Griffin Christopher T, Bano Lauren S, Turner Alan H, Smith Nathan D, Irmis Randall B, Nesbitt Sterling J
Department of Geosciences, Virginia Tech, Blacksburg, VA, USA.
Department of Biology, Virginia Tech, Blacksburg, VA, USA.
PeerJ. 2019 Feb 11;7:e6331. doi: 10.7717/peerj.6331. eCollection 2019.
Understanding growth patterns is central to properly interpreting paleobiological signals in tetrapods, but assessing skeletal maturity in some extinct clades may be difficult when growth patterns are poorly constrained by a lack of ontogenetic series. To overcome this difficulty in assessing the maturity of extinct archosaurian reptiles-crocodylians, birds and their extinct relatives-many studies employ bone histology to observe indicators of the developmental stage reached by a given individual. However, the relationship between gross morphological and histological indicators of maturity has not been examined in most archosaurian groups. In this study, we examined the gross morphology of a hypothesized growth series of femora (96.6-144.4 mm long), the first series of a non-dinosauriform dinosauromorph available for such a study. We also histologically sampled several individuals in this growth series. Previous studies reported that lacks well-developed rugose muscle scars that appear during ontogeny in closely related dinosauromorph taxa, so integrating gross morphology and histological signal is needed to determine reliable maturity indicators for early bird-line archosaurs. We found that, although there are small, linear scars indicating muscle attachment sites across the femur, the only rugose muscle scar that appears during ontogeny is the attachment of the , and only in the largest-sampled individual. This individual is also the only femur with histological indicators that asymptotic size had been reached, although smaller individuals possess some signal of decreasing growth rates (e.g., decreasing vascular density). The overall femoral bone histology of is similar to that of other early bird-line archosaurs (e.g., woven-bone tissue, moderately to well-vascularized, longitudinal vascular canals). All these data indicate that the lack of well-developed femoral scars is autapomorphic for this species, not simply an indication of skeletal immaturity. We found no evidence of the high intraspecific variation present in early dinosaurs and other dinosauriforms, but a limited sample size of other early bird-line archosaur growth series make this tentative. The evolutionary history and phylogenetic signal of gross morphological features must be considered when assessing maturity in extinct archosaurs and their close relatives, and in some groups corroboration with bone histology or with better-known morphological characters is necessary.
了解生长模式是正确解读四足动物古生物学信号的核心，但当生长模式因缺乏个体发育序列而受到的限制较小时，评估某些已灭绝类群的骨骼成熟度可能会很困难。为了克服在评估已灭绝的主龙类爬行动物（鳄形类、鸟类及其已灭绝的亲属）成熟度方面的这一困难，许多研究采用骨组织学来观察特定个体所达到的发育阶段的指标。然而，在大多数主龙类群体中，成熟度的总体形态学指标与组织学指标之间的关系尚未得到研究。在本研究中，我们检查了一组假设的股骨生长序列（长96.6 - 144.4毫米）的总体形态，这是首个可用于此类研究的非恐龙形恐龙omorph的序列。我们还对该生长序列中的几个个体进行了组织学采样。先前的研究报告称，[该物种名称缺失]缺乏在密切相关的恐龙omorph类群个体发育过程中出现的发育良好的皱纹状肌肉疤痕，因此需要整合总体形态学和组织学信号来确定早期鸟类谱系主龙类可靠的成熟度指标。我们发现，尽管股骨上有小的线性疤痕表明肌肉附着位点，但个体发育过程中出现的唯一皱纹状肌肉疤痕是[肌肉名称缺失]的附着处，且仅在采样的最大个体中出现。这个个体也是唯一具有组织学指标表明已达到渐近大小的股骨，尽管较小的个体具有一些生长速率下降的信号（例如血管密度降低）。[该物种名称缺失]的股骨整体骨组织学与其他早期鸟类谱系主龙类相似（例如编织骨组织、中等至良好的血管化、纵向血管管道）。所有这些数据表明，缺乏发育良好的股骨疤痕是该物种的自近裔性状，而不仅仅是骨骼不成熟的迹象。我们没有发现早期恐龙和其他恐龙形类中存在的高种内变异的证据，但其他早期鸟类谱系主龙类生长序列的样本量有限，这使得这一结论具有一定的不确定性。在评估已灭绝的主龙类及其近亲的成熟度时，必须考虑总体形态学特征的进化历史和系统发育信号，并且在某些类群中，需要与骨组织学或更知名的形态学特征进行确证。
