004, 302B:458468. The term includes the calcareous housings of sessile invertebrates such as clams but is most commonly applied to the chitinous integument of arthropods, such as insects, spiders, and crustaceans. The exoskeleton is made up of hard bones and malleable cartilage. (1993) [82] showed that the entire dermis, as well as the dermatocranial elements, is exclusively of neural crest origin (Figure5B). According to classical theory, transcendental morphologists and others believed that the anteroposterior segmentation of the roof of the dermatocranium merely reflected the pattern of cranial mesodermal segments of hypothetical ancestors (reviewed by [92,113,114]; Figure7A). Though bony endoskeletons are the best known, the animal kingdom features three other types of skeleton: exoskeletons, cartilaginous endoskeletons, and hydrostatic skeletons. Nature 2007, 445:307310. Numbers on the left neural fold indicate sites of injections. Patterson (1977) [7] proposed calling such intramembranous bones membrane bones and discriminated them from bones that developed within the dermis, or dermal bone. According Pattersons terminology, the endoskeleton consists of cartilage and membrane bones (Table1: Note that the above-mentioned Huxleys definition of membrane bone is different from that used in this paper, as he did not distinguish dermal bones from other intramembranously formed bones). Other types of arachnids include ticks, mites, chiggers, and scorpions whom like spiders, all have exoskeletons. Bamberg: Gbhardt; 1807. Festschr fr Carl Gegenbaur 1897, 3:349788. Even in non-model vertebrate species, including lampreys, similar results have been obtained [66,67] (also see [68,69]). 2 hours of sleep? By using this website, you agree to our Int Rev Cell Mol Biol 2010, 283:129234. Consequently, the interface between the neural crest- and mesoderm-derived parts of the exoskeleton again appears to be somewhere in the skull roof, and different results regarding its specific location have been obtained via different experimental methods in embryos of different taxa (reviewed by [8]; Figure5A, B). Much like the internal endoskeleton of vertebrates, the exoskeleton is responsible for supporting the structure of an animals body. New York: Columbia University Press; 1988:126. Rouxs Arch Ent mech Org 1959, 151:136158. Try It What kind of skeletal system does the kangaroo have? After about three days, the newly formed exoskeleton was tough enough to take back over again. Males of the deer family, such as moose and elk, grow antlersbony extensions of their skullsboth to show dominance and to attract mates. Their shells act not only as protection from predators but their actual homes. Does eating close to bedtime make you gain weight? Le Douarin NM, Dupin E: The neural crest in vertebrate evolution. This view is supported by recent histological data from placoderms (a taxon of stem-gnathostomes), indicating that the condition seen in extant chondrichthyans is derived. . Lethenteron japonicum For example, in armadillos, the osteoderm is produced by osteoblasts that are differentiated from the condensation of dermal cells, with the orientation of the primordial osteoderm parallel to that of the epidermis [48]. It can take several weeks for a new shell to totally harden, during which time the lobster is very vulnerable to predators. Huskey says skeletons are incredible feats of evolution, particularly in how each little piece is crucial to an animal and how it navigates the environment. Depending on the species the actual number of legs that these creatures can have greatly varies, what always holds true though is the fact that millipedes have 2 sets of legs per body segment while centipedes only have one pair of legs per segment. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The origination of part of the vertebrate cranium from the neural crest has been exemplified through several experimental embryologic analyses involving amphibian and avian models in which neural crest grafting experiments are possible (reviewed by [64,65]). In light of this understanding, we discuss the loose relationship between morphology and developmental basis and suggest that a frame shift in character identity occurred across cell lineages during the evolution of vertebrate skeletal systems. Consistently, a same set of gene expressions has been detected in endochondral ossifications of mesenchymal condensations both derived from neural crest and mesodermal cells [157]. While their stingers look pretty intimidating, they rely more on their massive claws to kill prey. Alternatively, perhaps exoskeletal bones in the ancestral condition were not associated with enameloid and dentine tissues. Furthermore, these current and previous findings coincide perfectly if we admit misidentification of the boundary between the frontal and parietal regions in mammals and avians: the supraoccipital region is the dorsal portion of a mesodermal element serially homologous with the vertebrae, and the interparietal region may not be present in avians (for the homology and evolution of the interparietal region, see [129] and references therein). Oxford: Oxford University Press; 1971. von Baer KE: Entwicklungsgeschichte der Thiere: Beobachtung und Reflexion. Bull Peabody Mus Nat Hist Yale 1971, 38:1109. Kangaroos give birth to a tiny, undeveloped joe y after a very short gestation period. In addition, because bones can be preserved as fossils, comparative research can include extinct vertebrates, thereby shedding light on evolutionary patterns and processes (e.g., [3]). The gastralia are a series of segmental rod-like bones that cover the ventral aspect of the abdomen in crocodilians and the tuatara, among living forms. Development 2007, 134:31333144. Volume 14: Deveolopment A. Edited by Gans C, Billett F, Maderson PFA. Huxley TH: Lectures on the elements of comparative anatomy. Edited by Andrews SM, Miles RS, Walker AD. The ventral cutaneous branch of the intercostal nerve (vcb) runs adjacent to the margin of the gastralium. There remains much room for investigation regarding the development of reptilian osteoderms. J Anat 2013, 222:4155. Kuratani S: Evolution of the vertebrate jaw from developmental perspectives. Cebra-Thomas JA, Terrell A, Branyan K, Shah S, Rice R, Gyi L, Yin M, Hu YS, Mangat G, Simonet J, Betters E, Gilbert SF: Late-emigrating trunk neural crest cells in turtle embryos generate an osteogenic ectomesenchyme in the plastron. However, as marsupials are one of the three subgroups of mammals, kangaroos are also mammals, or pouched mammals. Alligator mississippiensis Exoskeletons have evolved independently many times; 18 lineages evolved calcified exoskeletons alone. Importantly, as indicated by genomic analyses of turtles, the evolutionarily novel patterns of the skeletal system in vertebrates appear to arise through spatiotemporal developmental shifts after the establishment of the above-noted phylotype [162]. In these animals, both endo- and exoskeletons developed purely through intramembranous ossification, although the endoskeleton developed on the surface of cartilage (perichondral ossification; as for perichondral ossification in chondrichthyes, see [58]). Note that the trabecular plate (tp in B), generally derived from the premandibular crest cells, is mapped on the hyoid crest in Bombina. It was previously thought that the rostral neural crest (cephalic crest) yielded mesenchymal tissue throughout the bodies of vertebrates, whereas the posterior portion (that is, the trunk crest) typically gave rise to a more limited repertoire of tissues, including melanocytes and the peripheral nervous system [74-77]. (A- External skeletons, or exoskeletons, are the hard casings that protect arthropods, such as insects, crustaceans, and spiders. Dipnorhynchus sussmilchi Although the patterns of dermal bones and lateral lines are coupled developmentally, it is unclear whether the lateral line induces the dermal bones, or vice versa (see [116]). However, the gastralia embryonically develop in close association with the rectus abdominis muscle in a deep layer, whereas other trunk exoskeletal elements develop close to the epidermis [22,23] (see also Figure2). (D- J Morphol 2009, 270:13641380. Dorsal view of the chondrocranium (D), and left lateral (E), dorsal (F), and ventral (G) views of adult zebrafish. De Beer (1958, 1971) [61,62] later used Platts notion to refute von Baers germ layer theory [63], because mesoderm generally was believed to be the main source of skeletal tissue in animals. Am Sci 1978, 66:192201. Osteichthyes acquired endochondral ossification, in which bony tissues are produced within (as well as on top of) cartilage (Figure4C). In other words, we must identify parts or elements of the developmental program (for example, gene regulatory networks, modules, sets of regulatory genes and their regulatory elements) that can or cannot change when certain fixed phenotypic patterns are favored. Several evolutionary scenarios, not always mutually exclusive, may explain the situation regarding the origins of the dermatocranial roof: Morphological homologies of bony elements and the cell lineages that give rise to these elements are regulated at different, decoupled levels, and the bony elements can be conserved through evolution independent from the cell lineages, which are apt to change more rapidly. Zeit wiss Zool 1933, 144:510572. Two major skeletal systemsthe endoskeleton and exoskeletonare recognized in vertebrate evolution. Developmental origins of the dermal skull roof. known as an endoskeleton. Exoskeletal bones might be coated with enameloid and dentine tissues, but whether such a trait represents the ancestral or derived state is equivocal, on the sole basis of histological data. Article Evolutionary Morphology Laboratory, RIKEN, 2-2-3 Minatojima-minami, Chuo-ku, Kobe, Hyogo, 650-0047, Japan, You can also search for this author in
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