A reexamination of <i>Milosaurus mccordi</i>, and the evolution of large body size in Carboniferous synapsids

<i>Milosaurus mccordi</i> was described in 1970 as a large, pelycosaurian-grade synapsid from the latest Carboniferous of Illinois but has since received little attention. Here, the holotype and referred material of <i>Milosaurus</i> are reexamined and incorporated into a phylogenetic analysis. <i>Milosaurus</i> is resolved within Haptodontiformes, sharing with this clade a posterodorsally expanded ischium and a calcaneum with greater length than width. The more plesiomorphic ischium indicates a basal position within this clade. Most of the referred material shows very little overlap with the holotype and so most was not included in the cladistic analysis. However, what was originally described as a dorsal rib is here judged to be a femur that shares a highly distinctive morphology with the holotype. With its mass estimated as 41 kg, <i>Milosaurus</i> represents one of the largest Carboniferous synapsids. Large size evolved at least twice independently in Haptodontiformes during the Pennsylvanian: once in Sphenacodontidae and once in Edaphosauridae. Using phylogenetic comparative methods, <i>Milosaurus</i> is found to have evolved a large size independently of these two clades. As one of the largest Carboniferous amniotes, and the outgroup to one of the most diverse Paleozoic synapsid lineages, <i>Milosaurus</i> represents a crucial taxon for understanding early terrestrial ecosystems.麦克氏米蜥（Milosaurus mccordi）于1970年被正式描述，是一种产自美国伊利诺伊州晚石炭世的大型盘龙类（pelycosaurian-grade）合弓动物，但其自发表以来鲜有研究关注。本研究重新检视了米蜥属（Milosaurus）的正模标本与归入标本，并将其纳入系统发育分析。系统发育分析结果显示，米蜥属隶属于触形类（Haptodontiformes），与该演化支共享两项典型形态特征：后背侧扩张的坐骨（ischium）以及长大于宽的跟骨（calcaneum）。其坐骨保留更多祖征特征，表明该属在触形类演化支中处于基干位置。多数归入标本与正模标本的形态重叠度极低，因此大部分未被纳入支序系统发育分析。不过，原被描述为背肋的标本经重新鉴定为股骨，其形态与正模标本具有高度独特的相似性。经估算，麦克氏米蜥的体重可达41千克，是石炭纪体型最大的合弓动物之一。在宾夕法尼亚纪（晚石炭世）的触形类演化支中，大型体型至少独立演化过两次：分别发生于楔齿龙科（Sphenacodontidae）与背帆龙科（Edaphosauridae）。通过系统发育比较分析方法，本研究证实麦克氏米蜥的大型体型是独立于上述两个演化科演化而来的。作为石炭纪体型最大的羊膜动物之一，同时也是古生代最多样化的合弓动物演化支系的外类群，麦克氏米蜥是探究早期陆地生态系统的关键分类单元。