Both of jawless and jawed fish belong to the sub-phylum Vertebrata, also call as Craniate. Like the name, they have backbones. Vertebrates are one of the most successful groups of the animal at present. The successfulness is estimated according to a few topics such as having numerous families and species, distribution throughout the planet and various types of habitats, and the ability to survive from harsh conditions. It is mainly focused on adaptations. Adaptation is the biological mechanism by which organisms adjust to a new environment or to change their current environment. There are 3 different types of adaptations; behavioural, physiological and structural. We all know that at present, vertebrates are the most successful among all the animals. There are a lot of behavioural, morphological, and physiological variations among subphylum Vertebrata and it calls having a great diversity. The main adaptation that leads to the great diversity of vertebrates are improved respiration, protective and insulating body coverings, more efficient reproduction (especially on land), and paired, muscular appendages (having bilateral symmetry). Vertebrates are also known as craniate. It means they have a head and good cephalization. It leads them to be good predators and helps to win the competition for food easily.
As mentioned before, the sub-phylum Vertebrata are divided into 2 groups call Agnatha and Gnathostomata. Agnatha is a group of primitive jawless vertebrates which includes the two groups as Hyperotreti and Hypercartia. As mentioned above, since there is a great diverse in subphylum Vertebrata, there is no such diverse in group Agnatha. There are only around 150 species in this group. They don’t have jaws. Instead of jawed mouths, they have circular mouths. Having two-chambered heart, external or covered gill slits, a single nostril, and complete cartilaginous skeleton are the main characteristics in group Agnatha. There are fossil forms and extant forms of jawless fishes. Ostracoderms are the example of fossil type. But, extant lampreys and hagfishes are more primitive than Ostracoderms. Most of the jawless fishes’ origin is located in the Cambrian and Ordovician time periods. But a lot of them were extinct in the late Devonian period due to the Devonian mass extinction except hagfishes, lampreys, and Conodontas. Before this mass extinction, they were much more diverse (400 million years ago). Conodonts also extinct in the late Triassic period so Lampreys and hagfishes are the only exist jawless fishes at present. But their diversity is very low.
Fig 02. Lamprey sp. ©NatgeoWild
Both of these two groups are carnivores and it means there should be an overlap between diets. It would have led to a competition for food against each other. Here, jawed fishes have won because of their fast-moving ability and stronger jawed mouths. They were able to bite harder because their bite force is much higher than jawless fish and also crushing shells. In conclusion, they have become good and active predators but not Agnathans. All in all, climate changes re the main reason for the extinction of a large part of jawless fishes and the low diverse at present. On other hand, the origin of jawed fish also was a major reason to occur above phenomena. Jawed fishes adapted well and they have been survived but Jawless fish have not.
Fig 04. A group of Hagfish feed on a fish ©Smithsonian
Fig 05. Lamprey feeds on fish ©NatgeoWild

Check this out: Reasons for the success of Sponges (Phylum Porifera)


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