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JAWLESS FISH
     Two jawless fish are known from the Early Cambrian.  Myllokumingia had a head and trunk, a dorsal fin, a ventral fin (that might have been paired), 5-6 gill pouches, around 25 muscle segments (myomeres), a pharynx, an intestine, a notochord, and perhaps a pericardial cavity.  With such features it is similar in complexity to the most primitive fish alive today, the hagfish, but perhaps slightly more advanced (Shu, 1999). 
MYLLOKUNMINGIA HAIKUOICHTHYS
     Haikouichthyes possessed a head and trunk, a dorsal fin with radial structural elements, a ventral fin (that might have been paired), 6-9 gill arches, cartilage in its head area, a pericardial cavity, an intestine, and possibly gonads.  Its gill skeleton implies as an embryo it possessed neural crest cells—a type of cell which forms a number of vertebrate head structures and which is not found in animals other than vertebrates.  It is classified with lampreys and fossil jawless fish such as Jaymoytius (Shu, 1999).
jaymoytius
lamprey
One group of fossil jawless fish, the cephalaspids, include the lineage of the modern lamprey.

     Of the great diversity of primitive jawless fish, only two types of jawless fish survive today: hagfish (also known as slime eels, about 60 species) and lampreys.   Both are very derived and are not equivalent to their Paleozoic ancestors.

Hagfish

hagfish
Lamprey
lamprey lamprey
lamprey larva
     Are hagfish and lampreys vertebrates?  Most people assume that all fish are vertebrates and possess a vertebral column.  However, in primitive fish, the notochord was not merely an embryonic structure, it formed the major axial support structure in adults as well.  The notochord remained a continuous support rod through various lineages of fish and even into early amphibians.   In hagfish and lampreys, the notochord is still the major support in the adult fish.  What about vertebrae?  As will be discussed later, the solid structures we refer to as vertebrae evolved gradually over hundreds of millions of years.  Vertebrae began as small pieces of cartilage around the notochord which would gradually fuse and eventually replace the notochord.  Hagfish do have cartilaginous rays in their tails, but since they are restricted to the tail they do not qualify as primitive vertebrae.  Hence, hagfish are fish but they are not vertebrates.  Since their brain is much more developed than that of more primitive chordates, the term “craniate” is often used to designate the group composed of hagfish and true vertebrates. 
hagfish tail
tail
  Lampreys are classified as vertebrates because they possess cartilaginous rays of cartilage running the length of their backs over the notochord.  These rays are referred to as neural arches and all vertebrates, including ourselves, possess at least this portion of vertebrae. 
lamprey
     Hagfish possess a number of other very primitive conditions such as their inability to regulate water movement in their bodies which results in an internal salt concentration which is the same as seawater.   Although modern hagfish lack eyes, fossil hagfish from the Carboniferous (Myxinikela) possessed well developed eyes (Bardack, 1991).
hagfish fossil
teeth notochord

Hagfish have three sets of sensory barbels around their mouths to feed on dead or dying fish.  Their oral muscles can move their keratinous teeth to bite and cut their food (Wisner, 1988).  In hagfish, the braincase is composed of fibrous tissue with a basicranial band of cartilage and cartilage around the otic capsules.  In lampreys, the cartilage of the head region is more extensive and surrounds the brain laterally.  Lampreys possess a few other advanced characteristics not found in hagfish such as extrinsic eye muscles, radial muscles in fins, and nerves affecting the heart.   Some fossil jawless fish possess advanced characteristics not found in lampreys such as a large dorsal jugular vein, an occipital region of the braincase, a vagus nerve which exits through the occipital region, pectoral fins, and a sclerotic ring of bones around the eye (Janvier, from Ahlberg, 2001).

 

CONODONTS

     What are conodont elements?  Conodont elements, such as those in the images below, have been extensively studied and have been very useful in relative dating.  Until recently, it was not known what type of organism produced conodont elements.

conodont element conodont element conodont element
conodont element conodont element
conodont

    Euconodont elements are now known to be the teeth of some of the most primitive known vertebrates most of which were about 3-10 cm long.  The few fossils which preserve the soft anatomy include eyes, and external eye muscles, trunk muscles with chevrons, and fin rays.  One genus, Promissum, might have reached 40 cm in length.  They had eyes, teeth with enamel and dentin, a notochord, a dorsal nerve chord, calcified cartilage, external eye muscles, cellular bone, and segmented muscle.  Their body shape was similar to that of the most primitive known fish, the hagfish. (Ohno, S., from Muller, 1998; Gabbott, 1995; Donoghue, 2000).  Conodonts had enamel and a calcified dermal skeleton (Smith, from Ahlberg, 2001).  The microwear on conodont teeth indicate that they were used to crush and shear food, suggesting that the first vertebrates were active predators (Purnell, 1995). 

     Conodonts are more advanced than either hagfish or lampreys but more primitive than other fossil jawless fish. The traits which seem to be present in conodonts and jawed vertebrates which are absent in lampreys and hagfish include an olfactory tract, larger cerebellum, pretrematic branches of branchial nerves, flattened spinal cord, and a vertical semicircular canal (Donoghue, 2000).

Lampreys and hagfish lack the biomineralized tissues found in jawed vertebrates, such as bone, enamel, and dentin.  The only biomieralization of phosphate in lampreys and hagfish is the gravity-detecting statoliths, although there may be some biomineralization of the endoskeleton in lampreys and there may be some crystals embedded in the keratin of hagfish teeth (Donoghue, 2000).

     Although lampreys and hagfish have “teeth” in their mouths, these structures lack enamel and dentin.  The proteins of vertebrate enamel are not expressed in any other parts of the body and their function is unique, even among other biomineralized tissues.  Amelogenin is the major protein in vertebrate enamel, composing 90% of the organic portion of enamel.  One of the exons of the amelogenin gene (exon 2) is homologous to an exon of proteins found in protostomes (osteonectin) and deuterosotmes (SC1, hevin, and QR1).  Enamel definitely existed in Ordovician ostracoderms while its existence in Cambrian euconodonts and fish such as Anatolepis is less certain (Delgado, 2001).

cladogram

JAWLESS FISH WITH BONE

     The first vertebrate bone fragments are known from the Cambrian.  These fragments are only a few square millimeters in size and differ from the bone of modern vertebrates.   The fragments have been identified as belonging to a jawless fish, named Anatolepsis.  Anatolepis was probably only lightly armored and inhabited continental shelves at about the same time as the conodonts did (Smith, 1996).  The first complete specimen of a jawless fish with bone, Arandaspis, is known from the Ordovician.

fish arandaspis
fish

     A great variety of jawless fish evolved in both marine and fresh-water environments.  Most were only a few centimeters long but some reached 2 meters in length.   Bone existed in the head region, forming large head shields while the postcranial skeleton was composed of cartilage.  No internal bone is known from jawless fish; the bone of these jawless fish was external to most tissues.  In some species, the bone seemed to form only after the fish had reached full size.  Some later groups had growth rings in their head armor, indicating that they did not have to wait until full size was reached for the head armor to develop. 

     Many had slit-like and scoop-like mouths that probably adapted them for either filter feeding or feeding on the detritus of the ocean floor.  In some groups, the eyes were reduced or even absent, supporting the idea that they lived in the mud of the ocean floor.  In some later species, the armor was very much reduced, perhaps as an adaptation for a more active swimming lifestyle.

fish

     A later group developed a “third eye”-- a light sensing organ on the top of the brain.  This pineal organ sat underneath an opening between the skull bones.  A few modern animals still have a light-sensing pineal organ that rests under an opening between the skull bones (such as the lamprey and the tuatara, a lizard).  Light-sensitive pineal organs are not used for “vision” (such as seeing a vulture flying overhead) but instead send information to the brain regarding the duration of daylight hours.  The majority of early amphibians and reptiles also have this pineal opening in the skull (called the pineal foramen).  Even though most modern vertebrates no longer use the pineal gland to directly detect light, pineal glands often retain the ability to respond to light and maintain a daily rhythm of activity (Carroll, 1988; Forey, 1993).

   A group called the thelodonts were jawless fish that possessed stomachs, suggesting that the evolution of stomachs preceded the evolution of jaws.   These thelodonts seem to have been active predators given their large, forked tails for active swimming, large eyes, and stomach (Wilson, 1993). 

thelodont
fish
shields
shield
models

 POLYPLOIDY IN THE EARLY VERTEBRATES

     Accidents happen.  In genetics, accidents involving the genetic code for a living organism are called mutations.  Some mutations involve chromosome number in an organism.  Organisms which are closely related to each other commonly differ in the numbers of chromosomes in the nuclei of their cells.  Most of these variations are caused by the fissioning of large chromosomes or the fusion of smaller chromosomes.  There are also instances in which the variation in chromosome number is caused when an organism inherits an entire extra set of chromosomes.  This polyploidy plays a major role in the evolution of new species of plants.  In animals (including humans), polyploidy embryos usually fail to thrive, being spontaneously aborted during development or dying shortly after birth.  There are, however, animals which are polyploid compared to closely related animals.  If a polyploid individual survives, it possesses a large number of additional genes.  Many of these duplicate genes are typically lost but others can evolve new functions, increasing the complexity of the descendants.  Genetic comparisons suggest that two polyploidy events occurred early in the history of the vertebrates: once during the interval between the early chordates and the craniate fish and a second event after the separation of the lamprey lineage but before the evolution of jawed fish.  Vertebrates, as a group, are polyploidy and owe much of their complexity to the new roles given to the duplicate genes which resulted from these genome duplications.