The Question of a Presupracleithrum in Brachiopterygian Fishes

The presupracleithrum is an exoskeletal pectoral bone that occurs in Paleozoic and Mesozoic actinopterygian fishes. It has been equated more than once with an opercular element in brachiopterygian fishes. In recent cladistic analyses, this alleged homology is used to assign brachiopterygians to actinopterygians. However, a comparison of brachiopterygian and actinopterygian crania shows clearly that the former lack a presupracleithrum.     

The Question of a Dermohyal in Brachiopterygian Fishes

Since the turn of this century it has been suggested between whiles that brachiopterygians (i.e. the species of Polypterus and Calamoichthys) are actinopterygians. One of the arguments used recently by advocates of this widespread contention is the dermohyal, an exoskeletal branchial-arch element which they assert occurs in both groups. However, this argument has to be rejected: the dermohyal does not exist in brachiopterygian fishes. The writer proposes to maintain the view that brachiopterygians are not actinopterygians until better arguments to the contrary are forthcoming.     

The Morphology of the Organum Olfactus of a 32 mm Embryo of the Brachiopterygian Fish Polypterus senegalus

Anatomically the brachiopterygian organum olfactus is unique among craniates. As revealed by the present study of Polypterus senegalus , it consists of an incurrent and an excurrent tube, a body and six lobes. Luminally these divisions are extensively covered with cilia, the actions of which cause water to flow through the organ. The proximal extremities of the two tubes communicate directly with one another, allowing water to bypass the body and the lobes. The body is subdivided by means of seven infolds into a central part and two peripheral parts. Of these, the central part directs water to the lobes; the peripheral parts drain the lobes and lead water to the excurrent tube. Each lobe has an axial portion and two rows of alternating lobuli. Interlobularly infolds of the axial portion subdivide the lobar lumen in such a way that movement of water is possible. Olfactory neurosensory cells occur in the rear of the incurrent tube, as well as in the lobes. Probably the degree of stimulation of the tubal receptors determines whether the water taken in is to bypass the lobes.     

Two Intracranial Ligaments Supporting the Brain of the Brachiopterygian Fish Polypterus senegalus

Two hitherto unknown ligaments that assist in holding the brain in position occur in the brachiopterygian fish Polypterus senegalus. These rounded fibrous cords, placed one behind the other, are called ligamentum intracraniale transversum and ligamentum intracraniale obliquum. The former spans the orbitotemporal division of the cranial cavity and passes through the velum transversum. The latter, which is paired, extends from the roof to the sidewall of the endocranium and runs in the rhombomesencephalic sulcus in company with the trochlear nerve. It is argued that these intracranial ligments, like the spinal denticulate ligaments, are derivatives of a pair of fibrous bands which in early craniate phylogeny extended throughout the length of the neuraxial meninx and enhanced its tensile strength. It is also argued that at least the transverse intracranial ligament was already in existence some 350 million years ago.