Aragonite bias exhibits systematic spatial variation in the late Cretaceous Western Interior Seaway, North America

Preferential dissolution of the biogenic carbonate polymorph aragonite promotes preservational bias in shelly marine faunas. Whilst field studies have documented the impact of preferential aragonite dissolution on fossil molluscan diversity, its impact on regional and global biodiversity metrics is debated. Epicontinental seas are especially prone to conditions which both promote and inhibit preferential dissolution, which may result in spatially extensive zones with variable preservation. Here we present a multi-faceted evaluation of aragonite dissolution within the late Cretaceous Western Interior Seaway of North America. Occurrence data of molluscs from two time intervals (Cenomanian-Turonian boundary, early Campanian) are plotted on new high-resolution paleogeographies to assess aragonite preservation within the seaway. Fossil occurrences, diversity estimates and sampling probabilities for calcitic and aragonitic fauna were compared in zones defined by depth and distance from the seaway margins. Apparent range sizes, which could be influenced by differential preservation potential of aragonite between separate localities, were also compared. Our results are consistent with exacerbated aragonite dissolution within specific depth zones for both time slices, with aragonitic bivalves additionally showing a statistically significant decrease in range size compared to calcitic fauna within carbonate-dominated Cenomanian-Turonian strata. However, we are unable to conclusively show that aragonite dissolution impacted diversity estimates. Therefore, whilst aragonite dissolution is likely to have affected the preservation of fauna in specific localities, time averaging and instantaneous preservation events preserve regional biodiversity. Our results suggest that the spatial expression of taphonomic biases should be an important consideration for paleontologists working on paleobiogeographic problems.

生物成因碳酸盐矿物变体文石（aragonite）的优先溶解，加剧了带壳海洋动物群的保存偏差。现有野外研究已证实文石优先溶解对化石软体动物多样性的影响，但其对区域及全球生物多样性指标的作用仍存在争议。陆表海极易出现兼具促进与抑制文石优先溶解的环境条件，这可能形成空间分布广泛且保存状态各异的区域。本研究针对北美晚白垩世西部内陆海道内的文石溶解情况开展多维度评估。我们将两个时间间隔（森诺曼阶-土仑阶界线、早坎潘期）的软体动物化石产出数据，绘制于新的高分辨率古地理图中，以评估该海道内的文石保存状况。研究对比了按海道深度及距海道边缘距离划分的多个区域内，方解石质与文石质动物群的化石产出记录、多样性估算值及采样概率；同时还对比了可能受不同产地文石保存潜力差异影响的表观分布范围大小。研究结果与两个时间切片内特定深度带中文石溶解加剧的结论相符，且在以碳酸盐为主的森诺曼阶-土仑阶地层中，文石质双壳类的表观分布范围较方解石质动物群出现了统计学意义上的显著缩减。但本研究无法确凿证明文石溶解对多样性估算值造成了影响。因此，尽管文石溶解可能对特定区域的动物群保存造成了干扰，时间平均作用与瞬时保存事件仍保留了区域生物多样性的原始面貌。本研究结果表明，埋藏偏差的空间分布特征应成为研究古生物地理问题的古生物学者需重点考量的关键因素。