An echiuran from the Carboniferous of Mazon Creek and the evolution of spoon worms (Annelida: Thalassematidae)

Echiurans (Thalassematidae), the spoon worms, are an elusive group of unsegmented marine annelids with a convoluted taxonomic history. Their limited fossil record and ambiguous internal relationships present a challenge for reconstructing echiuran evolution. Here we describe the first unequivocal fossil member of the extant spoon worm subfamily Bonelliinae, and the second spoon worm fossil from the Palaeozoic. Mazobonelliia chenyangcaii gen. et sp. nov. from the middle Pennsylvanian Mazon Creek in Illinois possess a characteristic elongated proboscis with a bifid tip and, along with the only other previously known fossil echiuran Coprinoscolex ellogimus, documents that crown spoon worms diversified by the Carboniferous. The most extensive molecular dataset for echiurans published to date sampling five genes for 58 ingroup taxa was reanalysed using a better-fitting site-heterogeneous model to re-evaluate the phylogeny of the group and patterns of character evolution. Thalassematini was recovered a monophyletic, but Echiurini in its current state is paraphyletic and forms a clade with Bonelliinae. A revised classification of spoon worms is proposed, elevating Urechini stat. nov. and Echiurini sensu. nov. to tribal rank placed within a revised Bonelliinae sensu nov.螠虫动物（Echiurans，俗称勺蠕虫spoon worms）是一类行踪隐秘的不分节海洋环节动物，其分类历史错综复杂。该类群化石记录稀缺、内部系统发育关系模糊不清，为重建螠虫演化历程带来了显著挑战。本研究描述了首个确凿无误的现生博内利虫亚科（Bonelliinae，隶属于勺蠕虫类群）的化石标本，同时也是第二件古生代（Palaeozoic）勺蠕虫化石。产自美国伊利诺伊州宾夕法尼亚纪中期马宗溪（Mazon Creek）化石群的新属新种*Mazobonelliia chenyangcaii* gen. et sp. nov.，具有特征性的细长吻部且吻端分叉；结合此前仅知的另一件螠虫化石*Coprinoscolex ellogimus*，本研究证实冠群勺蠕虫在石炭纪（Carboniferous）时期就已发生分化。本研究对截至目前发表的规模最大的螠虫分子数据集进行了重新分析：该数据集涵盖58个内群类群的5个基因位点，研究团队采用拟合度更优的位点异质性模型，以重新评估该类群的系统发育关系与性状演化模式。分析结果显示，海猬族（Thalassematini）为单系群，但当前定义的螠族（Echiurini）为并系群，且与博内利虫亚科（Bonelliinae）构成一个演化支。本研究提出了修订后的勺蠕虫分类系统，将新等级的Urechini族（Urechini stat. nov.）与重新定义的螠族（Echiurini sensu nov.）提升至族级分类阶元，并归入修订后的博内利虫亚科（Bonelliinae sensu nov.）。