Oxygen isotope composition of teeth suggests endothermy and possible migration in some Late Cretaceous shark taxa from the Gulf Coastal Plain, USA

We analyzed the oxygen isotope composition of biogenic apatite phosphate (δ18Op) in fossil tooth enameloid to investigate the paleoecology of Late Cretaceous sharks in the Gulf Coastal Plain of Alabama, U.S.A. We analyzed six different shark taxa from both the Mooreville Chalk and the Blufftown Formation. We compared shark δ18Op with the δ18Op of a co-occurring poikilothermic bony fish Enchodus petrosus as a reference for ambient conditions. E. petrosus tooth enamel δ18Op values are similar between formations (21.3‰ and 21.4‰ VSMOW, respectively), suggesting minimal differences in water δ18O between formations. Most shark taxa in this study are characterized by δ18Op values that overlap with E. petrosus values, indicating they likely lived in similar habitats and were also poikilothermic. Ptychodus mortoni and Cretoxyrhina mantelli exhibit significantly lower δ18Op values than co-occurring E. petrosus (P. mortoni δ18Op is 19.1‰ VSMOW in the Mooreville Chalk; C mantelli δ18Op is 20.2‰ VSMOW in the Mooreville Chalk and 18.1‰ VSMOW in the Blufftown Formation). Excursions into brackish or freshwater habitats and thermal water-depth gradients are unlikely explanations for the lower P. mortoni and C. mantelli δ18Op values. The low P. mortoni δ18Op value is best explained by higher body temperature relative to surrounding temperatures due to active heating (e.g., mesothermy) or passive heating due to its large body size (e.g., gigantothermy). The low C. mantelli δ18Op values are best explained by a combination of mesothermy (e.g., active heating) and migration (e.g., from the Western Interior Seaway, low latitude warmer waters, or the paleo-Gulf Stream), supporting the hypothesis that mesothermy evolved in lamniform shark taxa during the Late Cretaceous. If the anomalous P. mortoni δ18Op values are also driven by active thermoregulation, this suggests that mesothermy evolved independently multiple in families of Late Cretaceous sharks.

本研究通过分析美国阿拉巴马州墨西哥湾沿岸平原晚白垩世鲨鱼化石牙釉质中的生物成因磷灰石磷酸盐氧同位素组成（δ¹⁸Oₚ），探究其古生态特征。研究样本采自穆尔维尔白垩组（Mooreville Chalk）与布拉夫敦组（Blufftown Formation），共涵盖6个鲨鱼类群。我们以同期共存的变温硬骨鱼佩氏剑吻鱼（Enchodus petrosus）的δ¹⁸Oₚ值作为环境参考标准，对比其与研究中鲨鱼的同位素数值。两个地层中佩氏剑吻鱼的牙釉质δ¹⁸Oₚ值均较为接近（分别为21.3‰与21.4‰，相对于维也纳标准平均海洋水VSMOW），表明不同地层间水体的氧同位素组成差异极小。本研究中多数鲨鱼类群的δ¹⁸Oₚ值与佩氏剑吻鱼的数值存在重叠，说明它们可能栖息于相似生境，且同样为变温动物。莫顿扁齿鲨（Ptychodus mortoni）与白垩尖吻鲨（Cretoxyrhina mantelli）的δ¹⁸Oₚ值显著低于同期共存的佩氏剑吻鱼：穆尔维尔白垩组中莫顿扁齿鲨的δ¹⁸Oₚ值为19.1‰ VSMOW；白垩尖吻鲨的δ¹⁸Oₚ值在穆尔维尔白垩组为20.2‰ VSMOW，在布拉夫敦组为18.1‰ VSMOW。通过进入咸淡水或淡水生境，以及水体温度-深度梯度效应，均无法合理解释莫顿扁齿鲨与白垩尖吻鲨较低的δ¹⁸Oₚ值。莫顿扁齿鲨的低δ¹⁸Oₚ值最合理的解释是其体温高于周围环境水温——这可能源于主动产热（即中温代谢mesothermy），或是因体型庞大产生的被动产热（即巨温现象gigantothermy）。白垩尖吻鲨的低δ¹⁸Oₚ值最合理的解释是中温代谢（主动产热）与迁移行为的共同作用：其迁移路径可能来自西部内陆海道、低纬度暖水区，或是古墨西哥湾流，这一结果支持了“晚白垩世鼠鲨目（lamniform）鲨鱼类群已演化出中温代谢能力”的假说。若莫顿扁齿鲨的异常δ¹⁸Oₚ值同样由主动体温调节所导致，则表明晚白垩世的鲨鱼类群中，中温代谢能力曾独立多次演化于不同的鲨科类群中。