Gastralia (: gastralium) are

dermal bone A dermal bone or investing bone or membrane bone is a bony structure derived from intramembranous ossification forming components of the vertebrate skeleton, including much of the skull, jaws, gill covers, shoulder girdle, fin rays ( lepidotrich ...

s found in the

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

body wall of modern

crocodilia Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchia ...

ns and

tuatara The tuatara (''Sphenodon punctatus'') is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is actually the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephal ...

, and many prehistoric

tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...

s. They are found between the

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

and

pelvis The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...

, and do not articulate with the

vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

e. In these reptiles, gastralia provide support for the

abdomen The abdomen (colloquially called the gut, belly, tummy, midriff, tucky, or stomach) is the front part of the torso between the thorax (chest) and pelvis in humans and in other vertebrates. The area occupied by the abdomen is called the abdominal ...

and attachment sites for abdominal muscles. The possession of gastralia may be ancestral for Tetrapoda and were possibly derived from the ventral scales found in animals like

rhipidistia Rhipidistia, also known as Dipnotetrapodomorpha, is a clade of lobe-finned fishes which includes the tetrapods and lungfishes. Rhipidistia formerly referred to a subgroup of Sarcopterygii consisting of the Porolepiformes and Osteolepiformes, a de ...

ns,

labyrinthodonts "Labyrinthodontia" (Greek language, Greek, 'maze-toothed') is an informal grouping of Extinction, extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras (about 390 to 150 million years ...

, and ''

Acanthostega ''Acanthostega'', from Ancient Greek ἄκανθα (''ákantha''), meaning "spine", and στέγη (''stégē''), meaning "roof", is an extinct genus of stem tetrapoda, stem-tetrapod, among the first vertebrates, vertebrate animals to have recogn ...

'', and may be related to ventral elements of

turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...

plastron The turtle shell is a shield for the ventral and dorsal parts of turtles (the Order (biology), order Testudines), completely enclosing all the turtle's vital organs and in some cases even the head. It is constructed of modified bony elements such ...

s. Similar, but not homologous

cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...

nous elements, are found in the ventral body walls of

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

s and anurans. These structures have been referred to as inscriptional ribs, based on their alleged association with the inscriptiones tendinae (the tendons that form the six pack in humans). However, the terminology for these gastral-like structures remains confused. Both types, along with sternal ribs (

ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

costal cartilages), have been referred to as abdominal ribs, a term with limited usefulness that should be avoided. Gastralia are also present in a variety of

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

animals, including

theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...

and

prosauropod Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the Sauropoda, sauropods and their ancestral relatives. Sauropods generally grew to very large siz ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

pterosaur Pterosaurs are an extinct clade of flying reptiles in the order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosaurs are the earli ...

plesiosaur The Plesiosauria or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period (geology), Period, possibly in the Rhaetian st ...

s, choristoderes and some primitive

pelycosaurs Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term mammal-like reptile was used, and Pelycosauria was considered an order, but this is now thoug ...

. In dinosaurs, the elements articulate with each other in a sort of zig-zag along the midline and may have aided in

respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...

. Gastralia are known to be present in primitive

ornithischia Ornithischia () is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek ...

n and

sauropodomorph Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had lo ...

dinosaurs. However gastralia are only known from heterodontosaurid ornithschians, and gastralia are lost in

eusauropoda Eusauropoda (meaning "True Lizard Foot") is a derived clade of sauropod dinosaurs. Eusauropods represent the node-based group that includes all descendant sauropods starting with the basal eusauropods of '' Shunosaurus'', and possibly '' Barapas ...

sauropods Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their b ...

Terminology

The term "gastralia" was proposed by Georg Baur in 1898. They had previously been termed "abdominal ribs", but because the term "abdominal ribs" has been applied to various structures, and the gastralia are not true ribs, this is not considered an appopriate term. In turtles, where the gastralia are incorporated into the

, each pair of gastralia gets a distinct name: the hyoplastra, hypoplastra, xiphiplastra, and in some taxa the mesoplastra.

Taxonomic distribution

Gastralia were ancestrally present in amniotes, but have been lost in many groups. Among extant taxa, they are only present in crocodylians and the

, and in modified form as part of the plastron of turtles. Gastralia are rarely preserved in

therapsids Therapsida is a clade comprising a major group of eupelycosaurian synapsids that includes mammals and their ancestors and close relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including li ...

, but have been identified in some

dinocephalians Dinocephalians (terrible heads) are a clade of large-bodied early therapsids that flourished in the Early Permian, Early and Middle Permian between 279.5 and 260 million years ago (Ma), but became extinct during the Capitanian mass extinct ...

and

gorgonopsians Gorgonopsia (from the Greek Gorgon, a mythological beast, and 'aspect') is an extinct clade of sabre-toothed therapsids from the Middle to the Upper Permian, roughly between 270 and 252 million years ago. They are characterised by a long and ...

and several anomodonts. However, they were probably genuinely absent in some dicynodonts, therocephalians, and

cynodonts Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extinct ances ...

. Most

ornithischian Ornithischia () is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek st ...

dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

lacked gastralia, but

heterodontosaurids Heterodontosauridae is a family (biology), family of ornithischian dinosaurs that were likely among the most Basal (phylogenetics), basal (primitive) members of the group. Their phylogenetic placement is uncertain but they are most commonly fou ...

, one of the earliest-branching lineages of ornithischians, retained them. Sauropods have been considered to lack gastralia. Elements interpreted as gastralia have been rarely found in sauropods, but it has been argued that these elements are more convincingly interpreted as sternal ribs. Modern birds lack gastralia, but they were present in early lineages of birds, as in other theropods.

Pathology

The ''

Allosaurus fragilis ''Allosaurus'' ( ) is an extinct genus of theropoda, theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic Geologic time scale, period (Kimmeridgian to late Tithonian Geologic time scale, ages). The first fossil ...

'' specimen USNM 8367 contained several gastralia which preserve evidence of healed fractures near their middle. Some of the fractures were poorly healed and "formed pseudoarthroses." An apparent subadult male ''

'' was reported by Laws to have extensive pathologies. The possible subadult ''A. jimmadseni'' specimen MOR 693 also had pathological gastralia. The left

scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...

and

fibula The fibula (: fibulae or fibulas) or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. ...

of an ''

'' specimen catalogued as USNM 4734 are both pathological, both probably due to healed fractures. The holotype of ''

Neovenator salerii ''Neovenator'' (Help:IPA/English, nˈiːə͡ʊvˌɛne͡ɪtə; "new hunter") is a genus of carcharodontosaurian Theropoda, theropod dinosaur. It is known primarily from several skeletons found in the Early Cretaceous (Hauterivian-Barremian) Wesse ...

'' had many pathologies, including pseudoarthrotic gastralia and a deviation to the right of the third and fourth neural spines of the neck vertebrae. An immature

dromaeosaurid Dromaeosauridae () is a family (biology), family of feathered coelurosaurian Theropoda, theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous period (geology), Period. The name Drom ...

specimen (which had not been described in the scientific literature as of 2001) from Tugrugeen Shireh was observed to have a "bifurcated" gastralium. In the ''

Gorgosaurus libratus ''Gorgosaurus'' ( ; ) is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period (geology), Period (Campanian), between about 76.5 and 75 million years ago. Fossil remains have been ...

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

( NMC 2120) the 13th and 14th gastralia have healed fractures. Another ''G. libratus'' specimen catalogued as TMP94.12.602 bears multiple pathologies, including a pseudoarthortic gastralium. The unidentified tyrannosaurid specimen TMP97.12.229 had a fractured and healed gastralium.

References

External links

Dinosaur gastralia and their function in respiration
{{Tetrapod osteology, P. Vertebrate anatomy Bones Thorax (human anatomy)