G&G Colloquium Spring 2018

Shigeru Kuratani

Laboratory for Evolutionary Morphology, RIKEN Center for Biosystems Dynamics Research, Kobe 650-0047, Japan

Living cyclostomes consist of hagfish and lamprey, and the monophyly of this group has been established by recent molecular analyses. Comparative morphology of these two cyclostomes, however, has contradicted the monophyly, especially for the embryonic craniofacial pattern of hagfishes: the hagfish adenohypophysis and related structures used to be suggested to arise from endoderm, unlike that of other vertebrates derived from the ectoderm. Thus, reexamination of hagfish embryology is critical to evaluate the anatomical traits of cyclostomes. By observing staged hagfish embryos, we show that the hagfish adenohypophysis arises ectodermally, as a posterior part of the medial placode, the hypophyseal plate, as in the lamprey larva. This finding allowed us to identify a craniofacial developmental pattern common to cyclostomes, but not to crown gnathostomes. From this cyclostome-specific developmental stage, lamprey and hagfish develop into distinct developmental trajectories, making it difficult to establish morphological homologies in adult anatomy of these animals. We also show that the comparison with gnathostomes, the out group of cyclostomes, implies that many of the hagfish peculiarities can be recognized as hagfish-specific derived traits (autoapomorphies). Thus the lamprey is likely to represent more ancestral state of cyclostomes, possibly reflecting the morphological and developmental pattern of the latest common ancestor of entire vertebrates. Based on the above developmental scheme, we first showed homologies of skeletal elements between lamprey and hagfish chondrocrania. Furthermore, observation of chondrocranial development in the hagfish has suggested that the enigmatic fossil animal, Palaeospondylus, could possibly represent an ancient hagfish lineage.

Earth’s core, the deepest region of our planet, stores heat used for plate tectonics and generates Earth’s magnetic field, yet details of its nature remain uncertain. The composition of the outer core, and the inner core which is slowly crystallizing at the center of the Earth, is not fully known — the core contains roughly a third of the mass of the Earth, yet we do not know which light elements are alloyed with the core’s iron and nickel. The outer core is thought to be well-mixed for the most part, but may contain stratified layers at its boundaries. The slowly growing inner core exhibits seismic heterogeneity at a variety of length-scales. Both short period body-wave data, comprising seismic waves which travel through either the outer core or both the outer and inner core, and long period normal mode frequencies, corresponding to vibrations of the whole Earth, can be used to enhance our understanding of the core. This colloquium will address a variety of the seismological features which are present in the core, from the radially varying velocity and density of the outer core, through to the possibility of “hemispherical” differences in anisotropic texture and smaller scale variation in the inner core.