New data on the Late Cretaceous lizard Dicothodon bajaensis (Squamata, Borioteiioidea) from Baja California, Mexico reveals an unusual tooth replacement pattern in squamates

Abstract Borioteiioids comprise an extinct family of squamates that inhabited the Northern Hemisphere during the Cretaceous and were characterized by varying patterns of tooth replacement and dental morphology. Understanding the evolution of these tooth replacement patterns has, however, been largely hampered by an extremely fragmentary fossil record. Here we present new information on Dicothodon bajaensisfrom the Campanian of Baja California (Mexico), so far known only from isolated teeth and jaw fragments. Among abundant new materials there are ten maxillae and five dentaries belonging to distinct ontogenetic stages. Whereas juveniles display active tooth replacement, older specimens show no evidence of replacement. Dicothodon bajaensis is therefore inferred to have had arrested tooth replacement later in ontogeny. This provides the first evidence of cessation of tooth replacement during late ontogeny in lizards (living or extinct). This replacement type is also an evolutionary intermediate between the typical lizard replacement pattern, observed in some borioteiioids (e.g., Bicuspidon) and the complete absence of tooth replacement since early ontogeny as observed in other borioteiioids (e.g., Polyglyphanodon).

摘要 波里泰蜥类（Borioteiioids）是一类已灭绝的有鳞目（Squamata）类群，生存于白垩纪的北半球，以多样的牙齿替换模式与牙齿形态为典型特征。然而，受限于极度零散的化石记录，学界对该类群牙齿替换模式的演化研究长期受到极大阻碍。本次研究报道了来自墨西哥下加利福尼亚白垩纪坎潘期的下加利福尼亚狄氏蜥（Dicothodon bajaensis）的新化石材料：此前该物种仅以孤立牙齿与颌骨碎片被记录。本次发现的丰富新材料包含10件上颌骨与5件齿骨，分别对应不同的个体发育阶段。研究显示，幼体个体展现出活跃的牙齿替换现象，而成熟标本则无任何替换痕迹。据此推断，下加利福尼亚狄氏蜥在个体发育后期会出现牙齿替换停滞的现象。这是首个在蜥蜴（现生或灭绝类群）个体发育后期出现牙齿替换停止的实证。该替换模式同时处于两类典型波里泰蜥类替换模式的演化中间态：一类是部分类群（如双齿龙属Bicuspidon）所具备的典型蜥蜴替换模式，另一类则是其他类群（如多尖齿龙属Polyglyphanodon）自早期个体发育阶段便完全缺失牙齿替换的模式。