Phylogeny of the Ordovician and Silurian members of the order Atrypida

The brachiopod order Atrypida originated in the Middle Ordovician and went
extinct in the Late Devonian. Few cladistic studies have been undertaken
for this group. Here we investigate their early evolution through the
Silurian time. We present a parsimony-based phylogenetic analysis of 41
characters and 70 genera representing all the early taxonomic groups
within the order. The stratigraphic record of the fossil genera analysed
here strongly supports the parsimony phylogeny. Most currently recognized
subfamilies and families may be identified within clades, except for the
Atrypinae, Idiospirinae, and the Septatrypidae. This warrants subdivision
of the former and redefinition of the two latter. The subfamily Atrypinae
is redefined and a new subfamily of the Atrypidae, the Protatrypinae is
split off. Tracing character evolution across the tree indicates that
calcified spiralia and a jugum or jugal processes characterize all members
of the Atrypida, except the most basal clade, the Cyclospiridae, that
lacks a calcified jugum. Ribs are a homoplasious character, occurring in
numerous clades throughout the tree. Darriwillian through Sandbian was a
time with rapid evolution when most new autapomorphies seen in the
atrypides evolved. This interval is part of the Great Ordovician
Biodiversification Event (GOBE). Solid teeth, dorsal direction of
spiralia, elaborate ornamentation, and frills evolved in the latest Katian
into early Silurian time. Solid teeth and dorsally directed spiralia, seen
in distantly related clades from the latest Katian on, may be analogous
features. The Late Ordovician Mass Extinction (LOME) is indicated in the
stratigraphic tree together with a possible event at the end of Aeronian.
The tree supports the long-held assumptions that the Plectatrypinae
evolved from the Spirigerininae. It further suggests that the genus
Tuvaella is included with the Davidsonioidea. Likewise, the redefined
Atrypinae is a sister group of the Lissatrypidae and a derived group in
the phylogenetic tree.

腕足动物（Brachiopod）的阿特里贝目（Atrypida）起源于中奥陶世（Middle Ordovician），并于晚泥盆世（Late Devonian）绝灭。针对该类群的分支分类学（cladistic）研究较为匮乏。本文通过志留纪（Silurian）时段探究其早期演化历程：我们对该目内所有早期分类类群的70个属、41个性状开展了基于简约法的系统发育分析。本次研究所涉及的化石属的地层记录，极大程度支持了该简约法系统发育树的结果。除阿特里贝亚科（Atrypinae）、异旋贝亚科（Idiospirinae）以及隔阿贝科（Septatrypidae）外，当前多数已被认可的亚科与科均可在演化支（clade）中得到对应。这一结果支持对前者进行细分，并对后两者重新定义。本文重新厘定了阿特里贝亚科，并从阿特里贝科（Atrypidae）中分出一新亚科——原阿特里贝亚科（Protatrypinae）。对系统发育树内性状演化的追踪显示：除最基干的演化支旋环贝科（Cyclospiridae）缺乏钙化腕锁外，阿特里贝目所有类群均具备钙化腕螺与腕锁或腕锁突。壳纹属于同塑性状（homoplasious character），在演化树的多个支系中独立出现。达里威尔阶（Darriwillian）至桑比阶（Sandbian）为快速演化时期，阿特里贝类多数新的自衍征（autapomorphy）均在此阶段产生，该时段属于奥陶纪生物大辐射事件（Great Ordovician Biodiversification Event, GOBE）的组成部分。坚固的齿、腕螺指向背方、精细的壳饰以及饰褶在卡皮坦阶（Katian）晚期至志留纪早期逐步演化形成。自卡皮坦阶晚期以来，在远缘演化支中出现的坚固齿与背向腕螺可能属于趋同演化特征。奥陶纪末大灭绝事件（Late Ordovician Mass Extinction, LOME）以及阿伦尼格阶（Aeronian）末期的潜在灭绝事件均在地层树中有所体现。系统发育树支持长期以来的学术假说：褶阿特里贝亚科（Plectatrypinae）起源于旋脊贝亚科（Spirigerininae）。本文进一步表明图瓦贝属（Tuvaella）应归入戴维逊贝超科（Davidsonioidea）。同样，重新厘定后的阿特里贝亚科是丽阿贝科（Lissatrypidae）的姊妹群，且为系统发育树中的衍生类群。