新疆北鲵骨骼系统及骨骼发育时序特征

CHARACTERISTICS OF SKELETAL SYSTEM AND SKELETON DEVELOPMENT SEQUENCE IN RANODON SIBIRICUS

为揭示新疆北鲵(Ranodon sibiricus)骨骼系统特征及骨骼发育时序规律,采用“软骨-硬骨”双染色法对2月龄、3月龄、4月龄、6月龄、1龄、2龄、3龄和9龄8个不同年龄段新疆北鲵骨骼系统进行详细观察和对比分析。结果表明新疆北鲵骨骼系统分为头骨、脊椎骨和附肢骨三大部分,共(217±1)块骨,其中头骨骨骼53块脊椎骨(48±1)块,附肢骨116块。此外,新疆北鲵头颅外形宽扁,犁骨齿排列幼体呈“/”型,成体则呈“■”型颅顶同时具有额顶囟和前颌囟,且前颌囟长与鼻骨长之比达或大于1/2,翼骨前端与上颌骨后端较接近等显著区别于其他小鲵科物种的骨骼特征,可作为种间分类重要依据。基于骨骼发育时序和骨化时间特征对比分析,北鲵骨骼数量和形态发生重大改变主要在6月龄幼体和2龄幼体两个时期。6月龄幼体的骨骼变化主要体现在头骨:新生出上颌骨、泪骨和前额骨,前颌囟形状逐渐清晰,腭骨退化,翼骨向后回缩,鳞骨向外延展。此外,腰带新生出前耻骨,暗示其进入骨骼变态发育阶段。2龄幼体的骨骼变化主要体现在舌器:第三、第四鳃弓退化,外鳃消失,暗示其变态发育接近尾声或结束。研究结果表明新疆北鲵变态发育时期从6月龄起至2龄结束,变态过程中骨骼发生的新生、退化和重塑主要为适应变态前后进食、呼吸和运动方式的改变,可为新疆北鲵骨骼功能发育研究及其系统进化分类提供寄出信息。

In order to adapt to the transition from aquatic to terrestrial,the skeletal system of amphibians has undergone tremendous changes before and after metamorphosis.The characteristics of the skeleton development sequence in amphibians reflect their evolutionary adaptation to lifestyle and are also ideal materials for studying vertebrate skeleton formation,growth,and remodeling.In order to reveal the characteristics of the amphibian skeletal system and the sequence of skeletal development,the process of skeletal system development of amphibians at different ages was observed and compared by the double staining of cartilage and bone procedure.The results showed that the skeletal system of Ranodon sibiricus was divided into the skull,vertebra,and appendicular skeleton,with a total of(217±1)bones,including 53 skull bones,(48±1)vertebra bones,and 116 appendage bones.Furthermore,the skeletal system of Ranodon sibiricus has some significant morphological features that are different from other species of Hynobiidae.The contour of the skull appears broad and flat,and the skull fragments are thin and weak.In addition to the fontanelle between the frontals in the dorsal center of the skull,there is also a premaxillary fontanelle in the dorsal front of the skull,and the ratio of the length of the premaxillary fontanel to the length of the nasal bone is up to or more than 1/2.The two rows of vomerine dentition are positioned between the nostrils and separated.The arrangement pattern of the vomerine pars dentition changed from‘/\’to accompanied by the metamorphosis from larva to adult of Ranodon sibiricus.The anterior of the pterygoid is very close to the posterior of the maxilla,even linked by the bone bridge at the larval stage until the bone bridge breaks after metamorphosis.The shape of the maxillary arch is arched,and the lower jaw is thin and weak.Squamosal is‘T’shaped and narrower.The first branchial arch is completely cartilage.The cornua of the basibranchial cartilage show two cylindrical branches gradually tapering anterolaterally to a pointed tip.The process of prearticular is not developed,with small articular.The caudal vertebrae are flat.The first three caudal vertebrae still have ribs,and the haemal arch appears from the fourth caudal vertebra.With 4 fingers and 5 toes,it usually has 8 carpals and 10 tarsals,respectively.At the age of 6 months,the morphology and structure of the skull of Ranodon sibiricus began to change significantly compared with the previous development stage.For example,the maxilla,lacrimale,and prefrontal were born.The outline of the premaxillary fontanelle is gradually clear.The palatine degenerates,the pterygoid retracts backward,the squamosal extends outward,and the angle formed by the outer edge of the left and right squamosal increases further.At the same time,the prepubis was born in the pelvic girdle.These massive newborn or remodeling bones at the skull and the pelvic girdle suggest that Ranodon sibiricus has entered the metamorphosis stage to adapt to the transition from aquatic to terrestrial.The second significant change in the skeletal number and morphology of Ranodon sibiricus occurs at the age of 2 years.Although most bones only showed the enhancement of ossification degree from 6 months to 2 years old,there were significant changes in the apparatus hyoideus.The third and fourth branchial arches degenerate or disappear,meaning the outer gills disappear.The disappearance of the outer gills also suggests that the development of metamorphosis is nearing its end.Based on the comparative analysis of the skeletal system of Ranodon sibiricus at different ages,it is speculated that the metamorphosis of Ranodon sibiricus occurred from 6 months and lasted until the age of 2 years.During the development of metamorphosis,the newborn,degeneration,and reorganization of bones are mainly to adapt to the changes in feeding,breathing,and movement before and after metamorphosis.The results of this study can provide a theoretical basis for studying the skeletal functional development of Ranodon sibiricus and its phylogenetic classification in Hynobiidae species.