Brusatte Stephen L, Carr Thomas D, Erickson Gregory M, Bever Gabe S, Norell Mark A
Department of Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA.
Proc Natl Acad Sci U S A. 2009 Oct 13;106(41):17261-6. doi: 10.1073/pnas.0906911106. Epub 2009 Oct 5.
Tyrannosaurid theropods are characterized by a generalized body plan, and all well-known taxa possess deep and robust skulls that are optimized for exerting powerful bite forces. The fragmentary Late Cretaceous Alioramus appears to deviate from this trend, but its holotype and only known specimen is incomplete and poorly described. A remarkable new tyrannosaurid specimen from the Maastrichtian (Late Cretaceous) of Mongolia, including a nearly complete and well-preserved skull and an extensive postcranium, represents a new species of Alioramus, Alioramus altai. This specimen conclusively demonstrates that Alioramus is a small, gracile, long-snouted carnivore that deviates from other tyrannosaurids in its body plan and presumably its ecological habits. As such, it increases the range of morphological diversity in one of the most familiar extinct clades. Phylogenetic analysis places Alioramus deep within the megapredatory Tyrannosauridae, and within the tyrannosaurine subclade that also includes Tarbosaurus and Tyrannosaurus. Both pneumatization and ornamentation are extreme compared with other tyrannosaurids, and the skull contains eight discrete horns. The new specimen is histologically aged at nine years old but is smaller than other tyrannosaurids of similar age. Despite its divergent cranial form, Alioramus is characterized by a similar sequence of ontogenetic changes as the megapredatory Tyrannosaurus and Albertosaurus, indicating that ontogenetic change is conservative in tyrannosaurids.
暴龙科兽脚亚目恐龙的特征是具有一般化的身体结构，所有已知的分类单元都拥有深邃且坚固的头骨，这些头骨经过优化以施加强大的咬合力。白垩纪晚期的分支龙化石似乎偏离了这一趋势，但其正模标本也是唯一已知标本并不完整且描述简略。来自蒙古马斯特里赫特阶（晚白垩世）的一个引人注目的新暴龙科标本，包括一个近乎完整且保存完好的头骨和大量的后肢骨骼，代表了分支龙属的一个新物种——阿氏分支龙。这个标本确凿地证明了分支龙是一种小型、纤细、吻部较长的食肉动物，在身体结构以及大概的生态习性方面与其他暴龙科恐龙不同。因此，它增加了这个最广为人知的已灭绝类群之一的形态多样性范围。系统发育分析表明分支龙在巨型掠食性暴龙科中处于较深的位置，并且在暴龙亚科分支内，该亚科还包括特暴龙和霸王龙。与其他暴龙科恐龙相比，其气腔化和装饰都很极端，并且头骨上有八个离散的角。新标本在组织学上显示年龄为九岁，但比其他同龄的暴龙科恐龙体型更小。尽管其头骨形态不同，但分支龙的个体发育变化序列与巨型掠食性的霸王龙和阿尔伯塔龙相似，这表明个体发育变化在暴龙科中是保守的。
