Jaw elements in Plumulites bengtsoni confirm that machaeridians are extinct armoured scaleworms

Abstract

Machaeridians are Palaeozoic animals that are dorsally armoured with serialized, imbricating shell plates that cover or enclose the body. Prior to the discovery of an articulated plumulitid machaeridian from the Early Ordovician of Morocco that preserved unambiguous annelid characters (segmental parapodia with chaetae), machaeridians were a palaeontological mystery, having been previously linked to echinoderms, barnacles, tommotiids (putative stem-group brachiopods) or molluscs. Although the annelid affinities of machaeridians are now firmly established, their position within the phylum and relevance for understanding the early evolution of Annelida is less secure, with competing hypotheses placing Machaeridia in the stem or deeply nested within the crown group of annelids. We describe a scleritome of Plumulites bengtsoni from the Fezouata Formation of Morocco that preserves an anterior jaw apparatus consisting of at least two discrete elements that exhibit growth lines. Although jaws have multiple independent origins within the annelid crown group, comparable jaws are present only within Phyllodocida, the clade that contains modern aphroditiforms (scaleworms and relatives). Phylogenetic analysis places a monophyletic Machaeridia within the crown group of Phyllodocida in total-group Aphroditiformia, consistent with a common origin of machaeridian shell plates and scaleworm elytrae. The inclusion of machaeridians in Aphroditiformia truncates the ghost lineage of Phyllodocida by almost a hundred million years.

A Cambrian crown annelid reconciles phylogenomics and the fossil record

“…the prostomium-peristomium head structure 3 and typically pygidial cirri 12 ) and are interpreted as stem-group annelids 6 . The lack of Cambrian crown annelids has prompted hypotheses of relatively late crown group diversification, perhaps late Cambrian/Ordovician 4 , when jawed polychaetes become diverse and abundant 16 .…”

New insights into the morphology and evolution of the ventral pharynx and jaws in Histriobdellidae (Eunicida, Annelida)

“…Annelids, one of the largest groups of lophotrochozoan animals [1], exhibit a wide variety of lifestyles and body forms [1,2]. Although recent phylogenomic analyses provide a solid backbone concerning our understanding of character evolution within Annelida, our knowledge for many annelid families is still fragmentary, and evolutionary hypotheses are often highly controversial [1][2][3]. The reasons for many unresolved questions are the scarcely available morphological and molecular data regarding certain taxa and the lack of fossil records for annelids.…”

“…The reasons for many unresolved questions are the scarcely available morphological and molecular data regarding certain taxa and the lack of fossil records for annelids. The soft body is very poorly and rarely preserved in a fossil state, such that evolutionary conclusions based on fossilized remnants are hardly possible for many annelid taxa [3]. Hence, the bulk of the fossil annelid material is represented by scolecodontsthe elements of the jaw apparatuses of ancient annelids [3].…”

“…The soft body is very poorly and rarely preserved in a fossil state, such that evolutionary conclusions based on fossilized remnants are hardly possible for many annelid taxa [3]. Hence, the bulk of the fossil annelid material is represented by scolecodontsthe elements of the jaw apparatuses of ancient annelids [3]. Nevertheless, complete jaw apparatuses are extremely rare in the fossil record, and the vast majority of fossilized scolecodonts are preserved as separate jaw plates [3,4].…”

“…Hence, the bulk of the fossil annelid material is represented by scolecodontsthe elements of the jaw apparatuses of ancient annelids [3]. Nevertheless, complete jaw apparatuses are extremely rare in the fossil record, and the vast majority of fossilized scolecodonts are preserved as separate jaw plates [3,4]. Within annelids, Eunicida is one of the most prominent jaw-bearing taxa, and numerous analyses of anatomical features of the jaw apparatus have been published thus far.…”