Bio 205, Lecture 10

History of skull components, part deux

Splanchnocranium - trends

jaw suspension

Muscles elevating the lower jaw will simply elevate the entire jaw as a unit unless the upperjaw is attached to the chondrocranium somehow. This is done in several ways in the different vertebrates:

Paleostyly: (agnathans) no suspension
Euautostyly: (placoderms, acanthodians) mandibular arch suspended by ligaments between chondrocranium and palatoquadrate
Amphistyly: (primitive sharks, some osteichthyes, crossopterygians) dual suspension by palatoquadrate and hyomandibula
Hyostyly: (modern sharks and actinopterygians): suspension by hyomandibula - provides more mobile upper jaw
Metautostyly: (most amphibians, reptiles) suspension by quadrate bone (ossification of posterior region of palatoquadrate cartilage). The ossification of the posterior part of meckel's cartilage is called the articular bone. So the primitive tetrapod condition is a quadrate-articular joint between the lower and upper jaw. The quadrate is sutured to the cranium in tetrapods so it doesn't look like part of the upper jaw. Remember that in tetrapods in general, the jaws are mostly dermal elements. Metautostyly frees up the hyomandibular for other functions (see below).
Craniostyly: (mammals) entire upper jaw incorporated into braincase and lower jaw is suspended. The mammals jaws are completely dermal. The Meckel's cartilage and palatoquadrate ossifications are in the middle ear (see below). Instead mammals have a dentary-squamosal jaw joint. The squamosal is a dermal bone of the temporal region of the skull. The dentary is one of the dermal bones of the lower jaw. In mammals, it is the only dermal bone of the lower jaw.

hyomandibular evolution

In osteicththyes, the posterior region of the palatoquadrate ossifies into the quadrate bone and the ascending process ossifies into the epipterygoid. The posterior end of Meckel's cartilage ossifies into the articular bone, which suspends the lower jaw to the upper jaw (the quadrate articular joint).

evolution of the tetrapod middle ear: In tetrapods, the mandibular arch is metautostylic, which frees the hyomandibula. This becomes the stapes (columella). In frogs and amniotes, the stapes inserts on the fenestra ovalis (oval window) on the wall of the otic capsule and transmits vibrations (sound) to the inner ear.

evolution of the mammalian middle ear: In a fossil mammal, Diarthrognathus, the quadrate-articular joint is supplemented by a more anterior dentary-squamosal joint (the dentary is a dermal bone covering part of Meckel's cartilage in the lower jaw and the squamosal is a dermal bone that is part of the temporal bone complex, which forms the lateral wall of the skull. In more derived mammals, including all modern mammals, the quadrate and articular become disassociated with the lower and upper jaw and become incorporated into the middle ear as the incus and malleus bones. Similarly, the angular, a dermal bone in the lower jaw of reptiles, becomes the tympanic in mammals, which holds the tympanic membrane and partially completes the middle ear (tympanic) cavity.

Fate of the arches

pharyngeal jaws

pharyngeal jaws in teleosts: In some teleosts, the paired fifth ceratobranchials have been modified into a lower phayngeal jaw which occludes with an upper pharyngeal jaw that is composed of dermal plates attached to the posterior epibranchials and pharyngobranchials. The lower jaw is slung by a set of muscles that elevate the lower jaw against the dorsal plates. Both upper and lower pharyngeal jaw have teeth whose morphology depends on the type of food that is processed. The evolution of pharyngeal jaws and its function in processing prey has allowed the oral jaws to differentiate into different forms related to capturing prey.

dermatocranium - trends

Review - Dermal series

facial	orbital	temporal	vault	palatal	mandibular
premaxilla	lacrimal	intertemporal	frontal	vomer	dentary
maxilla	prefrontal	supratemporal	parietal	palatine	splenials
nasals	postfrontal	tabular	postparietal	ectopterygoid	angular
	postorbital	squamosal		pterygoid	surangular
	jugal	quadratojugal		parasphenoid	prearticular
					coronoids

major trends

1. very difficulty to find homologies between actinopterygians, dipnoans, and tetrapods.

2. evolutionary loss and fusion of dermal elements

3. formation of secondary palate

palate evolution. In primitive tetrapods, including primitive amniotes, the internal nares open directly into the buccal cavity, that is the mouth and navsal cavity are not separated as they are in humans. Primitively, the internal nares are bordered by the maxilla laterally, the vomer medially, and the palatine posteriorly. In many different groups of amniotes, there has been a trend for the maxilla, and/or vomer, and/or palatine to form shelves that separate a dorsal air passage from the ventral buccal (mouth) cavity.

 

In the figures below, look at the variation in the position of the internal nares (in) on the palate. Air is breathed into the nasal or nasal/buccal cavity via the external nares. The internal nares provide the window to the pharynx. If these are anterior, then the animal has a single nasal/buccal cavity. If these are posterior, then the animal has a secondary palate that forms the floor of the nasal cavity and the roof of the buccal cavity. The internal nares (aka choanae), then, open into the pharynx.

Turtles.

 

Crocodiles

Synapisids (the lineage leading to mammals)

 

4. fenestration and emargination

Fenestration of palate in diapsids. The development of the suborbital fenestra on the palate and bounded by the palatine, pterygoid, and ectopterygoid creates a palatine-maxillary strut and a pterygoid-ectopterygoid strut. (see Fig. 7.11 above)

Emargination and Fenestration . Modern amniotes have much lighter skulls than primitive tetrapods and basal amniotes. In primitive tetrapods there are two lateral walls:

1. lateral wall of the neurocranium
2. lateral wall of the temporal fossa by the dermatocranium

Essentially, the solid, bony lateral and poterodorsal walls of the dermatocranium have been reduced to struts (or "arcades")in many different groups. There are two ways these struts have formed. The cheek or temporal fossa expands by emargination of the ventral border of the lateral wall

Emargination. This is most clearly seen in turtles although some people argue that it explains some of the pattern of the lateral wall in lizards. In turtles, the posttemporal fossa expands anteriorly by emargination of the posterior margin of the dermal roof. There may also be dorsal emargination of the ventral border of the lateral wall. In some cases, these emarginations can reduce the lateral wall to a narrow arcade or even completely remove the lateral wall

Temporal fenestration. Most amniotes other than turtles have reduced the lateral wall of the dermatocranium because of temporal fenestration, literally the opening of windows into the temporal region. It is this temporal fenestration that forms the basis for classifying the three major groups of amniotes.

Anapsids - no temporal fenestrae. Includes living turtles. Note that there are strong arguments that turtles have a highly modified diapsid skull.

Synapsids - single, lower temporal fenestra. Includes modern mammals.

Diapsids - traditionally, two temporal fenestrae, one dorsal and one ventral but the homology and even existence of the ventral fenestra is questionable so more radically, presence of at least a dorsal temporal fenstra.

Why temporal fenestration?

1. lightening the skull
2. more room for jaw muscles

The three lateral walls of the mammalian skull (actually, these walls are in all osteichthyes)

primary wall of braincase - formed by sphenoid and orbital cartilates. Severely reduced (essentially to dense connective tissue (the dura mater) in mammals).

secondary wall - formed by the alisphenoid (which is homologous to the epipterygoid of other tetrapods and to the ascending process of the palatoquadrate cartilage in fishes.

tertiary wall - formed by jugal (the zygomatic or malar of humans) and squamosal (part of the temporal bone in mammals). Reduced to the zygomatic arch. Primitively included the postorbital and quadratojugal.

5. Cranial Kinesis