Strange creatures called dipnoi (lungfish) fascinated the first naturalists to study the few species still alive in modern times.(44 kb)
Although their appearance might indicate otherwise, these are fish with lungs, not finned amphibians!
These curious sarcopterygian fish first appeared in Lower Devonian seas and reached maximum diversity by Upper Devonian time. Their diversity was revived during the Triassic Period before gradually declining once again. Only three genera exist today: Neoceratodus
in Australia, Lepidosiren
in Amazonia, and four Protopterus
species in tropical Africa.
The name dipnoi name means double breather, because in addition to gills, they all have one or two functional lungs, allowing them to breathe air. Both the American and African species of lungfish are dependant on air for survival, their gills having degenerated so much that they will drown if they cannot reach the surface.
Such strange features explain why researchers once suggested this group of fish was ancestral to all terrestrial vertebrates. For example, the Australian species still has fins strong enough to raise its body onto shore and struggle towards another water hole. But we now know, thanks to paleontology, that dipnoan fish are a lineage of sarcopterygians that developed these evolutionary adaptations separately from other groups an example of convergent evolution and they never evolved into terrestrial forms.
Various traits can be used to recognize dipnoi, but their skull anatomy is particularly distinctive. The skull and jaw apparatus tend to display a certain degree of consolidation, and there are no marginal teeth along the lower mandible. Food was ground between dental plates in the palate, which look somewhat like partly-fused teeth. These dense, hard plates fossilize well and are commonly all that is preserved from the original fish. In fact, almost half of the 300 known fossil dipnoi species have been described by their dental plates alone.
Fossils that preserved the entire body display a set of several separate fins towards the back of the body, which must have provided a powerful push through the water. In contrast, the dorsal, caudal and anal fins in living lungfish are fused into a single fin that surrounds the rear half of the animal.
The modern lungfish of Africa display a very unusual summer dormancy (estivation) behaviour. During a drought, they dig a burrow in the mud where they remain buried for several months. They stay in the ground, breathing with their lungs, even after the water has completely disappeared and the mud is bone dry. When the rains finally return, they emerge and resume swimming.
Fossilized burrows containing hibernating dipnoi have been found in layers dating back to the beginning of the Permian Period. It is thought that estivation behaviour, which implies the ability to breathe air, developed by the end of the Devonian Period, or during the Carboniferous, when this group invaded freshwater ecosystems.
Two dipnoi genera, Scaumenacia
, are found at Miguasha. It is unlikely that they dug burrows, although anatomical details suggest that they could gulp air like todays lungfish. But as with all the Devonian dipnoi, they depended primarily on breathing through their gills to obtain most of their oxygen.