Jawless Fish (Agnatha)
The earliest fish from the Cambrian and Ordovician periods lacked a lower jaw; such jawless fish are grouped in the superclass Agnatha (“no jaw”). Some of the agnathian families survived into the Devonian period. The first of the jawed fish (gnathostomes) developed in the Silurian period. Two groups of jawless fish comparable to Paleozoic fossil taxa remain alive today: hagfish and the lamprey.
constitute a single order called Petromyzontiformes.
lamprey is considered a
primitive marine vertebrate which may
one of the oldest extant groups of fish.
Lampreys are an order of
jawless, eel-like vertebrates, with a
Taxonomists place lampreys and hagfish in the subphylum
Vertebrata of the phylum Chordata. Lampreys
are not considered to be true fish, but represent very ancient lineage
vertebrates, possibly the most basal group of the Vertebrata. The
been extensively studied because its relatively simple brain is thought
respects to reflect the brain structure of early vertebrate ancestors.
Fig.1: External anatomy of Petromyzon, a sea lamprey
toothed, funnel-like sucking mouth. The common name "lamprey" is
derived from Latin lampetra ("stone licker" , from lambere "to
Pteraspida (“wing-shields”) is an abundant class of early jawless fish living from the Ordovician through Devonian periods (495-359 mya). They are characterized by massive dermal head armour, composed of large, median, ventral and dorsal plates or shields. Sometimes they are also covered by scutes or bony scales; these are the most common fossilized part to survive, and are therefore classified in some detail, to distinguish between taxa. Many Pteraspidans had hypocercal tails in order to generate lift to increase ease of movement through the water. They also had mouth parts used for sucking or eating, and some species may have lived in fresh water (Janvier 1998). Pterapsids are divided into three subclasses: Arandaspida, Astraspida, and Heterostraci, each with a number of taxa.
genus for this
subclass includes Arandaspis priontolepis
the oldest known vertebrates,
jawless fish that lived in the Early Ordovician period from 480 to 470
single known fossil was found at
Fig. 2: Arandaspis priontoleps, an Early Ordovician jawless fish from Australia (after Ritchie and Gilbert-Tomlinson 1977 ).
Arandaspis was about 15 cm (6 in) long, with a streamlined body covered in rows of knobbly armoured scutes. The front of the body and the head were protected by hard plates with openings for the eyes, nostrils and gills. Although it had no jaws, Arandaspis might have had some moveable plates in its mouth, serving as lips, sucking in food particles. The low position of its mouth suggests Arandaspis foraged the ocean floor. It had no fins; its only method of propulsion was its horizontally flattened tail. As a result, it probably swam in a fashion similar to a modern tadpole (Janvier 1998; White 2008).
Arandaspida also includes the South American genus Sacabambaspis, whose type species was found in Bolivia in a group of over 30 fossil specimens in a small area, probably caught in a mudslide or large storm (Gagnier et al. 1986). Other species have been found in Argentina, Oman, and Australia, showing a global distribution around the Ordovician land mass of Gondwanaland.
Fig. 3: Sacabambaspis janvieri, an Ordovician jawless fish from Sacabamba in Bolivia (after Gagnier et al. 1986).
Sacabambaspis (fig.3) had a head shield made from a large upper or dorsal plate that rose to a slight ridge in the midline, and a deep curved lower or ventral plate. Linking these along the sides were narrow branchial plates which covered the gill area. The rest of body was covered by long, strap-like scales behind the head shield.The tail consists of relatively large dorsal and ventral webs and an elongated notochordal lobe, a tail structure quite different from that of the Heterostracians (Pradel et al. 2013).
Sacabambaspis is also characterized by large, frontally positioned eyes. Between them are traces of two small nostrils, and a mouth armed with about 60 thin oral plates, whose function remains unclear. Like other Pteraspidans, Sacabambaspis had a lateral line system for sensing movements in the water, such as from predators (Gagnier et al. 1986).
the armored species Astraspis
desiderata (fig.4) from the
at ca. 450 mya. Examples have been
Fig. 4: Astraspis desiderata, an Ordovician jawless fish found in Colorado (after Sanson et al. 1997).
While no complete fossils of Astrapids have yet been found, adult forms are estimated to have been about 20 cm (8 in.) long. They had a mobile tail covered with small protective scales less than1 mm long, and a head region covered with larger scales, greater than 2 mm in length. Like Sacabambaspis, Astraspis shows clear evidence of having a lateral line sensory system, ennabling the fish to detect the direction and distance of movements in the water.
The Heterostracans also differed from other Paleozoic agnathan classes in the arrangement of their scales. Most heterostracans had two plates which form a large dorsal shield and a large ventral shield, and had series of scales arranged in various patterns on the sides of their bodies, the exact pattern differing from one group to another. In a few primitive forms, such as the genus Lepidaspis, the dorsal and ventral shields are composed of a mosaic of tiny scales. In most other known forms, however, these tiny scales have fused together to form the shield-plates typical of the Pteraspida.
As with many agnathan groups, heterostracans had no fins besides those of the tail (the caudal fin). In some pteraspids, especially in the psammosteids, the ends of the branchial plates which covered the gills were drawn out to form wing-like extensions.
The Pteraspidiformes are divided into five families, Anchipteraspididae, Protopteraspididae, Pteraspididae, Protaspididae, and the Psammosteidae.
The Anchipteraspididae (including the genera Anchipteraspis, Ulutitaspis,and Rhachiaspis) are small primitive forms from the Late Silurian and Early Devonian of the Canadian Arctic. They resemble the cyathaspidiforms in having only a single fused branchio- cornual plate, and lacking the separate plates that define all other pteraspidiforms. They are close to the ancestral stock from which other pteraspidiforms evolved.
Protopteraspididae are similar to the Pteraspididae. Some protopteraspidids, such as Doryaspis, have a sawfish-like snout or rostrum (formed by an elongated median oral plate). Doryaspis lived during the Early Devonian in northern Euromerica. Bony spines were set along the length of its rostrum, but unlike modern sawfish, the mouth of Doryaspis is situated above, rather than below, the rostrum. This purpose of this appendage is not clear. It may have been used to stir up the mud or sand at the ocean bottom, in order to find crustacea or other tiny invertebrates. Doryaspis, 15 cm in length, also had unusually long, lateral keels growing from the back of the head shield. The forward edges of these appendages were also armed with tooth-like spines.
The Pteraspididae are quite similar in overall morphology to the Protopteraspididae, but are distinguished by a supraorbital canal passing through the pineal plate (a feature shared in common with the Protaspididae and Psammosteida). It is likely therefore that the Pteraspididae evolved from the Protopteraspididae, rather than the reverse. Like the Protopteraspididae they were streamlined swimmers. One genus, Rhinopteraspis, resembles Doryaspis in having a very long rostrum and spinal plate, which in this case, is located in the normal position, above the mouth.
Pteraspis, the type genus and the most representative member of the family, was about 20 cm long, with a rostrum of more normal proportions. As with the Protopteraspididae, the rostrum may have served a dual purpose, both hydrodynamic while swimming, and used to probe the mud and sediment for small organisms. Significantly, the existence of an elongated rostral spine, and sometimes side spines or wings or cornua as well, seems to have been a recurring theme among the early Devonian ostracoderms.
Pteraspids became very
numerous and diverse during the Late Silurian and Early Devonian
paired fins, the pteraspids
were probably powerful swimmers. Stability
was provided by the wing-like outgrowths from the back of the head
A large spine over the back
as a kind
of dorsal fin, while 2 rigid 'wings' or keels functioned as pectoral
(Janvier 1997). The
flexible tail was also
hydrodynamic, with the lower lobe elongated to provide lift at the
front of the
body during swimming. Additional lift was provided by the elongated
which was drawn out into a bladelike 'rostrum', below which the mouth
It is thought that the
protopteraspids fed in mid-water or near the surface of the sea, among
shoals of planktonic, shrimp- like crustaceans.
The Pammosteida are the
youngest and largest
members of the “wing-shield” order. These
died out in the major
extinction event at the
end of the Devonian (359 mya). Among the possible ancestors of the
shaped fish whose fossils have been found in the fine-grained Devonian
Fig. 5: Drepanaspis gemuendensis, whose fossil was found in Devonian deposits along the Rhine River (after Elliot and Mark-Kurik 2005).
Cyathaspidiformes are divided into two suborders, the Amphiaspida of Early Devonian Siberia (grouped together with their relative Ctenaspis of Canada), and the Cyathaspidida (grouped together with their relative Nahanbiaspis.
The Anaspida (“no shield”) comprise another class of jawless fish who lived from the Early Silurian through Devonian periods (445-359 mya). All typical anaspids are Silurian in age (Janvier 1997). Anaspids are classically regarded as close relatives or ancestors of lampreys. However, all the characteristics which were considered as supporting this relationship also occur in some other fossil “ostracoderm” taxa. Only their slender body shape is somewhat suggestive of that of lampreys. Current analyses show anaspids mixed in with a heterogenous grouping of fish classes, largely due to the lack of information about their internal anatomy (Janvier 1997).
The exoskeleton of anaspids is made up of aspidine and contains no dentine. Some anaspids (e.g. Lasanius), however, are almost devoid of scales, retaining only postbranchial spines and a large median dorsal series of hook-shaped scute. Their endoskeleton was probably cartilaginous, and is not preserved. Some scale-less Silurian and Devonian forms often referred to as anaspids, including Jamoytius, Endeolepis, and Euphanerops, show imprints of a branchial skelton which may have been similar to the "branchial basket" of lampreys (Ritchie 1968; Janvier 1997).
Fig. 6: The Late Silurian Pharyngolepis, from the Baltic region (after Norsk Geologisk Forening).
More derived forms include the Rhyncholepididae and Birkeniidae, in which the anal fin is reduced or lost and the median dorsal ridge scales considerably enlarged. Rhyncholepis from the Late Silurian is also found in the area of southern
Fig. 7: Theolodont scales, from A) Paralogonia sp.; B) Shielia taiti; C) Lanarkia horrida (after Turner 1991).
Thelodont fossil specimens described in 1993 were the first to be found with a significant degree of three-dimensionality. Unexpectedly, these fossils revealed that these agnathians had stomachs; previously it had been thought that stomachs evolved after jaws. Distinctive fork-shaped tails, usually characteristic of the jawed fish (gnathostomes), were also found, further linking jawless and jawed fish to an unexpected degree.
Thelodonts were morphologically very similar, and probably closely related, to fish of the classes Heterostraci and Anaspida, differing mainly in their distinctive scales. The four orders of Theolodonts included Theleodontina, Loganiida, Katoporida, and Furcacaudiformes, comprising a total of 67 species.Theleodontina
Thelodontina lived in both freshwater and marine environments. They were predominantly bottom dwellers, although there is evidence that some species were also free-swimmers. They had paired pectoral fins combined with single, well-developed, dorsal and anal fins, a substantial innovation over the early Ordovician agnathians who had only tail fins. Thelodont fins resemble those of modern fish associated with both agile predation, and evasion.
Fig. 8: The Theolodont Loganellia, found in Scotland.Often the scale has linear ridges and spikes along its length, showing sometimes complex patterns, which together with the overall form of the scale are useful for identification. The scales are not necessarily a result of shared ancestry, however, but may have evolved independently by different groups. Thus the thelodonts may represent a polyphyletic or mixed-ancestry group, something much debated.
From the Late Silurian site
Fig. 9: The Late Silurian Lanarkia, found in Scotland..Cephalaspids
Cephalaspids (“head-shields”) are a class of armored, jawless fish who lived in the Silurian and Devonian periods (428-359 mya). Cephalaspis is the type genus this group, which contains a wide array of forms, including the large Parameteoraspis, with a headshield up to 45 cm wide. Their well-developed head shields served to protect the head, gills and anterior internal organs. The head shield had a series of grooves over its surface which formed an extensive lateral line organ, while their small eyes were located on top of their heads (fig.11). Outside of the head shield, no internal or axial skeleton is known; it probably consisted of an unsegmented notochord made of cartilage.
Fig. 10: The giant sea scorpion Pterygotus, a top predador of jawless fishes.
Increasingly, researchers now restrict the Cephalaspids to include only groups more clearly related to the Osteostraci, such as Galeaspida and Pituriaspida, taxa that were largely unknown in the 1920s (White 2008). Links among Osteostraci and Anaspida remain unclear, and the affinities of the lampreys are also contested. Evidence now suggests that lampreys acquired the characters they share with cephalaspids, such as the dorsal nostril, by convergent evolution (Janvier 2008). As such, many newer works about fishes classify lampreys in a separate group called Petromyzontida or Hyperoartia. Many researchers now define the Cephalaspids as a class with three subclasses, Osteostraci, Galeaspida, and Pituriaspida.
The Osteostraci (“bone shields”) include about 200 species which lived from the Early Silurian to the Late Devonian (ca. 430-370 mya). The bodies of osteostracans were covered with large, diamond-shaped scales. Sizes ranged from 1 m to 4 cm., with most osteostracans about 20 to 40 cm in length. Most have a characteristic horseshoe-shaped skull, covered with a shield of dermal bone.
On the dorsal or top surface of the head are closely-set eyes, a pineal foramen, and a keyhole-shaped nasohypophysial opening. An example is the small-sized Mimetaspis concordis, found in an Early Devonian strata exposed along the Dneister River in the Ukraine (fig.11.) In addition, there are peculiar depressions of the braincase, covered with loose platelets of dermal bone, regarded as either sense organs or electric organs. Some species also have hornlike processes protruding from the head shield. The mouth and gill openings are, as in the Galeaspida, on the lower or ventral side of the head. Osteostracans also have two large, pad-shaped dorsal fins (reduced to one or none in the more advanced forms). These dorsal fins contained muscles, and were attached to the the headshield (Janvier 1997).
Fig. 11: The headshield of Mimetaspis,
an small Early Devonian Cephalaspid from the
The Thyestiida are also small osteostracans, characterized by the special structure of their sensory-line canals. They include forms, the Tremataspididae, which have lost the paired fins and cornual process, their headshield becoming olive-shaped (fig.12A). This may represent an adaptation to burrowing habits.
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