Abstract
Agnatha are archaic vertebrates that appeared in the Earth’s biosphere in the early Paleozoic (Ordovician), earlier than fishes. The paper presents a reconstruction of the exterior of different groups of Agnatha and provides their general morphological characteristics. The internal structure of Diplorhina is analyzed in detail, including the endoskeleton, the olfactory apparatus, the brain and cranial nerves, and the respiratory system. The possibility of the transition from the endobranchial system of Agnatha to the ectobranchial system of the Gnathostomata is substantiated. A conclusion is made about the fundamental similarity of ontogenetic pathways in Diplorhina and Gnathostomata (sharks). The explanation of the structure of Heterostraci (Agnatha: Diplorhina) following the morphological pattern of hagfishes (Cyclostomata) is shown to be incorrect. It is shown that the phylogenetic proximity of Gnathostomata and Osteostraci (Agnatha), widely accepted in current publications, is not supported by paleontological data. The significance of fundamental structural features in determining the proximity of taxa of the highest rank is substantiated. Based on fundamental characters, Gnathostomata can be connected only with Diplorhina (Heterostraci, Thelodonti). Main structural features of the divergent branches of Gnathostomata and Cyclostomata,including humans (a branch of Gnathostomata), are clearly traced at the morpho-evolutionary level of agnathans.
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Notes
In whales (Gnathostomata), the nasal organ opens with one nostril. But this is the result of a very complex specialization. Their olfactory organ is not associated with the hypophysis.
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Novitskaya, L.I. The Internal Structure of Agnatha and Origin of Vertebrates Including Humanity. Paleontol. J. 55, 1319–1407 (2021). https://doi.org/10.1134/S0031030121120029
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DOI: https://doi.org/10.1134/S0031030121120029