title: The anatomy, affinity, and phylogenetic significance of Markuelia.
abstract: The fossil record provides a paucity of data on the development of extinct organisms, particularly for their embryology. The recovery of fossilized embryos heralds new insight into the evolution of development but advances are limited by an almost complete absence of phylogenetic constraint. Markuelia is an exception to this, known from cleavage and pre-hatchling stages as a vermiform and profusely annulated direct-developing bilaterian with terminal circumoral and posterior radial arrays of spines. Phylogenetic analyses have hitherto suggested assignment to stem-Scalidophora (phyla Kinorhyncha, Loricifera, Priapulida). We test this assumption with additional data and through the inclusion of additional taxa. The available evidence supports stem-Scalidophora affinity, leading to the conclusion that scalidophorans, cyclonerualians, and ecdysozoans are primitive direct developers, and the likelihood that scalidophorans are primitively metameric.
authors.affiliation: Department of Earth and Space Sciences, Peking University, Beijing 100871, China.
authors.forename: Xi-Ping
authors.initials: XP
authors.lastname: Dong
authors.forename: Philip C J
authors.initials: PC
authors.lastname: Donoghue
authors.forename: John A
authors.initials: JA
authors.lastname: Cunningham
authors.forename: Jian-Bo
authors.initials: JB
authors.lastname: Liu
authors.forename: Hong
authors.initials: H
authors.lastname: Cheng
mesh_terms: D000818:Animals; D005580:Fossils; D007448:Invertebrates / Q000196:embryology* / Q000648:ultrastructure; D010802:Phylogeny