Tetrapods – Skulls and Skeletons in Transition

Australia: The Land Where Time Began

Tetrapods – Skulls and Skeletons in Transition

In this book Clack¹ has listed a set of more accurate terms that refer to the orientation of the bodies of fishes as this is necessary to compare the evolution of particular body parts from fishes to, e.g., humans, where the bipedal stance of the human body changes the orientation of the analogous body part in the ancestral fish, such as front, back, etc.

Some common features that are considered to be primitive of the construction of the skull have been shared among early bony fishes, early tetrapods and some modern ray-finned fishes. In these forms the skull is comprised of an outer covering of bony armour, the dermal skull roof, and the braincase is separate, being a box, that is relatively small, containing the brain proper, the ear capsules, and the rots of the nerves that innervates the other sense organs. The lower jaw is hinged to the back of the dermal skull roof, while the palate forms the roof of the mouth. Much of the dermal skeleton present in members from the Palaeozoic has been lost in most modern fishes and tetrapods, which explains the difference in appearance between the skulls of the modern forms and those of the more ancient animals.

Dermal skull roof

There are several regions that make up the dermal skull roof. The skull table is a flat lid, which is more or less rectangular, at the top and back. This is comprised of paired bones that in some fishes and tetrapods have different names. A series of paired bones that form the interorbital region extends from the skull table passing between the eye sockets, or orbits. In Eusthenopteron the parietals are the main bones in this region, which surround the parietal foramen, also known as the pineal foramen. The parietals are attached to the snout, which has the external nostrils on the sides, at the front. The snout region is comprised of a series of a mosaic of small bones in Eusthenopteron.

There are 2 small holes, the nostrils or nares (pronounced as 2 syllables, and naris is the singular), on each side of the snout in most fishes. The anterior nostril, the incurrent nostril, allows water to enter the nasal capsule for olfaction (sense of small), the posterior nostril, the excurrent nostril, allows the water to pass back out. The arrangement is different in Eusthenopteron.

The cheek region extends back from the snout beneath the orbits and the skull table. The lachrymal, jugal, squamosal and quadratojugal are the main cheek bones. The region where the lower jaw is hinged to the roof of the skull at the back of the cheek, which is usually drawn back well beyond the rear edge of the skull table, the entire back region being sometimes called the suspensorium. Several platelike dermal bones, that are mostly flat, are joined or sutured together to form the various regions. The dermal bones form in the skin as the animal grows. The braincase is suspended beneath the skull table and is protected by the skull roof, and also encloses the eyes and nasal capsules and anchors the muscles that close the jaws. Of critical importance, the marginal arcade of teeth is on the premaxilla and maxilla.

A series of dermal bones, the operculogular series, are situated behind the main part of the skull of fishes, extending beneath the lower jaw. A flaplike extension to the back of the cheek, that is composed of 2 or more large dermal bones, protect the gill (or opercular) chamber that form a seal in the ventilatory cycle. This flap extends downward forming a flexible covering for the throat by an interlinked series of dermal plates that end with the gular plates beneath the chin.

There is a notch between the opercular bone and the skull table in Eusthenopteron, as also occurs in some modern ray-finned fishes, where the spiracle opening is located. The spiracle is all that remains of the gill pouch that was in earlier forms associated with the hyoid arch.

The lateral line canals pass over certain points on the skull bones.

Braincase

Many different embryonic and anatomical components make up the complex structure of the braincase; the braincase differs from the skull roof in being composed of bone that forms by the ossification of a cartilage precursor, endochondral bone, and the braincase is also part of the endoskeleton.

The braincase structure consists of 2 main parts, the back part forming the ear capsules and the occipital region which articulates with the vertebral column. In the embryo these components form separate units, as can be seen in all modern vertebrates, implied by the construction in fossil forms. The vestibular system and semicircular canals that are used to monitor the posture, balance and movement, as well as the sensory cells involved in hearing, when they are present, are housed in the ear (otic) capsules. In adults the bones that comprise the otic capsule are the pro-otics and the opisthotics. The occipital arch complex underlies this complex, which consists of paired parachordal cartilages, from which a series of arches or columns extend up that may or may not join up with the otic capsules. These structures form the basioccipital, the exoccipital, and supraoccipital bones in the adult. A gap, the lateral otic fissure separates the otic capsules from the occipital arch. The otoccipital region is the complete unit of the otic capsules plus occipital regions, where there are facets where the hyomandibular articulates, and the vestibular fontanelle, a large unossified gap, which separates the otic region from the occipital region.

A midline partition at the front part of the braincase separates the eyes and carries the olfactory nerves to the front of the snout where the nasal capsules that are also part of the braincase are situated. The front part consists of the ethmosphenoid in fishes, while in tetrapods it consists of the sphenethmoid, and a portion that is more posteriorly situated is the basisphenoid. There are articulations, the basipterygoid process, on this that allows the palate and braincase link up. The trabeculae, a pair of rods, underlie these components. The ventral cranial fissure is a gap that separates the front portion of the braincase from the back portion.

Palate

The palate region is suggested by Clack¹ to be the region that is the most difficult region to visualise and explain, possibly because of the multiplicity of its functions, as it has several functions, she points out that it is a product of its complex history. The front section of the palate forms the mouth roof, which supports the nasal capsules of the snout. It is fastened to the inner edge of the cheek along its outer edges. The basipterygoid process, which is a peg from the braincase supporting the braincase at a crucial point, is housed in a socket in the palate, located just behind the eyes and internal to them. A process, the epipterygoid, is sent from the palatal complex from behind the basal articulation, which passes upwards to contact either the otic region of the braincase or the underneath of the skull roof. In cross section the palate curves in this region from a component that is approximately vertical, near the midline of the skull, to the much less steeply angled or even almost horizontal plane at the edges. The vertical part extends backwards, in contact with the rear end of the suspensorium, the more horizontal one extends from its contact with the cheek, forming a gap, the subtemporal fossa, between the 2 regions, and the muscles of the jaw pass downwards through this gap. At the rear of the palate it is part of the palatal complex that provides the hinge where the lower jaw attaches, the quadrate.

A mixture of dermal and endodermal components comprise the palatal complex, which is even more complex in fishes, having more bones that are separate than are present in early tetrapods. Because of this multiplicity of bones in fishes those specialising in fossil fish use more and different names than do those specialising in tetrapods. In this chapter Clack¹ describes the tetrapod structures and uses the tetrapod names. The dermal pterygoid is the major contributor, forming the main part of the palate and the quadrate ramus, the part which clasps the quadrate. A row of bones bearing an inner row of teeth – from front to back, the vomer, palatine, and ectopterygoid, borders the pterygoid and links the palate to the cheek on each side. In some species the front end of the palate embodies a small endochondral component, though the epipterygoid, which forms the otic (vertical) process and the quadrate, the socket for the basal articulation, are the main endochondral elements.

The parasphenoid, a midline dermal element, which underlies the braincase, forms part of the palate. It bears teeth or denticles, as do the remainder of the palate. The internal nostril, or choana, is a hole in the palate, at the junction of the premaxilla, maxilla, vomer, and palatine.

Lower jaw

This is essentially a tube that is formed from dermal bones, of which some bear teeth. There is an outer arcade of teeth on the dentary which oppose those on the premaxilla and maxilla, and opposing the vomer, palatine and ectopterygoid, there is an inner row of teeth on a series of coronoids. In fish the infradentary series, on the outside of dentary, beneath it, and in tetrapods it is called the splenial series. The prearticular is located on the inside of the main bone, which is often covered with denticles which match those on the palate. The upper part of the jaw hinge, as also with the lower part, there is an endochondral bone, the articular, which is sandwiched between the inner and outer faces of the jaw bones. There is a hollow in front of the articular, between the inner and outer faces, the adductor fossa, and the muscles powering the jaws are inserted in it. In some cases there is a bar of endochondral Meckelian bone within the tube (lower jaw).

Hyobranchial Skeleton and Gill Openings

The bones and associated structures that support the gills are called the Hyobranchial skeleton. There is usually a series of gill bars with 5 joints, or branchial arches, that support the gill filaments, the first couple in the series hinging onto the braincase with the remainder attaching to the more anterior ones. They join at the base, under the “chin” of the fish, to a midline bone. The gills are allowed to move in and out and the expansion and contraction of the gill chamber as the fish breathers, by the flexible, elastic basket that is provided by this structure. The hyoid arch is a larger set that is highly modified in front of the standard set of gill arches. The hyomandibular is the upper element of this set and the ceratohyal is the lower part. An opening from the pharynx (the throat region) is associated with each gill arch which allows the water pass as it is pumped by the fish as it breathes. In the architecture of the skulls of most fishes the hyomandibula carries out a crucial, literally pivotal role. In most early fishes it was associated with a spiracle, a modified gill opening. The form and function of the bone has changed in the tetrapods where it is called the stapes.

As occurs with the braincase, the gill and hyoid arches are formed from endochondral bone, though in this case the embryonic origin of the cartilage differs from most other elements of the endoskeleton, the initiation of its formation being by the neural crest, a unique tissue in embryonic vertebrates. The trabeculae of the braincase, and the quadrate, articular, epipterygoid and other endochondral parts of the palate, which may ossify, are formed from this tissue. Among other reasons, this is considered by scientists to be a reason these elements of the jaw are members of the same series as the gill arches.

Sources & Further reading