A new adult specimen of the basalmost ornithuromorph bird Archaeorhynchus spathula (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 Archaeorhynchus spathula, 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 Archaeorhynchus 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.

现生鸟类的典型特征为快速生长，并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，但这类高度演化的发育策略的演化起源仍鲜为人知。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展开分析。匙吻古喙鸟是鸟胸类（Ornithuromorpha）中最基干的类群，而鸟胸类是包含现生鸟类但不包含中生代反鸟亚纲（Enantiornithes）的最大演化支。组织学分析显示，该标本的骨皮质由平行纤维骨构成，且带有三条生长停滞线（lines of arrested growth），表明该类群生长缓慢且每年都会出现生长中断。这类骨组织学特征与其他已知基干鸟胸类存在显著差异，但与反鸟亚纲类群的骨组织学特征相似，据此我们推测，鸟胸类与反鸟亚纲的共同祖先曾存在缓慢且持久的生长模式。长期以来，学界假设反鸟亚纲与鸟胸类的跗跖骨愈合序列存在差异，并据此认为反鸟亚纲并非鸟胸类的近亲。但由于缺乏记录早期个体发育阶段的化石，基干鸟胸类跗跖骨的发育过程至今仍不明确，使得该假设无法得到验证。本研究显示，匙吻古喙鸟的跗跖骨愈合序列与反鸟亚纲类群相似，且跗跖骨近端优先愈合乃是基干鸟类的祖征（plesiomorphic trait）。本研究结果还揭示了胸骨形态的个体发育变化，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。