Search results
Results From The WOW.Com Content Network
Agnatha (/ ˈ æ ɡ n ə θ ə, æ ɡ ˈ n eɪ θ ə /; [3] from Ancient Greek ἀ-(a-) 'without' and γνάθος (gnáthos) 'jaws') is a paraphyletic infraphylum [4] of non-gnathostome vertebrates, or jawless fish, in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct (conodonts, anaspids, and ostracoderms, among others).
Cyclostomi, often referred to as Cyclostomata / s ɪ k l oʊ ˈ s t ɒ m ə t ə /, is a group of vertebrates that comprises the living jawless fishes: the lampreys and hagfishes.Both groups have jawless mouths with horny epidermal structures that function as teeth called ceratodontes, and branchial arches that are internally positioned instead of external as in the related jawed fishes. [1]
The agnathans as a whole are paraphyletic, [13] because most extinct agnathans belong to the stem group of the gnathostomes, the jawed fish that evolved from them. [14] [15] Molecular data, both from rRNA [16] and from mtDNA [17] strongly supports the theory that living agnathans, known as cyclostomes, are monophyletic. [18]
Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the Early Silurian, and flourished until the Late Devonian extinction, [17] where most species, save for lampreys, became extinct due to the environmental upheaval during that time.
Cyclostomes were regarded as either degenerate cartilaginous fishes or primitive vertebrates. Cope (1889) [11] coined the name Agnatha ("jawless") for a group that included the cyclostomes and a number of fossil groups in which jaws could not be observed. Vertebrates were subsequently divided into two major sister-groups: the Agnatha and the ...
Traditionally, cyclostomes have been divided into two groups according to the skeletal organization. In free-walled (or double-walled) cyclostomes, the exterior frontal walls of the zooids are uncalcified; autozooids have either a polygonal aperture bounded by vertical interior walls, or a subcircular aperture in species with kenozooids filling the spaces between the autozooids.
Agnatha. The first vertebrates ("fish") appear: the Agnathans. They were jawless, had seven pairs of pharyngeal arches like their descendants today, and their endoskeletons were cartilaginous (then only consisting of the chondro cranium/braincase and vertebrae). The jawless Cyclostomata diverge at this stage.
Cyclostome is a biological term (from the Greek for "round mouth") used in a few different senses: . for the taxon Cyclostomi, which comprises the extant jawless fishes: the hagfish (Myxini) and the lampreys (Petromyzontidae).