Osteological systems of the skeleton of the head

 The skeleton of the head of a teleostean differs from the one in sarcopterygians (tetrapods and lobe-finned fishes, see classification) by the huge number of individualised bones. In sarcopterygians, there are multiple fusions between the bones.

Despite these differences, the head skeleton in both  groups is composed of two distinct parts :

- the neurocranium, which surrounds and protects the encephalon,
- the splanchnocranium, which comprises the bony elements surrounding the anterior part of the digestive tract.

Several bones of the splanchnocranium are attached to the neurocranium with ligaments.

In teleosteans, the appendicular skeleton  of the pectoral fins is associated to the neurocranium. In acanthomorphs, the skeleton of the other pair of fins (pelvic fins) is attached to the pectoral girdle.

These pages dedicated to the osteology of the head of the bluefin tuna present the bony elements, as discovered during an osteological dissection. In the present page are presented the various osteological systems of the skeleton of the head in an other scombrid species : the common mackerel, Scomber scombrus.

Morpho-anatomical lexicon
 

Head skeleton of a common mackerel  (Scomber scombrus, Scombridae)

Appendicular skeleton

The appendicular skeleton of teleosteans is composed by the pectoral (or scapular)  girdle and the pelvic girdle. Both sport paired fins and are, directly or not, in relation with the neurocranium.

A primary girdle of enchondral origin, and a secondary girdle of dermal origin form the pectoral girdle. This secondary girdle connects the endoskeleton of  the pectoral fin to the neurocranium. 

The splanchnocranium is composed by a succession of bony arches. In teleosteans, the two anteriormost bony arches are modified ; respectively the first one, the mandibular arch, forms the jaws and the second one, the hyoid arch bears the opercular bones and attaches posteriorly the jaws to the neurocranium . The five following arches form the skeleton  bearing the gills.

Surface sensory canals bones

These bones are dermal bones bearing sensory canals. Despite their close proximity to the bones of the neurocranium, they do not belong to it. Some of them are included in other osteological systems (n° 4 and 5 in the appendicular skeleton and n°14 in the splanchnocranium).

Neurocranium

The neurocranium is composed by a dermal endoskeleton -also called osteocranium -, here coloured in red and pink, and an enchondral endoskeleton - also called chondrocranium -, here coloured in blue hues. This choice of colours is a reference to the « cleared and stained » technique.

Neurocranium of a common mackerel (Scomber scombrus, Scombridae), left lateral view.
(A) dorsal view, (B) lateral view, (C) occipital view.
The dotted line shows the shape of the neurocranium, modified from G. Lecointre.

The ethmoid region is the anteriormost part of the neurocranium. Externally, it corresponds to the snout.

The orbito-temporal region corresponds to the area surronding the eyeballs and the orbits.

The otic region contains otoliths and the part of the encephalon ("the brain") involved in the analysis of information about equilibrium and position of the body in space.

The occipital region is holed by the foramen magnum large, through which the medulla enters and exits the neurocranium. The first vertebra is strongly imbricated on the posterior part of the neurocranium. It fixes the head to the vertebral column.