New contributions to the skull anatomy of spinosaurid theropods: Baryonychinae maxilla from the Early Cretaceous of Igea (La Rioja, Spain)

Spinosaurid dinosaurs show a wide geographical and temporal distribution, being present in Gondwanan and Laurasian landmasses, at least, during the Lower Cretaceous and the Cenomanian. European spinosaurids are more diverse than previously thought with five, and probably six, currently recognised genera. The revision of CPI 477, a fragment of a left maxilla from the Lower Cretaceous of La Rioja (Spain) previously assigned to Baryonyx, demonstrates that this specimen cannot be referred to this genus and likely belongs to an indeterminate baryonychine closer to Baryonyx than to Suchomimus. CPI 477 exhibits a tooth replacement process comparable to that of other theropods and, besides, similar to other large theropods, two replacement teeth could simultaneously be developed in the same alveolus, partly explaining high replacement ratios and the overabundance of spinosaurid teeth in some fossil sites. Furthermore, the Oxalaia maxilla shares the anteriorly located external naris with Irritator and baryonychine spinosaurids, which differ from the more retracted ones in African spinosaurines. Finally, in addition to the previously noticed differences between Baryonychinae and Spinosaurinae, the paradental groove of African spinosaurines and Oxalaia differs from that of baryonychines in being sinusoidal instead of straight.

棘龙类（Spinosaurid）恐龙具有广泛的地理分布与时间分布跨度，至少在早白垩世及森诺曼期期间，它们曾广泛分布于冈瓦纳（Gondwanan）与劳亚（Laurasian）大陆板块。欧洲棘龙类的多样性远超此前认知，目前已确认至少5个属，甚至可能达到6个。针对编号CPI 477标本的修订研究显示：该标本为一块采自西班牙拉里奥哈早白垩世地层的左上颌骨碎片，此前曾被归入重爪龙属（Baryonyx），但最新研究表明其无法被归属于该属，且大概率属于一种未定名的重爪龙亚科（Baryonychine）类群，其与重爪龙的亲缘关系近于似鳄龙属（Suchomimus）。CPI 477的牙齿替换过程与其他兽脚类（Theropods）恐龙相近；此外，与其他大型兽脚类类似，该标本所在个体的单个牙槽内可同时发育两枚替换齿，这一特征在一定程度上解释了较高的牙齿替换率，以及部分化石点中棘龙类牙齿化石大量富集的现象。此外，奥沙拉龙（Oxalaia）的上颌骨与激龙（Irritator）及重爪龙亚科棘龙类一样，拥有位置靠前的外鼻孔，这一特征与外鼻孔位置靠后的非洲棘龙亚科（Spinosaurinae）截然不同。最后，除此前已报道的重爪龙亚科与棘龙亚科间的差异外，非洲棘龙亚科及奥沙拉龙的齿旁沟呈正弦曲线状，而非重爪龙亚科的直型结构，该特征可作为两类群的区分依据之一。