Friday, June 12, 2020

[Paleontology • 2020] Dannychaeta tucolus • A Cambrian Crown Annelid Reconciles Phylogenomics and the Fossil Record

Dannychaeta tucolus 
Chen, Parry, Vinther, Zhai, Hou & Ma, 2020

The phylum of annelids is one of the most disparate animal phyla and encompasses ambush predators, suspension feeders and terrestrial earthworms. The early evolution of annelids remains obscure or controversial, partly owing to discordance between molecular phylogenies and fossils. Annelid fossils from the Cambrian period have morphologies that indicate epibenthic lifestyles, whereas phylogenomics recovers sessile, infaunal and tubicolous taxa as an early diverging grade. Magelonidae and Oweniidae (Palaeoannelida1) are the sister group of all other annelids but contrast with Cambrian taxa in both lifestyle and gross morphology. Here we describe a new fossil polychaete (bristle worm) from the early Cambrian Canglangpu formation that we name Dannychaeta tucolus, which is preserved within delicate, dwelling tubes that were originally organic. The head has a well-defined spade-shaped prostomium with elongated ventrolateral palps. The body has a wide, stout thorax and elongated abdomen with biramous parapodia with parapodial lamellae. This character combination is shared with extant Magelonidae, and phylogenetic analyses recover Dannychaeta within Palaeoannelida. To our knowledge, Dannychaeta is the oldest polychaete that unambiguously belongs to crown annelids, providing a constraint on the tempo of annelid evolution and revealing unrecognized ecological and morphological diversity in ancient annelids.

Holotype specimen YKLP 11382 of Dannychaeta tucolus.
 Part imaged using direct light. d, Anterior region of the part.
Scale bars = 2 mm. 

Dannychaeta tucolus, YKLP 11383b. Abdominal region within a dwelling tube.
Scale bars = 4 mm. 

Annelida Lamarck, 1809

Palaeoannelida Weigert & Bleidorn, 2016

Magelonidae Cunningham & Ramage, 1888

Dannychaeta tucolus gen. et sp. nov.

Etymology. Danny refers to Danny Eibye-Jacobsen, for his contributions to our understanding of early annelids; chaeta (Latin), bristle; tubus (Latin), tube; colus (Latin), dwelling in.

Holotype. YKLP 11382 part and counterpart (YKLP, Yunnan Key Laboratory for Palaeobiology)  

Horizon and locality. Canglangpu Formation, Cambrian stage, Hongjingshao Member (around 514 million years ago), at southwest of Guanshan reservoir, Chenggong, Kunming, China.

Diagnosis. Elongated, slender polychaetes with organic dwelling tubes. Head with anteriorly tapering spade-shaped prostomium, with paired palps attached ventrolaterally near the mouth. Body heteronomously segmented, with a wider thorax containing at least eight chaetigers. Parapodia with lateral lamellae in the posterior part of the abdomen. Abdominal parapodia biramous, unknown in thorax. Capillary chaetae in both rami, occurring in tight parallel bundles.

Life reconstruction showing Dannychaeta tucolus living in buried tubes.
Artwork by R. Nicholls. 

Hong Chen, Luke A. Parry, Jakob Vinther, Dayou Zhai, Xianguang Hou and Xiaoya Ma. 2020. A Cambrian Crown Annelid Reconciles Phylogenomics and the Fossil Record. Nature. DOI: 10.1038/s41586-020-2384-8