In this article we will discuss about the modern concept on the phylogenetic status of cyclostomata.
In this history of vertebrate evolution the cyclostomes occupy a doubtful status. Diverse opinions exist on the relative status of the cyclostomes in the vertebrate series. Cyclostomes are generally regarded to be lower in structural organisation than the fishes (Gnathostoma) but higher than Branchiostoma (Acrania). They are further regarded to be the degenerative end products of some groups of ostracoderms.
Fig. 5.23 depicts the idea that the cyclostomes have originated from the ostracoderms. But E. Jarvik (1968) in his article “Aspects of Vertebrate Phylogeny” in the book entitled, “Current Problems of Lower Vertebrate Phylogeny” edited by Tororig has put forward a new approach to the phylogeny of the cyclostomes (Fig. 5.26). Jarvik has solid positive evidences in support of his notion.
Jarvik has advanced the idea that the cyclostomes and the gnathostoms are “sister groups”. They have evolved from a common ancestor (Fig. 5.24). This concept eliminates the long-standing traditional view that cyclostomes are more primitive than the gnathostomes. The gnathostomes possess many primitive features than the earliest recorded cyclostomes.
This fact establishes the primitiveness of the gnathostomes than the cyclostomes. They are tentatively regarded as the primitive sister group. The surviving as well as the extinct cyclostomes have close structural similarities which indicate a common ancestry.
Cyclostomes exist in two distinct groups, petromyzontids or lampreys and myxinoids or hagfishes. Despite fundamental similarities they exhibit many differences which have already been discussed. The differences are mainly limited to organ systems which can be interpreted as the consequence of separation over vast periods of time.
Stensio (1968) showed the presence of structural similarities of cephalaspids (Ostracoderm) with the petromyzontids and separated them from the myxinoids. Based on the similarities Stensio has placed the petromyzontids, cephalaspids and anaspids under Cephalaspidomorpha and has put forward the idea that these groups have independently evolved from primitive cephalaspidomorph.
Based on his work on “electrical field”, Jarvik (1968) regards that the petromyzontids stand close to cephalaspids than to anaspids where “electrical fields” are reported to be absent. It advances the idea that the petromyzontids and cephalaspids have evolved from a common ancestor independent of anaspids.
Lack of specializations of nasohypophysial complex in myxinoids separated them from petromyzontids as well as cephalaspids. It is suggestive that the myxinoids are possibly the descendants of some early cyclostomes (Palaeozoic era) other than the petromyzontids.
Resemblances between Heterostraci and Myxinoidea:
The Heterostraci agree with the Myxinoidea in the following features:
1. The visceral component of the definitive snout does not extend forwards-upwards in front of cranial components.
2. In majority of the forms the nasohypophysial aperture is either terminally or ventrally placed.
3. Presence of a biting non-suctorial oral apparatus in actively feeding forms.
4. Retrogressive development of eyes.
5. Lack of superficial and principal portions of branchial constrictor muscles. Absence of these muscles means that the pumping of water to the gills is caused by the action of powerful velum.
Remark:
Based on the above similarities, Stensio has expressed the view that the Heterostraci and Myxinoidea have similar organisation and they represent an old stock of cyclostomes the Pteraspidomorphi.
Recent Cyclostomes are Di-phyletic in Origin:
The cyclostomes are represented by Petromyzontia and Myxinoidea. In the adult stage of the groups the definitive snout is composed of two different components.
They are:
(i) A superior cranial part consisting of the ethmoidal part of cranial division of head and
(ii) An inferior visceral part comprising of foremost upper part of visceral portion of head. The inferior visceral part contains the components derived from the pre-mandibular arches of both sides.
The definitive snouts in Petromyzontia and Myxinoidea are of two different types. In Myxinoidea the two components of the definitive snout either extend forward the same distance or the visceral component is shorter than the cranial one. This causes the placement of nasohypophysial opening terminal/sub-terminal/ ventral.
This type of snout is also found in Heterostraci. But in Petromyzontia the inferior visceral component of the definitive snout has undergone excessive forward- upward growth to form the principal part of the snout.
The superior cranial component becomes greatly suppressed. This type of definitive snout is found in Osteostraci and Anaspida. The structural organisation and composition of the definitive snout are highly specialised and differs from that in gnathostomes.
The cyclostomes are actually diplorhinal vertebrates. Separate nasal capsules become fused to form a secondary unpaired nasal capsule. The internasal wall is represented by a thin partition composing of soft tissues. Each nasal sac has independent external opening.
The secondarily unpaired nasal capsule has paired external nasal opening separated by internasal septum—a feature homologous with that in fishes. Because of presence of paired nasal sac, the view of including the cyclostomes under the group Monorhini (monorhinal vertebrates) is excluded.
The myxinoids differ in many important respects from cephalaspids and petromyzontids and they lack specializations in nasohypophysial complex. The myxinoids are the descendants of early Palaeozoic cyclostomes other than the petromyzontids. In this sense it may be suggested that the recent cyclostomes are di-phyletic in origin.
The Myxinoidea and Heterostraci have many similar characteristics. But the Heterostraci lacks a nasal-pharyngeal duct as we see in Myxinoidea. It may, therefore, be suggested that the Heterostraci are closely akin to Myxinoidea and share a common ancestor. Many zoologists are of the opinion that myxinoids are related to Cephalaspidomorphs.
But serious objections are raised against the view. Absence of rasping tongue, difference in jaw apparatus and presence of paired olfactory nerves and olfactory organ in myxinoids exclude them from Cephalaspidomorphs.
Evidences from fossil records show that the typical anaspids and the cephalaspids existed in mid-silurian period. The anaspids and cephalaspids exhibit great differences and it may thus be concluded that the common ancestor of the Cephalaspidomorphs was definitely much older. Fossils of Heterostraci were recorded in lowermost to mid-Ordovician period.
This establish that the Heterostraci must have possibly existed in the Cambrian period or much earlier. The common ancestors of all cyclostomes have certainly preceded the common progenitors of both Cephalaspidomorpha and Pteraspidomorpha. It is apparent that the common ancestor of the gnathostomata and the cyclostomata must have existed earlier.