Arandaspida is a taxon of very early, jawless

prehistoric fish The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fis ...

which lived during the

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

. Arandaspids represent the oldest known

craniates A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the m ...

, a proposed group of chordates that contain all chordates with a cartilage-derived skull (i.e.,

lampreys Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like su ...

, armored agnathans, and

gnathostome Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...

s), and

hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...

. The group represents a subclass within the class

Pteraspidomorphi Pteraspidomorphi is an extinct class of early jawless fish. They have long been regarded as closely related or even ancestral to jawed vertebrates, but the few characteristics they share with the latter are now considered as basal traits for al ...

, and contains only one order, the Arandaspidiformes. The oldest known genus of this group is ''

Sacabambaspis ''Sacabambaspis'' is an extinct genus of jawless fish that lived in the Ordovician period. ''Sacabambaspis'' lived in shallow waters on the continental margins of Gondwana. It is the best known arandaspid with many specimens. It is related to ...

'' found in

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...

Characteristics

The head armor of arandaspids is elongated, fusiform, with a rather flat dorsal shield, and a bulging ventral shield. In the anterior part of the dorsal shield are two closely set holes, which have been thought to be a paired pineal opening, but which are more likely the external openings of the endolymphatic ducts. The eyes, surrounded by a sclerotic ring, are housed in a notch at the anterior end of the dorsal shield. The nostrils are not clearly located, but may have been situated between the eyes. Ventrally, the ventral lip of the mouth is armed with long series of small oral plates which recall those of

heterostracan Heterostraci (Ancient Greek, ἕτερος+ὄστρακον "those itha different shell" i is pl. of -us is an extinct subclass of pteraspidomorph jawless vertebrate that lived primarily in marine and estuary environments. Heterostraci exi ...

s. The gill openings are probably numerous (more than 15) and minute. They opened between the diamond-shaped platelets which separate the dorsal from the ventral shield. The body is covered with rod-shaped scales arranged in chevrons, and the tail is probably pad-shaped and diphycercal. The dermal bones of arandaspids consist of aspidine (acellular bone) and are ornamented with oakleaf-shaped tubercles which seem to contain no dentine. The sensory-lines were housed in narrow grooves between the tubercles.

Taxonomy

Taxonomy based on the work of Mikko's Phylogeny Archive, Nelson, Grande and Wilson 2016 and van der Laan 2018. * Order † Arandaspidiformes Ritchie & Gilbert-Tomlinson, 1977 ** Family †Aseraspididae Halstead 1993 *** Genus †'' Aseraspis'' Dineley & Loeffler 1976 ** Family †Arandaspididae Ritchie & Gilbert-Tomlinson, 1977 orophoraspididae Halstead 1993; Sacabambaspidae*** Genus †'' Andinaspis'' Gagnier, 1991 non Ritchie & Gilbert-Tomlinson, 1977 *** Genus †''

Apedolepis ''Apedolepis'' (meaning "flat scale", from Greek ἄπεδοςi, ''apedos'', meaning 'even', 'flat', or 'level' and λεπίς, meaning 'scale') was an extinct genus of early jawless fish known from the Ordovician Stokes Formation of central Au ...

'' Young, 1997 *** Genus †'' Arandaspis'' Ritchie & Gilbert-Tomlinson, 1977 *** Genus †''

Areyongalepis ''Areyongalepis'' is a genus of prehistoric jawless fish that lived during the Ordovician period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Pe ...

'' Young, 2000 *** Genus †'' Pircanchaspis'' Erdtmann ''et al.'', 2000 *** Genus †'' Porophoraspis'' Ritchie & Gilbert-Tomlinson, 1977 *** Genus †'' Ritchieichthys'' Sansom ''et al.'', 2013 *** Genus †''

'' Gagnier, Blieck & Rodrico, 1986

References

Pteraspidomorphi Ordovician jawless fish Vertebrate subclasses Early Ordovician first appearances Late Ordovician extinctions {{ordovician-animal-stub de:Pteraspidomorphi#Arandaspida

Characteristics

Taxonomy

See also

References