SPECIAL ICHTHYOLOGY - R. S. Pentyliuk - 2011

1. ORIGIN AND EVOLUTION OF CYCLOSTOMES AND FISHES. SYSTEMATICS OF RECENT CYCLOSTOMES AND FISHES. SUBPHYLUM CRANIATA. INFRAPHYLUM AGNATHA. CLASS CYCLOSTOMATA

1.1. Origin and evolution of cyclostomes and fishes

Cyclostomes and Fishes mark a new chapter in the evolution of higher animals. According to paleontological evidence, they are the first vertebrates to appear in the Lower Silurian, approximately 400–440 million years ago. However, fossil remains indicate that complexly organized agnathans and fishes already existed during this period, possessing a prehistory of their own. Based on theoretical models developed by paleontologists, ichthyologists, and evolutionary ecologists (Stensio, 1927, 1932, 1948; Romer, 1939, 1946, 1959, 1966; Severtsov, 1939, 1945, 1967; Obruchev, 1945, 1949, 1964; Colbert, 1969), cyclostomes and fishes emerged much earlier—somewhere in the Ordovician, 450–500 million years ago. Theoretical concepts, grounded in embryological studies of lower Chordates as well as modern cyclostomes and fishes, suggest that they trace their ancestry back to primitive acraniates (Acrania primitiva). The primary craniates (Protocraniota) are directly derived from them. These organisms likely led a benthic lifestyle in silt-laden waters and required adaptations to protect their gill slits and develop more stable, differentiated respiratory Organs. The formation of such respiratory organs occurred in two ways. The simplest method involved the proliferation of pharyngeal outgrowths to form endodermal gills. At an early stage of endodermal gill development, a more complex process began—the formation of ectodermal gills through the rupture of epidermal Skin folds in the HEAD region. Thus, the first protocraniates evolved along two distinct evolutionary pathways that gave rise to the so-called Entobranchialia and Ectobranchialia. These served as the ancestral branches for the formation of Agnatha and Gnathostomata, which subsequently developed in parallel (Severtsov, 1939).

By the end of the Ordovician and the beginning of the Silurian, the primary entobranchials gave rise to the Pteraspidomorpha and Cephalospidomorpha. Numerous fossil remains of the first branch have been discovered, representing various thelodonts (Telodontia) and heterostracans (Heterostraci). The former possessed a robust armor composed of horny plates, particularly on the anterior part of the body. The best-known representative of this group is Thelodus.

The second group (Heterostraci) featured small horny plates and spines scattered across the body (Pteraspis). Within the second branch, the Osteostraci were widely distributed, with well-known fossil representatives such as Cephalaspis from the Lower Devonian and Kiaeraspis from the Middle Silurian. Their heads were covered dorsally with a solid bony shield, or both the head and body were encased in relatively small bone plates. Unlike the pteraspidids, these coverings were composed of true bone.

In the third quarter of the Silurian, the Anaspida branched off from the osteostracans; these were small agnathans characterized by a slightly elongated, spindle-shaped body. The heads of anaspids were covered with small bony plates, while their bodies bore large scales. They inhabited freshwater bodies of the Silurian and potentially ventured into marine environments. The best-known genus from the Silurian is Bircenia. Many paleontologists and zoologists believe that modern cyclostomes (Cyclostomata) diverged from birceniids at the end of the Silurian.

Thelodonts survived until the Early Devonian, whereas heterostracans, osteostracans, and anaspids persisted until the Late Devonian. Fish remains in the form of scale fragments are known only from the Late Silurian. Deposits of the Lower Devonian have yielded the remains of several fish groups already, allowing us to confidently conclude that fishes appeared sometime in the Late Ordovician to Early Silurian.

The ectobranchials gave rise to a primitive group, the Protognatostomata, which directly ancestral to placoderms (Placodermi). The best-known Examples of this group include Dinichthys, Bothriolepis, Pterichthyodes, Gemnuendina, and Rhamphodopsis. They encompassed both small and large fishes, with some reaching lengths of up to 6 meters. Their heads and the anterior parts of their bodies were encased in a complex armor of bony plates, and their gill slits were protected by processes of the head shield. Paired fins were well developed. Likely as early as the Lower Silurian, the Aphetohyoidea (or Acanthodii) branched off from them. These fishes were small to medium-sized, inhabited rivers, and were presumably highly active. Their heads and bodies were covered with small bony plates and numerous scales resembling placoid scales. Acanthurids—widespread at the beginning of the Permian—became extinct by the middle of the Devonian. It is probable that two major fish lineages originated from some primitive jawed fishes: one with a primitive transverse Mouth and another with a complex jaw apparatus (Euristomi). The former gave rise to cartilaginous fishes (Chondrichthyes), and the latter to bony fishes (Osteichthyes). In Chondrichthyes, robust placoid scales developed in connection with their cartilaginous Skeleton, whereas in Osteichthyes, ganoid and bone scales predominantly formed. The group of cartilaginous fishes did not undergo extensive diversification. The earliest cartilaginous fishes (Proselachii) gave rise to a series of evolutionary branches.

During the Late Silurian and Early Devonian, the Stegoselachii emerged (becoming extinct in the Early Carboniferous), along with the Cladoselachii and Bradiodontia, which at the beginning of the Permian (with remains found in the Lower Triassic) gave rise to the Holocephali. In the middle of the Devonian, the Xenacanthida branched off from the cladoselachians, becoming extinct by the middle of the Permian, alongside the important group of elasmobranchs (Elasmobranchii), which split at the beginning of the Jurassic into sharks (Selachomorpha) and rays (Batomorpha).

During the Late Silurian, the Palaeonisci branched off from the jawed fishes. From the very beginning, this Lineage gave rise to the Choanisci, which directly ancestored the Rhipidistia. In the middle of the Devonian, these yielded the lungfishes (Dipneustomorpha), lobe-finned fishes (Crossopterygimorpha), and the branch leading to the first amphibians, the Ichthyostegalia. Rhipidistians became extinct at the beginning of the Permian. All ray-finned fishes originated from these same palaeoniscids. The Polypteriformes emerged in the middle of the Carboniferous; Amia and gars (Lepidostei) at the beginning of the Permian; chondrosteans (Chondrostei) in the middle of the Permian; and teleosts (Teleostei) at the beginning of the Triassic, experiencing a major radiation starting from the mid-Cretaceous. Palaeoniscids became extinct at the beginning of the Cretaceous.