The stapes is commonly described as having been discovered by the professor Giovanni Filippo Ingrassia in 1546 at the University of Naples, although this remains the nature of some controversy, as Ingrassia's description was published posthumously in his 1603 anatomical commentary '. Spanish anatomist Pedro Jimeno is first to have been credited with a published description, in ' (1549). The bone is so-named because of its resemblance to a stirrup (), an example of a late Latin word, probably created in mediaeval times from "to stand" (), as stirrups did not exist in the early Latin-speaking world.
History
Stapes
Stapes | History
Yes, Giovanni Filippo Ingrassia although there is some controversy regarding it It is commonly described as having been discovered by the professor Giovanni Filippo Ingrassia . Giovanni Filippo Ingrassia Sorry. I cannot find the answer based on the context.
what are the 3 smallest bones in the human body
user
Strapes is one of them
assistant
what is the size of this above bone?
user
Roughly 3 x 2.5mm
assistant
which part of the body is this located?
user
Middle ear of humans
assistant
besides the human, is this bony structure present in other living species in the animal kingdom?
user
It is one of three ossicles in mammals, in non-mammalian four-legged animals its homologous form is called the columella
assistant
what are the functions this bony structure serves?
user
It transmits sound vibrations from the incus to the oval window, a membrane-covered opening to the inner ear.
assistant
are there any variations of this bony structure?
user
It appears to be relatively constant in size in different ethnic groups
assistant
can this bony structure get affected or impaired?
user
Yes, its impairment causes conductive hearing loss
assistant
is there any remedy to solve this?
user
Two common treatments are stapedectomy and stapedotomy
assistant
how is the first treatment that you mentioned - carried out?
user
It is the surgical removal of the stapes and replacement with an artificial prosthesis
assistant
what are some of the complications involved?
user
Causes fibrosis-related damage to the base of the bone, or obliterative otosclerosis, resulting in obliteration of the base.
assistant
what does the term otosclerosis mean?
user
A condition where one or more foci of irregularly laid spongy bone replace part of normally dense enchondral layer of bony otic capsule in the bony labyrinth.
assistant
what is the main difference between the treatment we discussed versus stapedotomy?
user
In the former the Stapes is replaced by artificial prosthesis whereas in the latter an artificial prosthesis is inserted by making a small hole in the base of the stapes
assistant
with reference to the tiny bone discussed earlier, when does the development of it begin?
user
From the second pharyngeal arch during the sixth to eighth week of embryological life.
assistant
is there any intellect who first observed and discussed this tiny bony structure?
user
The stapes develops from the second pharyngeal arch during the sixth to eighth week of embryological life. The central cavity of the stapes, the obturator foramen, is due to the presence embryologically of the stapedial artery, which usually regresses in humans during normal development.
Stapes | Development
The stapes is commonly described as having been discovered by the professor Giovanni Filippo Ingrassia in 1546 at the University of Naples, although this remains the nature of some controversy, as Ingrassia's description was published posthumously in his 1603 anatomical commentary '. Spanish anatomist Pedro Jimeno is first to have been credited with a published description, in ' (1549). The bone is so-named because of its resemblance to a stirrup (), an example of a late Latin word, probably created in mediaeval times from "to stand" (), as stirrups did not exist in the early Latin-speaking world.
Stapes | History
For example, the lower branch of the squamosal bone is shortened to offset the tall dorsal process of quadratojugal which connects to it. On the other hand, the rear branch jugal bone lengthens to fill some of the space left by the shortening of the anterior process of the quadratojugal. In archosauriforms, the jugal even re-encloses the lower temporal fenestra. The stapes are long, thin, and solid, without a perforating hole (stapedial foramen) present in the more robust stapes of other reptiles.
Archosauromorpha | Skull
Caudal to what will eventually become the stapes, Reichert's cartilage also forms the styloid process of the temporal bone. The cartilage between the hyoid bone and styloid process will not remain as development continues, but its perichondrium will eventually form the stylohyoid ligament.
Pharyngeal arch | Reichert's cartilage
The stapes is one of three ossicles in mammals. In non-mammalian four-legged animals, the bone homologous to the stapes is usually called the columella; however, in reptiles, either term may be used. In fish, the homologous bone is called the "hyomandibular", and is part of the gill arch supporting either the spiracle or the jaw, depending on the species. The equivalent term in amphibians is the "".
Stapes | Animals
The tympanic cavity and eustachian tube are derived from the first pharyngeal pouch (a cavity lined by endoderm). The distal part of the cleft, the tubotympanic recess, widens to create the tympanic cavity. The proximal part of the cleft remains narrow and creates the eustachian tube. The bones of the middle ear, the ossicles, derive from the cartilages of the pharyngeal arches. The malleus and incus derive from the cartilage of the first pharyngeal arch, whereas the stapes derives from the cartilage of the second pharyngeal arch.
Human embryonic development | Middle ear
The stapes is the third bone of the three ossicles in the middle ear. The stapes is a stirrup-shaped bone, and the smallest in the human body. It rests on the oval window, to which it is connected by an annular ligament. The stapes is described as having a base, resting on the oval window, as well as a head that articulates with the incus. These are connected by anterior and posterior limbs (). The stapes articulates with the incus through the incudostapedial joint. The stapes is the smallest bone in the human body, and measures roughly 3 x 2.5mm, greater along the head-base span.
Stapes | Structure
A much more detailed description of the first two ossicles followed in Andreas Vesalius' "De humani corporis fabrica" in which he devoted a chapter to them. Vesalius was the first to compare the second element of the ossicles to an anvil although he offered the molar as an alternative comparison for its shape. The first published description of the stapes came in Pedro Jimeno's "Dialogus de re medica" (1549) although it had been previously described in public lectures by Giovanni Filippo Ingrassia at the University of Naples as early as 1546. The term "ossicle" derives from , a diminutive of "bone" (; genitive ).
Ossicles | History
Embryonic pharyngeal slits, which form in many animals when the thin branchial plates separating pharyngeal pouches and pharyngeal grooves perforate, open the pharynx to the outside. Pharyngeal arches appear in all tetrapod embryos: in mammals, the first pharyngeal arch develops into the lower jaw (Meckel's cartilage), the malleus and the stapes.
Recapitulation theory | Haeckel
The configuration of the stapes is intermediate between non-amniote tetrapods and amniotes. On the one hand, its connection to the otic notch is unusual, since true reptiles and other amniotes have lost an otic notch, forcing the tympanum and stapes to shift downwards towards the quadrate bone of the jaw joint. On the other hand, the thin, sensitive structure of "Seymouria"'s stapes is a specialization over most non-amniote tetrapods, which have a thick stapes useless for hearing yet useful for reinforcing the skull. The inner ear of "Seymouria baylorensis" retains a cochlear recess located behind (rather than below) the vestibule, and its anterior semicircular canal was likely encompassed by a cartilaginous (rather than bony) supraoccipital.
Seymouria | Skull
Though it existed far earlier than "Archaeopteryx", its skeletal structure is allegedly more bird-like. "Protoavis" has been reconstructed as a carnivorous bird that had teeth on the tip of its jaws and eyes located at the front of the skull, suggesting a nocturnal or crepuscular lifestyle. Reconstructions usually depict it with feathers, as Chatterjee originally interpreted structures on the arm to be quill knobs, the attachment point for flight feathers found in some modern birds and non-avian dinosaurs. However, re-evaluation of the fossil material by subsequent authors such as Lawrence Witmer have been inconclusive regarding whether or not these structures are actual quill knobs.
Protoavis | Introduction
The morphology of the stapes suggests that the animal was capable of hearing high-frequency sound. "B. woodi" does not possess lateral line sulci or an ossified branchial system. The principal method of respiration was probably buccal (gulping air through mouth) rather than costal (expanding chest volume to take in air), indicated by the small straight ribs.
Balanerpeton | Introduction
The eyes pointed forwards to a degree, providing some binocular vision. Most of the skull consisted of bone struts. The presumed pygostyle was absent; investigating the real nine tail vertebrae instead of impressions showed that they were unfused, though very reduced. The wing finger lacked the fourth phalanx. According to Bennett a membrane, visible near the shin, showed that the wing contacted the ankle and was thus rather short and broad. Bennett also restudied the holotype, interpreting bumps on the jaws as an indication that hairs forming a protruding bristle were present on the snout.
Anurognathus | Description
Work on extinct theromorphs by Owen (1845), and continued by Seeley, Broom, and Watson, was pivotal in discovering the intermediate steps to this change. The transition between the "reptilian" jaw and the "mammalian" middle ear was not bridged in the "fossil" record until the 1950s with the elaboration of such fossils as the now-famous "Morganucodon". During embryonic development, the incus and malleus arise from the same first pharyngeal arch as the mandible and maxilla, and are served by mandibular and maxillary division of the trigeminal nerve. Recent genetic studies are able to relate the development of the ossicles from the embryonic arch to hypothesized evolutionary history.
Evolution of mammalian auditory ossicles | History
The earliest mammals were generally small animals, and were likely nocturnal insectivores. This suggests a plausible source of evolutionary pressure: with these small bones in the middle ear, a mammal has extended its range of hearing for higher-pitched sounds which would improve the detection of insects in the dark. The evidence that the malleus and incus are homologous to the reptilian articular and quadrate was originally embryological, and since this discovery an abundance of transitional fossils has both supported the conclusion and given a detailed history of the transition. The evolution of the stapes (from the columella) was an earlier and distinct event.
Evolution of mammalian auditory ossicles | Early mammals
A sacral length of around 6.5 is deduced. Dames observed 20 caudal vertebrae; Wellnhofer posits 21. Dames also noted the similarity in the long, rod-like structures of the tail to those in the flying pterosaur "Rhamphorhynchus", considering them to be ossified tendons. Thin, pointed cervical ribs begin at the third vertebra. The longest are about 50 mm and total eleven in number. Gastralia, or belly ribs, are arranged in 10 parallel rows and their position is suggested as evidence of post-mortem shrinking of the body. The scapulae are separated by about 17 mm and are flattened, narrow, and positioned much in their natural position.
Specimens of Archaeopteryx | Trunk
The stapes remained in contact with the quadrate bone, but functioned as auditory ossicles rather than supports for the brain case. As a result, the quadrate bones of therapsids likely had a dual function in both the jaw joint and auditory system.
Evolution of mammalian auditory ossicles | Early therapsid jaws and ears
As the dentary bone of the lower jaw continued to enlarge during the Triassic, the older quadrate-articular joint fell out of use. Some of the bones were lost, but the quadrate, the articular, and the angular bones became free-floating and associated with the stapes. This occurred at least twice in the mammaliformes. The multituberculates had jaw joints that consisted of only the dentary and squamosal bones, and the quadrate and articular bones were part of the middle ear. Other features of their teeth, jaws and skulls are significantly different from those of mammals.
Evolution of mammalian auditory ossicles | Fossil evidence for mammal-like jaws and ears
The bony palate is short, with its posterior end often located between the third molars. The alisphenoid strut, an extension of the alisphenoid bone of the skull that separates two foramina (openings) in the skull, is present in some individuals. The mastoid bone usually contains some openings. In the holotype, the head and body length is 130 mm, the tail length is 160 mm, the hindfoot length (without claws) is 31 mm, the ear length is 27 mm, and the skull length is 35.7 mm.
Nephelomys levipes | Introduction
The second pharyngeal arch is innervated by the facial cranial nerve. Muscles that arise from the arch are those involved with facial expression and the posterior digastric muscle. Skeletal structures that originate here are the cervical sinus, Reichert cartilage (stape) the styloid process of the temporal bone, the lesser cornu and the hyoid bone. The third pharyngeal arch is innervated by glossopharyngeal nerve. It molds the stylopharyngeus muscle and forms the skeletal structures of the greater horn and lower portion of body hyoid bone. The fourth and sixth arches are innervated by the vagus cranial nerve. Both arches will fuse to form the laryngeal cartilages.
Face and neck development of the human embryo | Pharyngeal arches
The jaws of early synapsids, including the ancestors of mammals, were similar to those of other tetrapods of the time, with a lower jaw consisting of a tooth-bearing dentary bone and several smaller posterior bones. The jaw joint consisted of the articular bone in the lower jaw and the quadrate in the upper jaw. The early pelycosaurs (late Carboniferous and early Permian) likely did not have tympanic membranes (external eardrums). Additionally, their massive stapes bones supported the braincase, with the lower ends resting on the quadrates. Their descendants, the therapsids (including mammalian ancestors), probably had tympanic membranes in contact with the quadrate bones.
Evolution of mammalian auditory ossicles | Early therapsid jaws and ears
The pterygoids and palatines were seemingly toothless, although this observance may be due to poor preservation. The tip of each ectopterygoid expands into a fan-shaped structure, and the suborbital fenestra is very short.
Prolacertoides | Description
Embryological origin is the third pharyngeal arch.
Stylopharyngeus muscle | Development
In the tetrapod and higher clades from the lower-middle Famennian there are several defining changes on the basis of anatomy of "Ichthyostega", "Tulerpeton", and "Acanthostega". In the cranium, there is a stapes derived from the hyomandibular of fishes; a single bilateral pair of nasal bones, and a fenestra ovalis in the otic capsule of the braincase. The opening of the otic wall of the braincase can be considered a paedomorphic feature for tetrapods and is linked to the stapes functionally. The stapes was thought to be just a structural support between the palate and the stapedial plate of the braincase.
Skeletal changes of organisms transitioning from water to land | Head changes in aquatic tetrapods
At the time, Standing noted similarities with the dentition of "Palaeopropithecus". Sixteen years after Standing's discovery, Charles Lamberton discovered the first and only complete cranium (skull) and associated mandible for "Archaeoindris", both of which were well preserved. He published this find in 1934. Only six postcranial specimens of "Archaeoindris" have been found. Two of these belonged to an adult and include a damaged humerus (upper arm bone) and an almost complete femur (thigh bone). The other four come from an immature individual and include a damaged humerus, a damaged ulna (lower arm bone), and two femurs, both lacking the epiphyses (rounded end of the bone) on both ends.
Archaeoindris | Taxonomic classification
These are small and fragmentary, and little information can be gained from them. It is possible that they are, in fact, stomach or throat contents of the actual "Dasygnathoides" fossil.
Dasygnathoides | Haemal arch & phalanx (ELGNM 15) and possible articular (ELGNM 29)
Skull material is very limited for "Nundasuchus"; only a right lower jaw and a right pterygoid bone (which formed part of the roof of the mouth) were preserved in the holotype. However, even these fragments are informative. The pterygoid is convex when seen from above and concave from below. This ventral (lower) concavity manifests in the form of a deep depression bounded by posterior (rear) and medial (inner) ridges. The portion of the pterygoid which would have been positioned along the midline of the palate is unusual compared to other archosaurs due to possessing teeth. These teeth were small and rounded, placed in three rows parallel to the midline of the mouth.
Nundasuchus | Skull
However, they were too incompletely preserved to estimate the light sensitivity of the eyes. The footplates of the bony stapes are large, associated with hearing low-frequency sounds underground, although its inner-ear is not as specialised as "Kawingasaurus".
Kembawacela | Burrowing
These are the most unusual of the braincase bones, and were previously misidentified as stapes. While they are not stapes, it is difficult to be certain what they are. As mentioned, they are sutured to the tubera basioccipitalia, and so could not have moved freely, meaning that they were not bones for the transmission of sound in this manner. Medially, they are sutured to the posterodorsal margins of the tubera basioccipitalia by a strongly serrate and partially intergrown suture. The posterior surface of this forms a steeply inclined trough running from the suture to the distal extremity of the paraoccipital process.
Phantomosaurus | Opisthotic bones
When neural cells migrate to the arches and surround them, they begin to increase in size. The six pharyngeal arches give rise to much of the skeletal and muscular tissue in the head and neck region. When the embryo is 42 days old, the mesenchymal arches can be recognized with its corresponding cranial nerve. The first pharyngeal arch forms maxillary and mandibular processes. It is innervated by the trigeminal nerve and molds muscles related to mastication such as temporal, masseter, medial, lateral, pterygoid bones, tensor palati, and tensor tympani. This arch originates maxillar and mandibular prominences, part of the temporal bone and Meckel's cartilage (malleus and incus) as skeletal structures.
Face and neck development of the human embryo | Pharyngeal arches
Air vibrations could not set up pulsations through the skull as in a proper auditory organ. The spiracle was retained as the otic notch, eventually closed in by the tympanum, a thin, tight membrane. The hyomandibula of fish migrated upwards from its jaw supporting position, and was reduced in size to form the stapes. Situated between the tympanum and braincase in an air-filled cavity, the stapes was now capable of transmitting vibrations from the exterior of the head to the interior. Thus the stapes became an important element in an impedance matching system, coupling airborne sound waves to the receptor system of the inner ear.
Tetrapod | Hearing
An alisphenoid strut separates two foramina (openings) at the base of the skull, the buccinator-masticatory foramen and the foramen ovale accessorium. The pattern of grooves and foramina on the head indicates that the circulation of the arteries in the head of "T. ucucha" follows the primitive pattern. The tegmen tympani, the roof of the tympanic cavity, overlaps the suspensory process of the squamosal bone. At the back of the mandible (lower jaw), there is a capsular process to receive the root of the lower incisor, which is absent in "T. hylophilus".
Thomasomys ucucha | Description
In one of the earliest phylogenetic analyses of the group, Gardiner (1983) recognized five characteristics that made Temnospondyli a clade: a bone at the back of the skull, the parasphenoid, is connected to another bone on the underside of the skull, the pterygoid; large openings called interpterygoid vacuities are present between the pterygoids; the stapes (a bone involved in hearing) is connected to the parasphenoid and projects upward; the cleithrum, a bone in the pectoral girdle, is thin; and part of the vertebra called the interdorsal attaches to the neural arch. Additional features were given by Godfrey "et al." (1987), including the contact between the postparietal and exoccipital at the back of the skull, small projections (uncinate processes) on the ribs, and a pelvic girdle with each side having a single iliac blade.
Temnospondyli | Phylogeny
Following on the ideas of Étienne Geoffroy Saint-Hilaire (1818), and studies by Johann Friedrich Meckel the Younger (1820), Carl Gustav Carus (1818), Martin Rathke (1825), and Karl Ernst von Baer (1828), the relationship between the reptilian jaw bones and mammalian middle-ear bones was first established on the basis of embryology and comparative anatomy by Karl Bogislaus Reichert (in 1837, before the publication of "On the Origin of Species" in 1859). These ideas were advanced by Ernst Gaupp, and are now known as the "Reichert–Gaupp theory". The discovery of the link in homology between the reptilian jaw joint and mammalian malleus and incus is considered an important milestone in the history of comparative anatomy.
Evolution of mammalian auditory ossicles | History
The stapes or stirrup is a bone in the middle ear of humans and other mammals which is involved in the conduction of sound vibrations to the inner ear. This stirrup-shaped bone is connected to the oval window by its annular ligament, which allows the footplate to transmit sound energy through the oval window into the inner ear. The stapes is the smallest and lightest bone in the human body, and is so-called because of its resemblance to a stirrup ().
Stapes | Introduction
The earliest claimed date for the beginnings of an upright spine and a primarily vertical body plan is 21.6 million years ago in the Early Miocene with "Morotopithecus bishopi".
Australopithecus afarensis | Classification
The hindmost part of the surangular had a small foramen placed in the same position as similar openings in the mandibles of non-bird theropods and modern birds. The splenial bone was three-pronged (as in some modern birds, but unlike the simple splenial of "Archaeopteryx"), and its lower margin followed the lower margin of the mandible. There was a large rostral mandibular fenestra and a small, rounded caudal fenestra behind it. Though only five specimens preserve parts of the beak's keratinous covering, these show that there would have been differences between species not seen in the skeleton. The holotype of "C.
Confuciusornis | Skull
An forward-directed extension of the supratemporal formed the inner-rear edge of the supratemporal fenestra, an opening through the skull roof behind the eyes. The parietal, which would have formed the inner margin of the supratemporal fenestra, had a complex front margin that was interdigitating with either the frontal or postfrontal bones, which are not preserved in the known specimens. The quadrate bone, which connected to the lower jaw to form the jaw joint, was C-shaped in side view. Two probable hyoid bones (tongue bones) are preserved with specimen SNHM1284-R; these bones were rod-like, with one end spatula-shaped. The stapes (bone in the ear region that is used for hearing) had a shaft that was more slender than in any other ichthyosaur, and its head was large and square—these features are regarded an autapomorphy (unique feature) and serve to distinguish the genus from related genera.
Acamptonectes | Skull
When "Incisoscutum ritchiei" was first described, bony plates of smaller arthrodires were discovered within the body cavity of two specimens. Due to their disorganised arrangement, posterior position behind the trunk shield, apparent gastric etching and the fact that one of the small arthodires was posterior facing relative to the adult, these bony plates were thought to represent the stomach contents of the adult "Incisoscutum". However spurred by the discovery of preserved embryos inside the ptyctodonts "Materpiscis attenboroughi" and "Australoptyctodus gardineri", the ‘last meals’ of the "Incisoscutum" have now been reinterpreted as embryos, 5 cm in length, within pregnant adult females.
Incisoscutum | Embryos
The presence of pharyngeal arches and clefts in the neck of the developing human embryo famously led Ernst Haeckel to postulate that "ontogeny recapitulates phylogeny"; this hypothesis, while false, contains elements of truth, as explored by Stephen Jay Gould in "Ontogeny and Phylogeny". However, it is now accepted that it is the vertebrate pharyngeal pouches and not the neck slits that are homologous to the pharyngeal slits of invertebrate chordates. Pharyngeal arches, pouches, and clefts are, at some stage of life, found in all chordates. One theory of their origin is the fusion of nephridia which opened both on the outside and the gut, creating openings between the gut and the environment.
Pharyngeal slit | Introduction
The reason for this evolutionary change is still unknown; however, it may be to assist in grasping small objects and thin branches.
Sesamoid bone | Other animals
The cephalopharyngeal skeleton is 0.57–0.61 mm long. Mandibular sclerites (hardened portions of the exoskeletons) exist as long, narrow, mouth-hooks that appear triangular from a side-view. Dentary sclerites located next to the mandible are pointed down, and are lightly pigmented towards the back of the fly. Small sclerites are present between the mandibular ones, and an additional sclerite is found on the posterior end near the hypostome (a structure found near the mouth which allows an animal to anchor itself firmly on another to suck, as in the tick's hypostome). The third instar stage is 4.10–9.42 mm long and 0.76–2.13 mm wide.
Dryomyza anilis | Larva
In those other mammals, the supra-condyloid foramen is an opening in the bone that important structures, the median nerve and the brachial artery, run through. Struthers observed that when his ligament was present in humans, the nerve and artery did run through it. Darwin took this to mean that the human structure was homologous with the foramen in other mammals, and that therefore humans and other mammals had a common ancestor. He used Struthers' work as evidence in Chapter 1 of his "Descent of Man" (1871):
John Struthers (anatomist) | Evolution and Struthers' ligament
Each arch develops its own artery, nerve that controls a distinct muscle group, and skeletal tissue. The arches are numbered from 1 to 6, with 1 being the arch closest to the head of the embryo, and arch 5 existing only transiently. These grow and join in the ventral midline. The first arch, as the first to form, separates the mouth pit or stomodeum from the pericardium. By differential growth the neck elongates and new arches form, so the pharynx has six arches ultimately.
Pharyngeal arch | Structure
Such structures include a shelf at the rear of the surangular and a thick articular bone which has a tall retroarticular process with an inner prong pierced by a "chorda tympani" foramen.
Prestosuchus | Skull
The structure was originally depicted by Tiedemann, and later by Knox in the early 19th century, but John Struthers was the first to draw attention to this structure in 1848 as a "peculiar process" that bore curious resemblance to anatomy that he had seen in cats. This observation was one of many that Struthers made in subsequent investigations of vestigial and rudimentary structures, and in sharing these observations with his contemporary, Charles Darwin, provided significant evidence for the theories of evolution. Charles Darwin took the ligament to mean that humans and other mammals had a common ancestor, and used Struthers' work as evidence in Chapter 1 of his "Descent of Man" (1871).
Struthers' ligament | Historical significance
MacPhee found problems with either interpretation and suggested that the true mandibular fossa was part of the area Lamberton identified as such, at the side of the braincase. The fossa is small and low, suggesting that the animal was not capable of powerful biting. At the back of this fossa is a pseudoglenoid proces, which is more prominent in "P. germainepetterae". In "P. germainepetterae" but not "P. madagascariensis", a small opening, perhaps the vascular foramen, is present next to the foramen ovale. The petrosal bone forms a relatively large portion of the roof of the tympanic cavity, which houses the middle ear; parts of the petrosal are more developed in "P.
Plesiorycteropus | Skull
The lower margin formed an angle at the level of the front margin of the nasal foramen, which indicates how far back the rhamphotheca of the beak extended. The dentary had three processes that extended backwards into other bones placed further back in the mandible. The articular bone at the back of the mandible was completely fused with the surangular and prearticular bones. The mandible extended hindwards beyond the cotyla (which connected with the condyle of the upper jaw), and this part was therefore similar to a retroarticular process as seen in other taxa. The surangular enclosed two mandibular fenestrae.
Confuciusornis | Skull
Their vertebrae were convex on the two main articulating surfaces, and their bony palates were little developed. The mesosuchians saw a fusion of the palatine bones to form a secondary bony palate and a great extension of the nasal passages to near the pterygoid bones. This allowed the animal to breathe through its nostrils while its mouth was open under the water. The eusuchians continued this process with the interior nostrils now opening through an aperture in the pterygoid bones. The vertebrae of eusuchians had one convex and one concave articulating surface, allowing for a ball and socket type joint between the vertebrae, bringing greater flexibility and strength.
Crocodilia | Evolution
Some researchers however, contend that the lower arm did not allow pronation, a rotation of the radius around the ulna, so that the hand could not be directed downward, making the animal an obligate biped. The skull is small, pointed and triangular. There are four teeth in the premaxilla. The neck is only moderately elongated but thin. The neural spines of the tail are low. The second finger is longer than the third finger. The first toe is not strongly reduced. Von Huene identified a cluster of stomach stones (gastroliths) in association to specimen SMNS 12667. Yates identifies two unique derived traits (autapomorphies): the presence of a raised crescent-like ridge on the upper part of the inner side of the pubis shaft; and the presence of a vaulted bony web between two lower extensions of a braincase bone, the "processus basipterygoidei", with a raised central bony platform on top of the vault.
Efraasia | Description
The opisthotics were thickest at the base and generally similar to those of "Milleretta". They each connected to a thick yet complex stapes which possessed a conspicuous footplate, stapedial foramen, and a dorsal process. A knob on the outer edge of the stapes likely connected to a characteristic spur on the quadrate. What can be seen of the lower jaw indicates that it was primarily formed by the dentary in its front half, and the low, elongated surangular and angular in its rear half. The coronoid had a low peak and the tall articular had a small retroarticular process. Only one tooth was exposed, and it was similar to those of the maxilla, albeit smaller.
Australothyris | Description
The bone flap represents the first and second epiparietal pairs, the "P1s" and "P2s", as is evidenced by the osteoderm points still being visible, those of the first pair pointing to the front near the midline, those of the second pair forming the corners of the flap. The third epiparietals, pointing to behind, together form a pincer-like construction around the notch. At each rear corner of the frill a very large osteoderm is present. As it spans the suture between the squamosal and the parietal, this has been called an epiparietosquamosal or "EPS".
Spiclypeus | Skeleton
Marsh, like Seeley, defended the animal as representing a real bird, albeit the most reptilian one known. Geologist John Evans also studied the fossil in 1881, and observed that the feathers on the hind limbs of the animal appear to have the same structure as those on the wings and may have acted as lifting structures in flight. This idea later formed the basis for American zoologist William Beebe's prediction that a four-winged ancestor played a pivotal role in avian evolution.
Specimens of Archaeopteryx | Historical reviews
It lived in forested and riverbank environments before the evolution of grasslands. Like all entelodonts, the genus had typical artiodactyl legs but lacked specializations for fast running; though it supported its weight on cloven hooves, the foot bones remained unfused, and the toes could spread as camelid feet do. This structure, unique to entelodonts, may have helped the living animal move on soft ground. The head was unusually large, and the high spines on the vertebrae above the shoulders supported strong neck muscles and tendons to handle the weight of the head. The brain was tiny, but had relatively large olfactory lobes, suggesting that the animal had a keen sense of smell.
Archaeotherium | Paleobiology
The holotype of "E. primordialis" is the uncrushed and mostly intact anterior half of an individual with the articulating head and trunk armor preserved in three dimensions. The holotype is about long, and the live animal is estimated to have been over long. In overall form, the animal resembles primitive arthrodires, but the anatomy of the jaws strongly suggests the anatomies of bony fish and tetrapods. Specifically, this is the first stem gnathostome with dermal marginal jaw bones. These bones are the premaxilla, maxilla, and dentary. Most known placoderms had simple beak-like jaws made of bone plates. The researchers' cladistic diagram suggests that "E.
Entelognathus | Specimen and taxonomy
As evolution later attached the cranium of terrestrial vertebrates to the rest of the skull, the hyomandibula lost its supportive function and became an interior organ, the stapes, and thus its secondary function had become its primary function.
Hyomandibula | Evolutionary context
In vertebrates, the pharyngeal arches are derived from all three germ layers. Neural crest cells enter these arches where they contribute to craniofacial features such as bone and cartilage. However, the existence of pharyngeal structures before neural crest cells evolved is indicated by the existence of neural crest-independent mechanisms of pharyngeal arch development. The first, most anterior pharyngeal arch gives rise to the oral jaw. The second arch becomes the hyoid and jaw support. In fish, the other posterior arches contribute to the brachial skeleton, which support the gills; in tetrapods the anterior arches develop into components of the ear, tonsils, and thymus.
Pharyngeal slit | Pharyngeal arches in Vertebrates
The posterolateral borders were rounded and lacked spines. The compound eyes appear as anterolateral reniform (bean-shaped) elevations in the surface of the prosoma. The opisthosoma increased slightly its width in the third and fourth segments, then decreased slightly until the seventh segment. All the tergites (the dorsal part of the segments) were similar, superimposing strongly on each other, with the half of the first segment being hidden under the prosoma. The rest of tergites were more slightly superimposed, indicating a great flexibility and even the ability to roll up itself. The telson was trapezoidal, but Størmer mistook it as the twelfth segment of the body, suggesting a hypothetical lanceolate (lance-shaped) or fan-shaped telson.
Paleomerus | History of research
In young "Stegoceras", the dome is flat; however, in even the smallest "Foraminacephale" specimens, the parietal is already slightly domed. The supratemporal fenestrae on the top of the skulls of "Foraminacephale" are slit-like, as opposed to "Stegoceras" where they are round, and they seal shut very early in ontogeny as opposed to remaining open more-or-less throughout the animal's lifespan. The amount of empty space in the skull roof was also smaller relative to "Stegoceras", and more similar to "Acrotholus". Furthermore, in "Foraminacephale", the squamosal and postorbital bones became part of the dome much quicker, the dome thickened at a slower rate, and the sides of the dome were less angled than in "Stegoceras".
Foraminacephale | Ontogeny
Early labyrinthodont groups had massive stapes, likely primarily anchoring the brain case to the skull roof. It is a question of some doubt whether early terrestrial labyrinthodonts had the stapes connected to a tympanum covering their otic notch, and if they had an aerial sense of hearing at all. The tympanum in anurans and amniotes appear to have evolved separately, indicating most, if not all, labyrinthodonts were unable to pick up airborne sound.
Labyrinthodontia | Sensory apparatus
The holotype of "Archaeornithoides" was a very small individual. The head fragment as preserved measures just twenty-seven millimetres in length, indicating an original skull length of about five centimetres. The body length was estimated at fifty to sixty centimetres, making the "Archaiornithoides" type one of the smallest known non-avian dinosaurs. Adult length is uncertain. The snout of "Archaeornithoides" features a long antorbital fenestra, stretching over three quarters of the length of the maxilla. The maxilla bears at least eight teeth. These are small, conical and smooth, lacking wrinkles, serrations or carinae. The palatine bone seems to show the presence of a secondary fenestra.
Archaeornithoides | Description
Approximately in the middle of the parietal bones was a small hole known as a pineal foramen, which may have held a sensory organ known as a parietal eye. The pineal foramen is smaller in "Seymouria" than in other seymouriamorphs. The stapes, a rod-like bone which lies between the braincase and the wall of the skull, was tapered. It connected the braincase to the upper edge of the otic notch, and likely served as a conduit of vibrations received by a tympanum (eardrum) which presumably lay within the otic notch. In this way it could transmit sound from the outside world to the brain.
Seymouria | Skull
The trabecular cartilages were first described in the grass snake by at 1839. In 1874, Thomas Henry Huxley suggested that the trabecular cartilages are a modified part of the splanchnocranium: they arose as the serial homologues of the pharyngeal arches. The vertebrate jaw is generally thought to be the modification of the mandibular arch (1st pharyngeal arch). Since the trabecular cartilages appear anterior to the mandibular arch, if the trabecular cartilages are serial homologues of the pharyngeal arches, ancestral vertebrates should possess more than one pharyngeal arch (so-called "premandibular arches") anterior to the mandibular arch. The existence of premandibular arch(es) has been accepted by many comparative embryologists and morphologists (e.g., Edwin Stephen Goodrich, Gavin de Beer).
Trabecular cartilage | History
Ten thoracic vertebrae were found, increasing in size until the sixth then rapidly decreasing, and they have little connection with the carapace. The three vertebrae of the sacrum are short and flat. It probably had eighteen tail vertebrae; the first eight to ten (probably in the same area as the carapace) had neural arches, whereas the remaining did not. Its tail likely had a wide range of mobility, and the tail is thought to have been able to bend at nearly a 90° angle horizontally. The humeri in the upper arms are proportionally massive, and the radii and ulnae of the forearms are short and compact, indicating the animal had strong flippers in life.
Archelon | Description
The stapedius emerges from a pinpoint foramen in the apex of the pyramidal eminence (a hollow, cone-shaped prominence in the posterior wall of the tympanic cavity), and inserts into the neck of the stapes.
Stapedius muscle | Structure
Because the species was based on multiple specimens (a syntype series), CMN 515 was designated as the lectotype specimen by John Bell Hatcher in 1907. As no similar remains had been found in the area before, Lambe was unsure of what kind of dinosaur they were, and whether they represented one species or more; he suggested the domes were "prenasals" situated before the nasal bones on the midline of the head, and noted their similarity to the nasal horn-core of a "Triceratops" specimen. In 1903, Hungarian palaeontologist Franz Nopcsa von Felső-Szilvás suggested that the fragmentary domes of "Stegoceras" were in fact frontal and nasal bones, and that the animal would therefore have had a single, unpaired horn.
Stegoceras | History of discovery
The limbs were not equal in length as the forelimbs were about 70% of the hindlimbs. The toe bones (phalanges) are poorly preserved and the only well known bone is a fifth metatarsal (bone in hindlimbs attached to the toe bones) which was hooked in shape. However, hypotheses suggest that probably each forelimb had four toes and each hindlimb five. "Batrachotomus" had a tall and narrow skull estimated at in length. It had five pairs of fenestrae (skull openings), two pairs of which were for the eyes (called orbits) and the nostrils. Behind the orbits were two temporal fenestrae. These holes probably helped to reduce the weight of the skull and enabled the jaw to open more widely.
Batrachotomus | Description
As the dentary increased in size over million of years, two of these postdentary bones, the articular and angular, became increasingly reduced and the dentary eventually made direct contact with the upper jaw. These postdentary bones, even before their articular function was lost, probably transmitted sound vibrations to the stapes and, in some therapsids, a bent plate that might have supported a membrane capable of detecting vibrations developed on the angular. Eventually, it developed into the ectotympanic ring which supports the tympanic membrane in the ears of modern mammals.
Angular bone | Introduction
"Styloichthys" is characterized by large pores on the cosmine surface. These pores are often spoon-shaped and arranged in parallel grooves. There is a jagged margin between ethmosphenoid and otoccipital shields, an otoccipital with a wide flat surface carrying no vestibular fontanelle. The snout bends sharply downward and is slightly concave. The lower jaw has a ventrally protruding flange formed by prearticular and meckelian bone. Also characteristic of "Styloichthys" is the lyre-shaped trajectory of the supraorbital canal, a fenestra ventralis, small internasal cavities, three coronoids in the lower jaw, an eyestalk and a slender postorbital pila which connects the basipterygoid process and the side of the braincase wall.
Styloichthys | Description
The holotypic specimen was disarticulated prior to fossilisation, with many elements of its skeleton scattered throughout the assemblage: none of the vertebral column is preserved in articulation and those vertebrae that are preserved consist of separated neural arches and centra, signifying that the holotype was an immature individual. Due to the relative low-quality preservation of many of the skeletal elements, numerous pieces discovered have been difficult to identify: these include unidentified cranial elements, as well as an “ulna” which has since been recognised as the distal part of the tibia. Before the proper identification of this fragment, "Eotyrannus" was reconstructed with much longer tibiae, which influenced the early reconstructions of the animal.
Eotyrannus | Description
Osteohistological analysis shows that it was smaller than contemporary caenagnathids, and it is distinguished by fusion of the foot bones.
Citipes | History
This system had evolved independently within several different amphibian lineages. The impedance matching ear had to meet certain conditions to work. The stapes had to be perpendicular to the tympanum, small and light enough to reduce its inertia, and suspended in an air-filled cavity. In modern species that are sensitive to over 1 kHz frequencies, the footplate of the stapes is 1/20th the area of the tympanum. However, in early amphibians the stapes was too large, making the footplate area oversized, preventing the hearing of high frequencies. So it appears they could only hear high intensity, low frequency sounds—and the stapes more probably just supported the brain case against the cheek.
Tetrapod | Hearing
The eye socket was slightly overshadowed in its front part by a brow ridge that has been seen as the prefrontal bone. In 2019, Norman concluded that it was a fused palpebral bone. Behind it, the upper rim of the eye socket was formed by the supraorbital bone. A study by Susannah Maidment e.a. concluded that juvenile specimens show that this bone was a fusion of three elements, one in front, the next in rear, and the third at the inner side. The premaxilla, the bone forming the snout tip, was short and no predentary, the bone core of the lower beak on the tip of the stout lower jaws, has been found, so the horny beak that is assumed present with all ornithischians was likely very short.
Scelidosaurus | Skull
This inclusion is based on the fusion of the frontals as well as the inclusion of the prootic bone in the "recessus scalae tympani", a structure of the inner ear. They also found that a number of characteristics were shared by Barbatteiidae and Teiidae, including the overlap of the squamosal by the postorbital; the origin of the temporal muscles on the upper portion of the parietals, which is overlapped by the contact between the parietals and the ectopterygoid bones; and the weakly-developed shelf on the inside of the tooth row. The phylogenetic tree is reproduced below.
Oardasaurus | Classification
"Pinacosuchus" would have been an archaic crocodilian because it lacked the procoelous vertebrae (concave articulation surface on the anterior face of the bone, and convex articulation on the posterior face) of more derived crocodilians, instead having amphicoelous vertebrae (concave articulations on both faces). "Pinacosuchus" would have been a very small crocodylomorph. The sacrum (the vertebra supporting the hips) of USNM 16592 was only long. Gilmore regarded the specimen as an adult because the vertebrae lacked sutures for their neural arches, a sign of maturity. This assessment was supported by O'Neill and colleagues, comparing "Pinacosuchus" to their new spiked crocodilian "Akanthosuchus".
Pinacosuchus | History and description
The bone is not very well preserved; for most of its length only the anterior surface remains. What remains of the trochleae is still preserved in good detail however. Altogether, the bone is very similar to that of the sympatric and probably contemporary "Palaeochenoides mioceanus", only appearing a bit more albatross-like. The spread of the toes must have resembled that found in a fulmar quite a lot, by contrast. The thin-walled bone has a second toe trochlea that attaches notably kneewards from the others and is angled slightly outwards while the hallux was vestigial or missing, as is typical for the pseudotooth birds.
Tympanonesiotes | Description
Embryologically it is derived from the first pharyngeal arch along with the incus.
Malleus | Development
The fifth cartilage does not appear to have any contribution to adult anatomy and disappears.
Face and neck development of the human embryo | Pharyngeal arches
Remains attributable to "Pachycephalosaurus" may have been found as early as the 1850s. As determined by Donald Baird, in 1859 or 1860 Ferdinand Vandeveer Hayden, an early fossil collector in the North American West, collected a bone fragment in the vicinity of the head of the Missouri River, from what is now known to be the Lance Formation in southeastern Montana. This specimen, now ANSP 8568, was described by Joseph Leidy in 1872 as belonging to the dermal armor of a reptile or an armadillo-like animal. It became known as "Tylosteus". Its actual nature was not found until Baird restudied it over a century later and identified it as a squamosal (bone from the back of the skull) of "Pachycephalosaurus", including a set of bony knobs corresponding to those found on other specimens of "Pachycephalosaurus".
Pachycephalosaurus | History of discovery
For example, the basipterygoid processes (a pair of plates at the bottom of the braincase which connect to the roof of the mouth) were short, blade-like, and tilted forwards. In addition, the exoccipitals (a pair of braincase bones adjacent to the foramen magnum, the main exit for the spinal cord) were wide and edged by a pronounced ridge next to the exit holes for the hypoglossal nerve. Bonaparte (1975) additionally described squamosal and quadrate bones similar to those of "Euparkeria" attached to PVL 3872's braincase, although these were not mentioned by later studies.
Marasuchus | Skull
These struts locked the two bones together, suggesting that force was then transmitted from the nasals to the lacrimals. "Tarbosaurus" lacked these bony struts, and the connection between the nasals and lacrimals was weak. Instead, a backwards projection of the maxilla was massively developed in "Tarbosaurus" and fit inside a sheath formed from the lacrimal. This projection was a thin, bony plate in North American tyrannosaurids. The large backwards projection suggests that force was transmitted more directly from the maxilla to the lacrimal in "Tarbosaurus". The lacrimal was also more firmly anchored to the frontal and prefrontal bones in "Tarbosaurus".
Tarbosaurus | Skull mechanics
It is plausible that it was lost through evolution (as is the case with the antorbital and supratemporal fenestrae), but there is also the possibility that it was too small to be noticeable in the preserved "Vancleavea" skull. A small and sometimes overlooked mandibular fenestra is present in "Proterosuchus", one of the first archosauriforms to evolve.
Vancleavea | Lower jaw
Burdosh, and the block had been brought to the Museum. When the rock was chipped away, the insignificant external lump proved to belong to a fairly complete skull of a fossil crocodilian allied to the alligators; and on one posterior corner it bore a triangualr horn-like knob which proved to be identical with the mysterious separate fragments."
Ceratosuchus | Introduction
Its skull was long, low, perfectly arched, and lacked a "keel" or any other crest sometimes seen in related species. The nasal opening and the large opening typically present between the nose and eye openings of archosaurs (the "antorbital fenestra") had joined together in species such as this to create a single oval opening that occupied nearly one half the length of the skull. The beak was long, thin, sharply pointed, and lacked teeth. The cervical vertebrae were elongated. The first six dorsal vertebrae had fused into a notarium. Several pairs of belly ribs were preserved. Its upper leg bone was half the size of its upper arm bone, and strong and thin.
Zhejiangopterus | Discovery
Also, like ducks, geese and swans pelagornithids only have two and not three condyles on the mandibular process of the quadrate bone, with the middle condyle beakwards of the side condyle. Their basipterygoid articulation is similar to that of the Galloanseres. At the side of the parasphenoid lamina, there is a wide platform as in Anseriformes. The bony-toothed birds' attachment of the coronoideal part of the external mandible adductor muscles was located at the midline, the rostropterygoid process had a support at its base and the mesethmoid bone had a deep depression for the caudal concha, just as in waterfowl.
Pelagornithidae | Systematics and phylogeny
Living mammal species can be identified by the presence in females of mammary glands which produce milk. Other features are required when classifying fossils, since mammary glands and other soft-tissue features are not visible in fossils. Paleontologists therefore use the ossicles as distinguishing bony features shared by all living mammals (including monotremes), but is not present in any of the early Triassic therapsids ("mammal-like reptiles"). Early amniotes had a jaw joint composed of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). All non-mammalian amniotes use this system including lizards, crocodilians, dinosaurs (and their descendants the birds) and therapsids; so the only ossicle in their middle ears is the stapes.
Evolution of mammalian auditory ossicles | Defining characteristic of mammals
The femur is held by the Muséum national d'histoire naturelle in Paris (No.7978); the location of the scapular fragment is unknown.
Protopelicanus | Introduction
The coracoid, which comprises the other half of the pectoral girdle, is only well-preserved on the left side but shows a flattened, rectangular structure. Small fragments of the furcula are also identified, and as in other "Archaeopteryx" specimens this is a single boomerang-shaped structure. Some controversy surrounded the sternum when first described. Dames (1897) first identified the triangular structure and Petronievics (1925) later attempted to reconstruct it as being keeled. The sternum identified in the London specimen, by contrast, appeared unkeeled and this discrepancy led Petronievics to classify the Berlin specimen as a different genus, "Archaeornis". American paleontologist John Ostrom later identified the structure as part of the right coracoid based on X-ray photographs of this specimen.
Specimens of Archaeopteryx | Trunk
A peculiar characteristic of this group were the auditory bulla, protective structures of the bones of the ear : they were very large, like those that are found today in small mammals that live in open and dry environments. This evolved cranial anatomy was in stark contrast to the rest of the skeleton, rather primitive: the cainotherids still possessed four non-reduced toes (an ancestral condition of the artiodactyls), even if the lateral fingers were shorter; they ended in long claws, as in modern rabbits. The hind legs were much longer than the front ones, and indicate a remarkable adaptation to running and jumping.
Cainotheriidae | Introduction
The bone has a length of seven centimetres. This manual element shows a prominent dorsomedial process, a feature shared with the much smaller "Ornitholestes" which occasioned the specific name. The process with "Ornitholestes" is much less distinctive though. On its upper end there is only one cotyle, from which von Huene deduced it must have been a metacarpal. However, several coelurosaurian groups lack a second cotyle on the first phalanx also. If "Rapator" had a build like "Australovenator", it would have attained a considerable size: a body length of nine metres (30 ft) has been estimated.
Rapator | Description
The dorsal process of the stapes in "Theriognathus" is greatly reduced or entirely absent. The dentary is short, shaped like a banana, and continuously tapers anteriorly to a narrow edge. The dentarys’ lateral surface is smooth. The posterodorsal terminal end of the coronoid process is more rounded, and the dorsal extent terminates below the middle of the orbit in adults. The postdentary bones are reduced to form a free standing coronoid process. The reflected lamina is spade shaped and does not extend below the dentary The area between left and right dentaries remains relatively long and narrow just posterior to symphyseal region.
Theriognathus | Lower jaw
The preservation is so exceptional that studies have shown the species' numerous small ommatidia, distributed over the large eyes, could reduce the angle between ommatidia, thus improve their ability to detect small objects. Of the arguments above, it is posited by opponents that eyes are complex structures, and those in the Thylacocephala display clear and numerous affinities with compound eyes in other arthropod fossils, down to a cellular level of detail. The 'cephalic sac' structure itself is poorly preserved in Osteno specimens, a possible reason for interstitial 'sclerites'. The structural analogy with a cirripede peduncle lost supporting evidence when the 'ovaries' were shown to be alimentary residues, and the sac muscular system could be used to support the eyes.
Thylacocephala | Disagreements
The world's first stapedectomy is credited to Dr. John J.Shea Jr. who performed it in May 1956 on a 54-year-old housewife who could no longer hear even with a hearing aid. Significant contributions to modern stapedectomy techniques were then made by the late Dr. Antonio De La Cruz of the House Ear Institute in Los Angeles; by the late Professor Henri André Martin of the Hôpital Edouard Herriot in Lyon, France, including calibrated platinotomy (stapes footplate rather than whole surgery) and trans-footplate piston surgery that also paved the way for modern stapedotomy; and by the late Dr. Jean-René Causse of the eponymous clinic in Béziers, France, who pioneered the use of Teflon piston prostheses (also critical progress for stapedotomy) and, with his late son Dr.
Stapedectomy | History
Within the same period, paleontologist Charles Lewis Camp collected over a hundred "rauisuchian" bones, from what is now the Petrified Forest National Park of Arizona, which belong to at least seven individuals (UCMP A296, MNA 207C). Later, more remains came to light. In 1943, Case again described a pelvis along with a pubis (UM 23127) from the Dockum Group of Texas, which dates from the Carnian through the early Norian stages of Late Triassic period. These early findings, from 1932 to 1943, were initially referred to as a new phytosaur reptile, but assigned forty years later to "Postosuchus". The first articulated skeleton referred to "P.
Postosuchus | Putative occurrences
"Betasuchus" is known only from a single incomplete femur, so its exact relationships with other theropods have been difficult to determine. In 1972 Dale Russell confirmed Von Huene's opinion that "Betasuchus" was an ornithomimosaurid, but also considered the name a "nomen vanum": a failed emendation. Some workers in reference to the material still use "M. bredai" instead of "Betasuchus". David Norman in 1990 listed "Megalosaurus bredai" as a "nomen dubium". Jean le Loeuff and Eric Buffetaut in 1991 concluded it was a small abelisaurid, close to "Tarascosaurus" and that it was distinct enough not to consider it a "nomen dubium": they mentioned a more narrow femoral neck, a lack of orientation of the femoral head towards the front, the lack of an opening, or foramen, under the lesser trochanter and that the anterior face is narrower.
Betasuchus | Phylogeny
The external mandibular fenestra is large. The dentary, lower jaw, teeth are quite pointed and have no front edge denticles; their rear edge is very straight. The holotype preserves a furcula and a basket of fifteen pairs of gastralia. The arms are weakly developed, with a slender humerus and ulna. The front top of the third metatarsal is not fully overgrown by the second and fourth metatarsals. The sickle claw of the second toe is relatively large and long for a troodontid.
Sinornithoides | Description
Only on the edges of the outer trochleae does significant material seem to be lost. Altogether, the bone is very similar to that of the sympatric and probably contemporary "Tympanonesiotes wetmorei", only appearing a bit less albatross-like. The spread of the toes must have resembled that found in a fulmar quite a lot, by contrast. The thin-walled bone has a second toe trochlea that attaches notably kneewards from the others and is angled slightly outwards while the hallux was vestigial or missing, as is typical for the pseudotooth birds. The fossil is about one-quarter larger than that of "Tympanonesiotes", with a maximum end width of – probably a bit more in life, as the trochlea rims are eroded away.
Palaeochenoides | Description and systematics
A remarkable feature seen on its back was a row of paired, flattened bony plates. "Batrachotomus" was possibly an early relative of "Postosuchus", which lived during the dawn of the dinosaurs.
Batrachotomus | Introduction
Ezcurra "et al." (2014) performed bone histology on the remains of "Aenigmastropheus" in order to better understand the paleobiology of early archosauromorphs. As evident from the short period of fibrolamellar bone formation, "Aenigmastropheus" exhibited high growth rates during its early development. However, it was growing more slowly for most of its life, as indicated by the steady decrease in vascularization as well as the increase in spatial organization of the bone matrix up until the formation of lamellar-zonal bone. Ezcurra "et al." (2014) noted that the rapid growth during early development observed in "Aenigmastropheus" resembles those of "Prolacerta broomi" and the basal archosauriforms "Proterosuchus fergusi", "Erythrosuchus africanus" and "Chanaresuchus bonapartei", while extant lepidosauromorphs and the basal archosauromorphs "Trilophosaurus buettneri" and rhynchosaurs exhibit a slow overall growth pattern.
Aenigmastropheus | Paleobiology
The middle ear and its components develop from the first and second pharyngeal arches. The tympanic cavity and auditory tube develop from the first part of the pharyngeal pouch between the first two arches in an area which will also go on to develop the pharynx. This develops as a structure called the tubotympanic recess. The ossicles (malleus, incus and stapes) normally appear during the first half of fetal development. The first two (malleus and incus) derive from the first pharyngeal arch and the stapes derives from the second. All three ossicles develop from the neural crest. Eventually cells from the tissue surrounding the ossicles will experience apoptosis and a new layer of endodermal epithelial will constitute the formation of the tympanic cavity wall.
Ear | Middle ear