Department of Botany, Connecticut College, New London, CT, 06320, USA.
Sci Rep. 2022 Mar 22;12(1):4896. doi: 10.1038/s41598-022-09006-1.
Mallomonas is the largest and most speciose genus within the Synurales, a monophyletic clade of siliceous scale-bearing organisms within the class Chrysophyceae. The genus consists of unicellular, motile, photosynthetic organisms found in freshwater localities worldwide. Mallomonas diverged from other synurophytes during the lower Cretaceous at approximately 130 Ma. Recent discoveries of fossil species were used to examine shifts in scale and cell size over geologic time. On average, scales of fossil species were 2.5 times larger than those produced by modern species. However, a smaller subset of extinct fossil taxa lacking modern analogs had scales over four times larger than modern species, and the largest recorded specimens were six times larger. Data from modern species were further used to develop a model relating scale size to cell size, and applied to the fossil specimens. Based on the model, the mean size of fossil cells was almost twice as long and 50% wider compared to modern species, and cells of taxa lacking modern analogs close to three times as large. These large cells, covered with robust siliceous scales, were likely slow swimmers requiring significant energy to maintain their position in the water column, and possibly prone to increased predation.
盘藻目是硅质鳞片生物类群中最庞大和最多样化的一目，属于金藻门。该目由单细胞、能动、光合作用的生物体组成，分布于世界各地的淡水环境中。盘藻目在白垩纪早期大约 1.30 亿年前与其他合尾藻目生物分化。最近发现的化石物种被用来研究鳞片和细胞大小随地质时间的变化。平均而言，化石物种的鳞片比现代物种大 2.5 倍。然而，一小部分没有现代对应物的灭绝化石类群的鳞片比现代物种大 4 倍以上，而记录到的最大标本则大 6 倍。现代物种的数据进一步用于建立一个将鳞片大小与细胞大小相关联的模型，并应用于化石标本。根据该模型，化石细胞的平均大小与现代物种相比，长度几乎长了一倍，宽度宽了 50%，而没有现代对应物的类群的细胞则接近 3 倍大。这些大细胞被坚固的硅质鳞片覆盖，可能游动速度较慢，需要大量能量来维持在水柱中的位置，并且可能更容易受到捕食。
