The effects of skeletal asymmetry on interpreting biological variation and taphonomy in the fossil record

Biological asymmetry is present in all bilaterally symmetric organisms as a result of normal developmental instability. However, fossilized organisms, which have undergone distortion due to burial, may have additional asymmetry as a result of taphonomic processes. To investigate this issue, we evaluated the magnitude of shape variation resulting from taphonomy on vertebrate bone using a novel application of fluctuating asymmetry. We quantified the amount of total variance attributed to asymmetry in a taphonomically distorted fossil taxon and compared it to that of three extant taxa. The fossil taxon had an average of 27% higher asymmetry than the extant taxa. In spite of the high amount of taphonomic input, the major axes of shape variation were not greatly altered by removal of the asymmetric component of shape variation. This presents the possibility that either underlying biologic trends drive the principal directions of shape change irrespective of asymmetric taphonomic distortion, or that the symmetric taphonomic component is large enough that removing only the asymmetric component is inadequate to restore fossil shape. Our study is the first to present quantitative data on the relative magnitude of taphonomic shape change and presents a new method to further explore how taphonomic processes impact our interpretation of the fossil record.

生物不对称现象普遍存在于所有两侧对称生物中，其源于正常的发育不稳定。然而，因埋藏作用发生变形的化石生物，可能会因埋藏学过程额外产生不对称性。为探究这一问题，我们创新性地将波动不对称（fluctuating asymmetry）方法应用于脊椎动物骨骼，以此评估埋藏学作用引发的形态变异程度。我们量化了经埋藏学变形的化石类群中，由不对称性导致的总变异量，并将其与三个现生类群的对应指标进行对比。结果显示，该化石类群的不对称性平均较现生类群高出27%。尽管埋藏作用带来的不对称性占比较高，但去除形态变异中的不对称成分后，形态变异的主轴线并未发生显著改变。这一发现存在两种潜在可能：其一，内在的生物学趋势主导着形态变化的主方向，不受埋藏学不对称变形的影响；其二，对称的埋藏学成分占比足够高，仅去除不对称成分不足以恢复化石的真实形态。本研究首次提供了埋藏作用引发的形态变化相对程度的定量数据，并提出了一种新方法，用以进一步探究埋藏学过程如何影响我们对化石记录的解读。