Australia: The Land Where Time Began
Lungfish - Dipnoi (dipnoan)
One of 3 main classes of Osteichthyans. The skull roof of early lungfish was composed of a mosaic of bones that was not comparable to the skull-roof patterns in other osteichthyans. In most forms the teeth were in the form of hard crushing plates or a fine shagreen of denticles covering the palate, lower jaw bones and on bones resting on the ventral gill arches. Some of the known forms are Dipterus and Neoceratodus.
This was a family of Devonian lungfish that had a well-developed tooth plates with many rows of teeth, 2 dorsal fins, and they retained cosmine in parts of the dermal skeleton. Dipterus belonged to this family.
The ability to produce a powerful bite is the one adaptation that unites all the anatomical features found in early dipnoans. The many tightly interconnected bones of the skull roof relate to powerful jaw muscles that insert on the inside of the skull roof. The massive size of the symphysis, the region where the lower jaws meet, the fusion of the palate and braincase, the heavily strengthened structure of the gill arch bones, the specialised dental tissues, all suggest a very powerful bite.
In dipnoans, the braincase is heavily ossified as a single piece, the palate being fused to its lower surface. The braincase of primitive dipnoans has struts supporting the skull roof, creating large chambers for the passage of jaw muscles, attaching to the inside of the skull roof and to the lower jaw. The parasphenoid bone, in the centre of the palate, is short and plough-share-shaped in primitive forms. In later lineages it became expanded with a long posterior stalk. More room in the mouth for air bubble gulping became available as a result of this elongation.
There are visible grooves for the incurrent nostril in the snout of lungfish, along the upper border of the mouth. The excurrent nostril connects directly to the palate from the nasal capsule, and there are no bones covering the nasal capsule. There is a complex system of minute tubules running through the bone of the snout, ending in the lower jaws in the primitive dipnoans. Some have interpreted these as an electrosensory system, as occurs in some modern fish, where it is used to find food in muddy environments. Some think it was part of a nutritive system used to feed the skin and sensory-line canals of the snout.
Lungfish fed by 1 of 2 methods, either crushing food with hardened tooth-plates, or denticulates (or denticle shedders or "denticulates") that had a mouth covered with small denticles that were periodically shed. A powerful crushing bight characterises the oldest dipnoans, as seen in the large area where the lower jaws meet in the midline, and the large areas for jaw musculature attachment.
Included among the primitive biters are forms with palates covered with shiny dentine, found below the enamel layer in most vertebrate teeth. These dentine-plated forms gave rise to tooth-plates, rows of teeth being organised on both ridges.
The denticle-shedders have powerful gill arch bones lined with denticle-covered bones, apparently used for rasping food against the denticle-covered palates. There are large ceratohyal elements and the hyomandibular doesn't take part in the jaw articulation in the gill arches of lungfish, as in other osteichthyans. In denticle-shedding lungfish, many toothed bones accompany the gill arch series.
In early lungfish, the bodies have 2, equally-spaced dorsal fins, a separate anal fin and a heterocercal caudal fin, with long feathery paired pectoral and pelvic fins. The trend changed to have a shorter first dorsal fin and a larger second dorsal fin throughout the Devonian, with eventual merging of the median fins and the tail fin. Lungfish had acquired the body shape and fin plan that they kept for the rest of their evolution by 355 Ma, the end of the Devonian.
Some unique features characterise the soft anatomy of lungfish. Specialisations of the nervous system, such as the concentrically layered olfactory bulb, and the Mauthner cells of the brain. They have a 3-chambered heart, with partial petition of the atrium, and in Neoceratodus, it is barely visible. Their lungs have developed as an outpocketing of the gut, having developed from the osteichthyan swim bladder. The modification of the swim bladder to form a functioning lung involved the internal surface area for improved gas exchange, so wasn't a very complex evolutionary step.
In the Early to Middle Devonian, lungfish had cosmine-covered thick rhombic scales, each housing a system of pores and interconnecting canals in the dentine beneath. By the Late Devonian, most dipnoans had lost the cosmine layer, the bodies of these fish being covered by thinner, rounded scales. In primitive cosmine-covered dipnoans, the snout was ossified as a strong single unit. This snout often broke away after death, leading to the "loose-snout problem" of fish palaeontology. The ossified snout was replaced by one of soft tissue, sometimes with special small bones covering the dorsal surface, as cosmine disappeared from the skeleton.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|