Five longitudes in chordate body

Area of Science:

• Evolutionary biology
• Developmental biology
• Paleontology

Background:

• The evolutionary origin of chordates and their body plan remains a significant scientific debate.
• Current theories inadequately explain key chordate features beyond the dorsal central nervous system.
• Revisiting morphological data is crucial for interpreting new fossil evidence and identifying molecular targets for chordate origin research.

Purpose of the Study:

• To propose a novel hypothesis for chordate origins based on generalized vertebrate features.
• To introduce the concept of a pentaradiate stage in chordate evolution, linking it to echinoderms.
• To challenge the theory of dorsoventral inversion and propose a new ancestral subphylum, Pentachordata.

Main Methods:

• Generalization of previously underestimated vertebrate morphological features.
• Identification of five longitudinal lines in chordate trunks as representatives of ancestral pentaradiality.
• Morpho-functional analysis to reconstruct the proposed ancestral Pentachordata.

Main Results:

• Chordate evolution may have involved a pentaradiate stage, with five longitudinal lines forming the blueprint for ectoderm-mesoderm interactions.
• These lines can develop various outgrowths (e.g., fins, limbs) and invaginations (e.g., neural tube), supporting the pentaradial model.
• The hypothetical Pentachordata subphylum is characterized by five neural tubes and five muscular notochords, potentially explaining early chordate forms.

Conclusions:

• The study discourages the long-standing theory of dorsoventral inversion in favor of an ancestral pentaradial symmetry.
• Pentachordates are hypothesized as sessile, active sediment-feeders, with two problematic fossils tentatively assigned to this group.
• The Pentachordata hypothesis offers a new framework for understanding chordate evolution and suggests avenues for experimental verification.