Trilobite Ontogenetic Study

The trilobites have been well studied in the systematics, evolution and biostratigraphy, but their ontogeny has received little research attention. The study on the trilobite ontogeny can not only provide abundant material for their classification and evolutionary origin, but also be important on the study of the phylogeny and evolution of other arthropods. A large amount of complete exoskeletons together with numerous disarticulated sclerites of trilobites have been collected from the lower Cambrian (Stage3, Series2) Shuijingtuo Formation in Yichang and Changyang, Hubei Province, South China, and the Yu'anshan Member of the Heilinpu Formation, in Kunming, Yunnan Province, southwest China, which comprised7species: eodiscinid trilobites Tsunyidiscus acutus (Sun,1983) and Sinodiscus changyangensis Zhang in Zhou et al.,1977; redlichiid trilobites Metaredlichia cylindrica Zhang,1953and Eoredlichia intermedia Lu.1940; ptychopariid trilobites Estaingia sinensis (Chang,1953) and Yunnanocephalus yunnanensis Mansuy,1912and corynexochid Hunanocephalus(Duotingia) duotingensis Chow,1974. Ontogenetic sequence of each species is established on the basis of the immature and mature exoskeletons, including the previously unknown protaspides and meraspides in particular.The material of T. acutus reveals some prominent morphological changes such as the number of pygidial axial rings, genal spines retained throughout ontogeny and pygidial pleurae from furrowed to unfurrowed. Changes that took place during the meraspid period of S. changyangensis include the addition of post-cephalic segments and prominent pygidial larval notches in early meraspid development which became progressively less distinct and disappeared in meraspid degree2. Two holaspid stages are recognized in both species according to the addition of a new pygidial segment, indicating that the start of the holaspid phase preceded the onset of the epimorphic phase and accordingly, their developmental modes are attributed to the protarthrous pattern. Moreover, the trunk segmentation schedule of these two primitive eodiscoid trilobites are discussed, respectively, which is similar to the other early occurring eodiscinids, i.e. as the boundary between the thorax and pygidium migrated posteriorly, there is no change in the number of the trunk segments. The processes of the liberation of thoracic segment and segment insertion into the pygidium are separated from one another, and the two different mechanisms, somitogenesis and tagmosis, progress independently during the ontogenetic development of the post-cephalic region of these primitive eodiscinids.The material of M. cylindrica and E. sinensis include numerous complete protaspides, of which two stages can be differentiated respectively according to the appearance of a shallow furrow that separates the protopygidial area from the protocranidium. Additionally, the descriptions of the subsequent ontogenetic phases of these two species, including meraspides and holaspides, mainly depend on the isolated cranidia which displayed prominent morphological changes such as the contraction of frontal glabellar lobe, disappearance of the fixigenal spines and migration of facial sutures from proparian to opisthoparian. Incorporating the whole ontogenetic sequence of these two species allows us not only to trace the developmental trends of certain structures with growth, but to ascribe the protaspides to their adults accurately, with the help of the meraspid morphology in particular.Furthermore, a detailed exploration of morphology and ontogeny of the trilobite Eoredlichia intermedia Lu,1940is presented. The new material of E. intermedia comprises a relatively complete ontogenetic series ranging from early meraspid to holaspid period, based upon which more details on the morphological variations are revealed, such as the appearance of bacculae in some holaspides, contraction and disappearance of fixigenal spines and macropleural spines of the first and second thoracic segments, which are all documented for the first time and can also be used as developmental markers defining their ontogenetic phases. Two distinct morphotypes, possibly an expression of sexual dimorphism or intraspecific variation, are recognized according to the morphology of the macropleural spines of the second thoracic segment in the meraspid degree14and holaspides. The trunk segmentation schedule of E. intermedia is also discussed and the distinction of the trunk region into the two batches, prothorax and opisthothorax, can be observed throughout ontogenetic development, which might be considered as a reference for a better understanding on the relationship of tagmosis and growth segmentation among the Cambrian redlichiid trilobites.A recently discovered degree9meraspis of Y. yunnanensis reveals some morphological changes through meraspis-holaspis transition, among which the most prominent one is the migration of the facial sutures from proparian to gonatoparian. Consequently, it can be concluded that during the ontogeny of this species the posterior sections of facial sutures moved backward from cutting lateral cephalic margin to bisecting the genal angle, and not to move adaxially to or cut posterior cephalic margins anymore. Similar to proparian trilobites, gonatoparian trilobites might be also derived from the opisthoparian trilobites due to heterochrony.The new material of H. duotingensis comprises a complete ontogenetic series ranging from meraspid to holaspid period, in which more details on the morphological variations are revealed. The addition of any new pygidial segment was not recognized in the holaspid period, indicating that the start of the holaspid phase coincides with the onset of the epimorphic phase and accordingly, its developmental mode is attributed to the synarthromeric pattern. Additionally, compared with the ontogeny of other early Cambrian corynexochid trilobites, the realization that tagmosis and somitogenesis can each progress independently during ontogeny of these corynexochids provides a useful tool in attempts to establish the mechanisms that have led to different patterns of tagmosis and somitogenesis.