Is Cyclocardia (Conrad) a wastebasket taxon? Exploring the phylogeny of the most diverse genus of the Carditidae (Archiheterodonta, Bivalvia)

The carditid genus Cyclocardia is currently the most diverse genus of the
family, including nearly 180 nominal species from a wide stratigraphical
(Cretaceous–Recent) and geographical range (Antarctica, South and North
America, Europe, Africa, Alaska, Russia, Japan, and New Zealand). Due to
the lack of autapomorphies in the diagnosis of the genus and its large
account of species, we re-evaluated the systematic and phylogenetic status
of Cyclocardia. We carried out three approaches: bibliographic revision,
phylogenetic analysis, and morphological disparity exploration. We used a
shell-character matrix including 65 taxa (two outgroups, 29
non-Cyclocardia carditids, and 28 species of Cyclocardia) for phylogenetic
and disparity analyses. The morphological disparity analysis was conducted
using Maximum Observable Rescaled Distances for calculate distance matrix,
and comparing Cyclocardia species and other carditid groups. According to
our results, Cyclocardia is a ‘wastebasket taxon‘, chiefly because its
diagnosis was based mainly on plesiomorphic characters. Also, Cyclocardia
represents a non-monophyletic taxon. The European species C. kickxi and C.
chameformis are placed within Scalaricardita, and the previously proposed
genus Crassicardia is monophyletic (including C. crassidens, C.
crebricostata, C. isaotakii, and C. rjabininae). Three new genera are
proposed for new groups identified by the phylogenetic analysis: South
American Oesterheldia nov. gen. (including O. cannada and O. dalek),
Western North American Coanicardita nov. gen. (including C. ventricosa and
C. occidentalis), and North Pacific Hippocampocardia nov. gen. (including
H. barbarensis, H. hamiltonensis, and H. yakatagensis). The newly defined
monophyletic Cyclocardia is restricted to the Atlantic Ocean species C.
borealis, C. novangliae, and C. compressa.

圆帘蛤属（Cyclocardia）目前是心蛤科（Carditidae）中物种多样性最高的属，包含来自宽泛地层（白垩纪—现代）与地理分布范围（南极洲、南美洲、北美洲、欧洲、非洲、阿拉斯加、俄罗斯、日本及新西兰）的近180个定名物种。由于该属的鉴别特征缺乏独有衍征，且物种数量庞大，我们重新评估了圆帘蛤属的系统分类与系统发育地位。本次研究采用三种研究方法：文献修订、系统发育分析以及形态差异度探索。我们使用包含65个类群的壳形特征矩阵（其中外类群2个、非圆帘蛤属心蛤类29个、圆帘蛤属物种28个）开展系统发育与形态差异度分析。形态差异度分析采用最大可观测重标距离法计算距离矩阵，并对圆帘蛤属物种与其他心蛤类群进行比较。研究结果显示，圆帘蛤属属于"废物筐类群"，这主要是因为其鉴别特征主要基于近祖性状。同时，圆帘蛤属并非单系群。欧洲物种C. kickxi与C. chameformis被归入Scalaricardita属，而此前提出的厚心蛤属（Crassicardia）为单系群（包含C. crassidens、C. crebricostata、C. isaotakii及C. rjabininae）。基于系统发育分析结果，我们为三个新类群建立了新属（nov. gen.）：南美洲的Oesterheldia新属（包含O. cannada与O. dalek）、北美西部的Coanicardita新属（包含C. ventricosa与C. occidentalis），以及北太平洋的Hippocampocardia新属（包含H. barbarensis、H. hamiltonensis与H. yakatagensis）。重新界定的单系圆帘蛤属仅局限于大西洋物种C. borealis、C. novangliae及C. compressa。