Cambrian lobopodians shed light on the origin of the tardigrade body plan

Document Type

Article

Publication Date

7-11-2023

Subject Area

Animals; Tardigrada (genetics); Phylogeny; Arthropods; Biological Evolution; Invertebrates; Fossils

Abstract

Phylum Tardigrada (water bears), well known for their cryptobiosis, includes small invertebrates with four paired limbs and is divided into two classes: Eutardigrada and Heterotardigrada. The evolutionary origin of Tardigrada is known to lie within the lobopodians, which are extinct soft-bodied worms with lobopodous limbs mostly discovered at sites of exceptionally well-preserved fossils. Contrary to their closest relatives, onychophorans and euarthropods, the origin of morphological characters of tardigrades remains unclear, and detailed comparison with the lobopodians has not been well explored. Here, we present detailed morphological comparison between tardigrades and Cambrian lobopodians, with a phylogenetic analysis encompassing most of the lobopodians and three panarthropod phyla. The results indicate that the ancestral tardigrades likely had a Cambrian lobopodian-like morphology and shared most recent ancestry with the luolishaniids. Internal relationships within Tardigrada indicate that the ancestral tardigrade had a vermiform body shape without segmental plates, but possessed cuticular structures surrounding the mouth opening, and lobopodous legs terminating with claws, but without digits. This finding is in contrast to the long-standing stygarctid-like ancestor hypothesis. The highly compact and miniaturized body plan of tardigrades evolved after the tardigrade lineage diverged from an ancient shared ancestor with the luolishaniids.

Publication Title

Proceedings of the National Academy of Sciences of the United States of America

Volume

120

Issue

28

First Page

e2211251120

Digital Object Identifier (DOI)

10.1073/pnas.2211251120

PubMed ID

37399417

E-ISSN

1091-6490

Language

eng

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