R code from A toothless dwarf dolphin (Odontoceti: Xenorophidae) points to explosive feeding diversification of modern whales (Neoceti)

Toothed whales (Odontoceti) are adapted for catching prey underwater and possess some of the most derived feeding specializations of all mammals, including the loss of milk teeth (monophyodonty), high tooth count (polydonty), and the loss of discrete tooth classes (homodonty). Many extant odontocetes possess some combination of short, broad rostra, reduced tooth counts, fleshy lips, and enlarged hyoid bones—all adaptations for suction feeding upon fish and squid. We report a new fossil odontocete from the Oligocene (approx. 30 Ma) of South Carolina (Inermorostrum xenops, gen. et sp. nov.) that possesses adaptations for suction feeding: toothlessness and a shortened rostrum (brevirostry). Enlarged foramina on the rostrum suggest the presence of enlarged lips or perhaps vibrissae. Phylogenetic analysis firmly places Inermorostrum within the Xenorophidae, an early diverging odontocete clade typified by long-snouted, heterodont dolphins. Inermorostrum is the earliest obligate suction feeder within the Odontoceti, a feeding mode that independently evolved several times within the clade. Analysis of macroevolutionary trends in rostral shape indicate stabilizing selection around an optimum rostral shape over the course of odontocete evolution, and a post-Eocene explosion in feeding morphology, heralding the diversity of feeding behaviour among modern Odontoceti.

齿鲸亚目（Odontoceti）是适应水下捕食的鲸类，拥有哺乳类中最为特化的摄食特化特征之一，包括丧失乳齿（单齿型，monophyodonty）、高齿数（多齿型，polydonty）以及齿类分化的消失（同齿型，homodonty）。多数现生齿鲸类兼具短宽吻突、减少的齿列数量、肉质唇以及膨大的舌骨，上述特征均为适应以鱼类和鱿鱼为食的抽吸式摄食的特化结构。我们报道一件产自美国南卡罗来纳州渐新世（约3000万年前）的全新化石齿鲸类——无吻突鲸（Inermorostrum xenops，gen. et sp. nov.），该物种具备抽吸式摄食的特化特征：无齿以及短缩的吻突（短吻型，brevirostry）。其吻突上的膨大骨孔（foramina）暗示该物种拥有膨大的肉质唇，甚至可能生有触须（vibrissae）。系统发育分析明确将无吻突鲸归入异索兽科（Xenorophidae）——这是一支早期分化的齿鲸类支系，以长吻、异型齿海豚为典型特征。无吻突鲸是齿鲸亚目中已知最早的专性抽吸摄食类群，而抽吸式摄食模式在该支系中曾独立演化多次。对吻突形态的宏观演化趋势分析表明，在齿鲸类的演化历程中，存在围绕最优吻突形态的稳定选择；同时始新世之后的摄食形态爆发式演化，预示了现代齿鲸类多样的摄食行为模式。