Chordate Zoology - Textbook - Y. V. Tsaryk - 2013

Chapter 4. DIVISION GNATHOSTOMATA, or ECTOBRANCHIATA. SUPERCLASS FISHES PISCES

4.2. CLASS BONY FISHES OSTEICHTHYES

4.2.3. Systematics of the Class of Bony Fishes and Characteristics of Representatives of Individual Orders

4.2.3.1. Subclass Lobe-finned Fishes Sarcopterygii

4.2.3.1.1. Superorder Tassel-finned Fishes Crossopterygimorpha

The neurocranium is divided into two parts: the olfactory and the cerebral proper. The Skull is amphistylic. Secondary jaws are well-developed, with strong, sharp Teeth. The Skeleton of the paired fins has a shortened central axis and a well-developed basal element at the base. They were freshwater predators that repeatedly invaded the seas.

Order Rhipidistiformes. The neurocranium ossifies (completely in some groups). Internal nares (choanae) are present. The fins have a powerful, fleshy base. They were likely already adapted to inhabiting various types of Water bodies in the Early Devonian. They possessed a complex system of lateral line canals on the HEAD. The Development of Lungs allowed them to survive in water bodies with poor oxygen conditions (due to an Abundance of decaying organic matter) and, during droughts, to crawl to other water bodies using their paired fins. It is believed that as early as the Middle Devonian, primitive amphibians branched off from one of the freshwater groups of rhipidistians. During the Carboniferous period, the abundance and diversity of rhipidistians declined sharply, and they went extinct in the Early Permian. This was driven by increased competition and direct pressure from other fish groups (coelacanths, lungfishes, and ray-finned Fishes, whose representatives colonized almost all types of water bodies during the Carboniferous) and, in particular, primitive amphibians, whose diversity was rapidly expanding.

Order Coelacanthiformes. Representatives of this order apparently branched off from primitive rhipidistians. Choanae are absent. The tail is diphycercal, with an additional middle lobe. The fins have strong but short bases and elongated lobes. The swim bladder is poorly developed. They inhabited freshwater bodies and invaded the seas during the Paleozoic. However, they were displaced by ray-finned fishes in the Triassic. Coelacanth remains are still found in the Jurassic and Cretaceous, but have not been detected in later strata. Consequently, they were considered an entirely extinct group.

In 1938, an unknown fish was caught off the coast of South Africa, which J. L. B. Smith identified as a coelacanth and named Latimeria chalumnae (Fig. 4.39). It turned out that coelacanths live in the northern part of the Mozambique Channel (between Madagascar and Africa) at depths of up to 300 m, leading a demersal lifestyle. The population of Latimeria is small: slightly over a hundred catches of this fish have been documented.

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Fig. 4.39. Coelacanth Latimeria chalumnae

The well-developed notochord has a thick, dense, and elastic fibrous sheath. The Brain occupies no more than one percent of the volume of the braincase, which is filled mainly with a fat-like substance. The swim bladder, similar to the lungs of lungfishes, arises from the ventral side of the Esophagus; it is short and tube-like, 5-8 cm in length, transitioning into a fat-surrounded cord. There is a spiral valve in the intestine and a conus arteriosus in The Heart. In a mature female weighing 78 kg, 19 eggs with a diameter of 8-9 cm and a mass of about 300 g each were found in the right Ovary (the left one is reduced); they are ovoviviparous. Latimeria is a typical relict species, and its survival to the present day in a restricted area of the World Ocean is the result of a coincidental combination of favorable conditions.