III. Pisces-09. Subclass "Sarcopterygii"

Subclass “Sarcopterygii” — Lopatoperi
Lopatoperi is a group of fishes that became ancestral to tetrapods. From the viewpoint of many systematists (who follow the postulates of cladistics, the school of systematics founded by V. Henning), only those groups should be recognized in a system that contain a certain common ancestor and all its descendants. From this perspective, the group Sarcopterygii includes not only several groups of extant and extinct fishes but also all tetrapods. In our course we use a classification in which the highest taxa (superclasses and classes) are built on different principles. As we have already noted, because of this we place the names of paraphyletic groups (those that do not include all descendants of a single ancestor) in quotation marks. In all other respects we employ here the fish system from the 5th edition of Nelson’s compilation.
In the fifth edition of Nelson and co‑authors the following views on the phylogeny of lopatoperi are accepted.
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We will examine in detail only those groups whose representatives have survived to the present, as well as Elpistostegalia — a group of fishes that is closest to tetrapods.
Thus we use the following system of lopatoperi (taxa highlighted in brick colour are those that must be clearly visualized by second‑year students).
Subclass “Sarcopterygii” — Lopatoperi
Infraclass Actinistia
Order Coelacanthiformes — coelacanth‑like
Family Latimeriidae — latimeriids
†Infraclass Onychodontida
Infraclass Dipnomorpha — dipnomorphs
†Superorder Porolepimorpha
Superorder Dipnoi — lungfishes
Order Ceratodontiformes — dipnoan‑like
Family Neoceratodontidae — neoceratodontids
Family Lepidosirenidae — lepidosirens
Family Protopteridae — protopterids
†Infraclass Rhizodontida
†Infracl ass Osteolepidida
†Infraclass Elpistostegalia
Note that, according to the presented phylogenetic scheme, the name Rhipidistia can be used for representatives of the infraclasses Dipnomorpha, Rhizodontida, Osteolepidida and Elpistostegalia. Accordingly, modern lungfishes can also be considered rhipidistians, but we will use this name only for the Paleozoic members of the mentioned infraclasses.
Features of lopatoperi
To understand the conditions to which lopatoperi fishes are adapted, it is necessary to establish how their fins differ from the fins of ray‑finned fishes.
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Although the schematics shown on the slide are simplified, they depict two polar variants of fin construction. In one case the musculature that drives fin movement is located outside the body contour, in the other – inside it. The lopatoperi fin provides much stronger strokes, whereas the ray‑fin is more precisely controlled. The lopatoperi fin is optimal for pushing off a substrate with force, while the ray‑fin is suited for producing delicate movements that enable the fish to navigate through algal thickets or among tangled coral reefs.
The origin of lopatoperi is probably linked to shallow waters rich in organic matter. It was in such environments that ambush predators lived, some of which later migrated to open sea or switched to feeding on invertebrates with dense coverings.
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Living in shallow, organic‑rich water caused these fishes to experience oxygen shortage. This shortage was compensated by a natural mechanism also demonstrated by many modern fishes. Aquarists are well aware of various gouramis and catfishes that gulp air bubbles and rise to the surface. Paleozoic fishes did the same.
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The air bubble swallowed by the fish ends up in a specific section of the esophagus. In modern fishes this is simply a highly vascularized expansion. In Paleozoic fishes it transformed into lungs.
In addition to facilitating gas exchange, the lungs helped maintain body position in the water column by performing a hydrostatic function. When lung‑bearing fishes transitioned to marine life, the lungs became a swim bladder.
Another peculiarity of lopatoperi fishes is the presence of choanae – internal nostrils (modern latimeriids have lost them a second time). Most ray‑finned fishes retain external nostrils (presumably a more primitive condition). In most ray‑finned fishes both the inlet and outlet nostril openings are on the surface of the head. Water passes through the olfactory sac during swimming.
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In lopatoperi the outlet opening pierces the palate and opens into the oral cavity. Thus water can enter the oral cavity (and subsequently the pharynx and gills) not through the mouth but via the olfactory sac and choanae.
The composition of the group whose members are called rhipidistians is controversial. We will use this name for that Paleozoic lopatoperi group that is closely related to the ancestors of tetrapods. Typical rhipidistians can be considered members of the order Osteolepiformes, which belonged to the infraclass Osteolepidida. These were ambush predators with a torpedo‑shaped body about 1 m long, known from the Middle Devonian to the Early Permian.
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The best‑known genus among them is Eusthenopteron.
The teeth of typical rhipidistians indicate adaptation for feeding on large prey, probably comparable in size to the fishes themselves. To strengthen the teeth against breakage, rhipidistians could not increase the thickness of the enamel layer (enamel contains no living cells and cannot be formed as a thick layer), but they developed folds in the enamel. Possibly, grooves between these folds allowed poisonous saliva to flow onto the prey (this makes sense when the prey is very large and strong relative to the predator). Note: one of the teeth bears a harpoon‑like hook that prevents the prey from escaping!
Such teeth are called labyrinthodont; the labyrinthodonts (a subclass of amphibians) inherited them from rhipidistians.
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Another remarkable feature of rhipidistians was the division of their axial skull into two blocks that were movable relative to each other: the otico‑occipital and the ethmoidal. The notochord passed through the otico‑occipital part and entered a cavity in the ethmoidal part; these two sections were joined by a joint, and their mutual movements were powered by a specialized musculature.
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What was the purpose of this mechanism? To open the mouth by raising the upper jaw rather than lowering the lower one (in shallow water this can be advantageous, preventing the lower jaw from “plowing” the substrate). To pump water through the oral cavity and gills by smooth joint movements within the axial skull, without creating sudden water disturbances that potential prey could detect via the lateral line.
Osteolepidida are the ancestors of Elpistostegalia, which included highly specialized “semi‑terrestrial” fishes. Today three genera of this group are known: Elpistostega, Panderichthys and Tiktaalik. The latter two are the more studied and popular.
Panderichthys – a slightly dorsoventrally flattened fish adapted to life in shallow waters and probably to crawling over terrestrial patches.
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On land the somewhat flattened body of Panderichthys allowed it to crawl by bending laterally.
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Even more adapted for crawling is Tiktaalik. Its relatively recent discovery in Alaska…
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… delighted many scientists, inspiring even a cartoon in which Darwin embraces the fish.
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Tiktaalik possessed a crocodile‑like elongated body and fins whose skeleton resembles the limb skeleton of tetrapods.
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It is likely that this animal was well suited for terrestrial locomotion.
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According to many views, Tiktaalik was close to our ancestors. The discussion of this will be in the next lecture; here we will review modern lopatoperi fishes.
Infraclass Actinistia
Order Coelacanthiformes — coelacanth‑like
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Well represented in the fossil record from the Devonian to the Cretaceous, the group is mostly marine. Nine families of coelacanths are distinguished, eight of which are known only from fossils.
In 1938 Marjorie Courtenay‑Latimer, curator of the museum in East London, South Africa, examined a fishermen’s catch taken in the ocean near the mouth of the Chalumna River. She found an unusual fish that she could not identify. Latimer sent the specimen for model making and wrote a letter about it to ichthyologist James Leonard Bryce Smith. Below is a drawing from her letter.
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Smith recognized the coelacanth imprint, such as this Triassic specimen. Remarkably, from Latimer’s drawing he was able to identify a coelacanth!
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The species described by Smith was named Latimeria chalumnae. After many years of searching, Smith established that this fish lives near the Comoros Islands off eastern Africa. At the end of the 20th century another species was described from the Indian Ocean – Latimeria menadoensis. It inhabits the waters around Sulawesi in Indonesia.
As can be seen, the latimeriid is not very different from Triassic coelacanths.
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The next video shows a living, albeit slightly sleepy, latimeriid.
Lobe‑like fins help the latimeriid “walk” on a rocky seabed, but former coelacanths, as can be inferred from their body shape, led a different lifestyle.
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Infraclass Dipnomorpha — dipnomorphs
Superorder Dipnoi — lungfishes
Order Ceratodontiformes — dipnoan‑like
Numerous fossil remains from the Lower Triassic … and six extant species belonging to three genera and three families.
Lungfishes are a group of specialized sclerophagous animals (those that feed on hard food). Their teeth have become crushing plates that efficiently break crustacean shells and mollusk shells.
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In Australia lungfishes are common, in Africa – Protopteridae, and in South America – Lepidosirenidae.
Suborder Ceratodontoidei — dipnoan‑like
Family Neoceratodontidae — neoceratodontids
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Covered with large scales, these fishes have a single lung. Their fins are lobed, typical of lopatoperi. Neoceratodus forsteri, the Australian lungfish, is a large (up to 175 cm), sluggish fish that feeds on invertebrates. When Australian water bodies dry out and become foul ponds, the lungfish survives by breathing atmospheric air. Unlike members of the second suborder, it cannot enter aestivation during drought.
In this video neoceratodes swim near the feet of David Attenborough.

Next to it are shown the Devonian Dipterus and the modern Neoceratodus.
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Suborder Lepidosirenoidei — lepidosiren‑like
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Very slender. Body elongated, paired fins paddle‑like, used mainly as oars. They inhabit waters that completely dry out in summer. They survive drought by burrowing into the substrate and forming a protective cocoon from dried mucus.
Family Lepidosirenidae — lepidosirens
One species, Lepidosiren paradoxa, occurs in Brazil and Paraguay. Body is even more elongated than in the next family, reaching up to 125 cm. Small scales are embedded in the skin.
Family Protopteridae — protopterids
Four species in Africa. Maximum length up to 1.8 m. A fish excavated from dried ground gradually re‑absorbs water and revives, as shown in this video where David Attenborough extracts a protopterid.

In modern fauna lungfishes are the closest relatives of tetrapods. However, it should be noted that the emergence of a new superclass was somewhat more complex than in this video, where a lungfish seemingly easily transforms into a tailed amphibian.

In the current fauna, lungfishes are the closest relatives of tetrapods. However, it should be noted that the emergence of the new superclass was somewhat more complex than shown in this video, where a lungfish somehow easily transforms into a tailed amphibian.