A new adult specimen of the basalmost ornithuromorph bird <i>Archaeorhynchus spathula</i> (Aves: Ornithuromorpha) and its implications for early avian ontogeny

Most living birds characteristically grow rapidly and reach adult size within a year. Nevertheless, little is known about how such an advanced developmental strategy evolved despite many discoveries of early fossil birds. Here we assess the long-bone histology from a new adult specimen of <i>Archaeorhynchus spathula</i>, the basalmost taxon of Ornithuromorpha. Ornithuromorpha is the most inclusive clade containing extant birds but not the Mesozoic Enantiornithes. Histological analysis reveals that the cortex is composed of parallel-fibred bone with three lines of arrested growth, indicative of slow and annually interrupted growth for this taxon. Such bone histology is significantly different from that of other known basal ornithuromorphs, but resembles that of enantiornithines, which leads us to suggest protracted slow growth in the common ancestor of Ornithuromorpha and Enantiornithes. The fusion sequence of the tarsometatarsus between Enantiornithes and Ornithuromorpha has long been hypothesized to be different and regarded as indicative that enantiornithines are not close relatives of ornithuromorphs. Due to a lack of fossils recording early ontogenetic stages, little is known about the development of the tarsometatarsus in basal ornithuromorphs, making this hypothesis impossible to test. Here we show that the fusion sequence of the tarsometatarsus in <i>Archaeorhynchus</i> is similar to that of enantiornithines, and that the proximal-early fusion in tarsometatarsus represents a plesiomorphic trait for basal birds. Our findings also shed light on ontogenetic variation of sternal morphology, which highlights the importance of ontogeny in the taxonomic and phylogenetic study of early birds.现生鸟类普遍具有快速生长的特征，可在一年内达到成体体型。尽管早期鸟类化石已有诸多发现，但我们对这类先进发育策略的演化历程仍知之甚少。本研究对一件新发现的匙吻古喙鸟（Archaeorhynchus spathula）成体标本的长骨组织学进行分析，该物种是今鸟型类（Ornithuromorpha）最基干的类群。今鸟型类是包含现生鸟类但不包含中生代反鸟类（Enantiornithes）的最大演化支。组织学分析显示，其骨皮质由平行纤维骨构成，并带有三条生长停滞线，表明该类群的生长速度缓慢且每年出现生长中断。这类骨组织学特征与其他已知基干今鸟型类存在显著差异，但与反鸟类相似，据此我们推测今鸟型类与反鸟类的共同祖先具有缓慢且持续时间较长的生长模式。长期以来，学界假设反鸟类与今鸟型类的跗跖骨愈合序列存在差异，并以此作为反鸟类并非今鸟型类近亲的依据。由于缺乏记录早期个体发育阶段的化石，基干今鸟型类跗跖骨的发育情况至今不明，使得该假说无法得到验证。本研究发现，匙吻古喙鸟的跗跖骨愈合序列与反鸟类相似，且跗跖骨近端优先愈合的特征是基干鸟类的祖征。本研究结果还揭示了胸骨形态的个体发育变异，凸显了个体发育在早期鸟类分类学与系统发育研究中的重要性。