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Early animal evolution: a morphologist's view.

Claus Nielsen¹

¹The Natural History Museum of Denmark, University of Copenhagen, Zoological Museum, Universitetsparken 15, DK-2100 Copenhagen, Denmark.

Royal Society Open Science

|August 17, 2019

Summary

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Morphological analysis strongly supports the Porifera-first hypothesis for early animal evolution, suggesting sponges are the sister group to all other metazoans. This contrasts with the Ctenophora-first hypothesis, which implies significant character losses.

Area of Science:

Zoology
Evolutionary Biology
Phylogenetics

Background:

Recent phylogenomic studies debate early metazoan radiation hypotheses.
Key hypotheses include 'Porifera-first' and 'Ctenophora-first'.

Purpose of the Study:

To analyze morphological characters to resolve the controversy in early metazoan evolution.
To evaluate the implications of both 'Porifera-first' and 'Ctenophora-first' hypotheses.

Main Methods:

Comparative morphological analysis of key characters.
Evaluation of character homoplasies and losses under different phylogenetic scenarios.

Main Results:

The 'Porifera-first' hypothesis implies no major character losses or homoplasies.

Keywords:

animalia large molecules morphology ultrastructure

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The 'Ctenophora-first' hypothesis suggests independent evolution or loss of complex traits like digestive systems.

Morphological evidence does not support significant adaptive advantages for losses implied by the 'Ctenophora-first' hypothesis.

Conclusions:

Morphological data provide strong support for the 'Porifera-first' hypothesis.
The 'Ctenophora-first' hypothesis requires substantial, unparsimonious character evolution or loss.