A puzzling frog crab (Crustacea: Decapoda: Brachyura) from the Early Cretaceous Santana Group of Brazil: frog first or crab first?

The evolutionary origin of frog crabs (Raninoida) remains puzzling partly due to their astonishing morphological disparity, ranging from broad and heavily ornamented ‘crab-like’ extinct families (necrocarcinids and allies), to elongate and smoother ‘frog-like’ extant ones (raninids and allies). However, an ancient Cretaceous clade (Palaeocorystidae) displays a combination of plesiomorphic and apomorphic traits that might advocate for either evolutionary scenario: from ‘crab-like’ to ‘frog-like’, or vice versa. This lack of agreement is partly fuelled by the scarcity of Early Cretaceous fossils, a time from which the first raninoidans are known. A close re-examination of an Early Cretaceous fossil from the Santana Group of Brazil, <i>Araripecarcinus ferreirai</i> Martins-Neto, 1987, combined with phylogenetic analysis including all main clades of podotreme crabs, reinforces its raninoidan condition, and rejects the initial hypothesis of a Portunoidea affinity. Furthermore, comparisons with other raninoidans support the hypothesis that a more ‘crab-like’ body plan is the plesiomorphic condition for raninoidans, and that the ‘frog-like’ architecture of Palaeocorystidae, and perhaps the Raninoidea as a whole, reflects a derived condition related to a specialized burrowing lifestyle. Phylogenetic analyses are fundamental to evaluate the position of Palaeocorystidae with respect to raninoidean and necrocarcinid-like families, helping to better resolve the Raninoida evolutionary tree of life, and to gain a broader understanding on their relatedness by common ancestry throughout geological time.

蛙蟹（Raninoida）的演化起源始终成谜，部分原因在于其惊人的形态差异：从宽阔且纹饰繁复的"蟹形"已灭绝类群（Necrocarcinidae及其近缘类群），到修长光滑的"蛙形"现生类群（Raninidae及其近缘类群）。然而，一个古老的白垩纪演化支（Palaeocorystidae）兼具祖征（plesiomorphic）与衍征（apomorphic）特征，可为两种演化场景提供支撑：即从"蟹形"向"蛙形"演化，或反之。学界对此尚未达成共识，部分原因在于早白垩世（Early Cretaceous）化石的稀缺——目前已知的最早蛙蟹类（Raninoida）化石即产自该地质时段。对产自巴西桑塔那群（Santana Group）的早白垩世化石<i>Araripecarcinus ferreirai</i> Martins-Neto, 1987进行的重新细致检视，结合涵盖所有主要绵蟹派（Podotremata）蟹类演化支的系统发育分析，进一步确认了其蛙蟹类的分类地位，并否定了最初认为其隶属于梭子蟹总科（Portunoidea）的假说。此外，与其他蛙蟹类类群的对比分析支持如下假说：更为"蟹形"的体型构型是蛙蟹下目（Raninoida）的祖征状态，而古棒蟹科（Palaeocorystidae，或许还包括整个蛙蟹总科Raninoidea）的"蛙形"体型则反映了与特化穴居生活方式相关的衍征状态。系统发育分析是评估古棒蟹科（Palaeocorystidae）在蛙蟹总科（Raninoidea）及尸蟹科（Necrocarcinidae）类群中的分类地位的核心手段，有助于更清晰地解析蛙蟹下目（Raninoida）的演化系统发育树，并更全面地理解其在地质历史中通过共同祖先所建立的亲缘关系。