Summers A P
Organismic and Evolutionary Biology, University of Massachusetts, Amherst, Massachusetts, USA.
J Morphol. 2000 Feb;243(2):113-26. doi: 10.1002/(SICI)1097-4687(200002)243:2<113::AID-JMOR1>3.0.CO;2-A.
The stingray family Myliobatidae contains five durophagous (hard prey specialist) genera and two planktivorous genera. A suite of morphological features makes it possible for the hard prey specialists to crush mollusks and crustaceans in their cartilaginous jaws. These include: 1) flat, pavement-like tooth plates set in an elastic dental ligament; 2) multiple layers of calcified cartilage on the surface of the jaws; 3) calcified struts running through the jaws; and 4) a lever system that amplifies the force of the jaw adductors. Examination of a range of taxa reveals that the presence of multiple layers of calcified cartilage, previously described from just a few species, is a plesiomorphy of Chondrichthyes. Calcified struts within the jaw, called "trabecular cartilage," are found only in the myliobatid genera, including the planktivorous Manta birostris. In the durophagous taxa, the struts are concentrated under the area where prey is crushed, thereby preventing local buckling of the jaws. Trabecular cartilage develops early in ontogeny, and does not appear to develop as a direct result of the stresses associated with feeding on hard prey. A "nutcracker" model of jaw function is proposed. In this model, the restricted gape, fused mandibular and palatoquadrate symphyses, and asynchronous contraction of the jaw adductors function to amplify the closing force by 2-4 times.
鲼科包含五个食硬壳猎物（硬猎物专家）属和两个食浮游生物属。一系列形态特征使硬猎物专家能够用其软骨颌压碎软体动物和甲壳类动物。这些特征包括：1）扁平的、类似铺路石的齿板镶嵌在弹性牙韧带中；2）颌表面有多层钙化软骨；3）钙化支柱贯穿颌部；4）一个杠杆系统，可放大颌内收肌的力量。对一系列分类群的研究表明，多层钙化软骨的存在（此前仅在少数物种中描述过）是软骨鱼类的一个原始特征。颌内的钙化支柱，称为“小梁软骨”，仅在鲼科各属中发现，包括食浮游生物的双吻前口蝠鲼。在食硬壳猎物的分类群中，支柱集中在猎物被压碎区域的下方，从而防止颌部局部弯曲。小梁软骨在个体发育早期就已形成，似乎并非因捕食硬猎物所产生的压力直接发育而成。提出了一个颌功能的“坚果钳”模型。在这个模型中，有限的口裂、融合的下颌和腭方骨联合以及颌内收肌的异步收缩，其作用是将闭合力放大2至4倍。
