THE PAST AND PRESENT RECORDS OF SAND DOLLARS (ECHINOIDEA; CLYPEASTEROIDA) FROM TAIWAN: ORIGINATION, DISPERSAL, PHYLOGENY AND BIOGEOGRAPHY

Clypeasteroids and stem group forms a monophyletic clade known as Neognathostomata, and their fossil records are rich in the Cenozoic strata worldwide. The Clypeasteroida comprises two suborders, Clypeasterina and Scutellina. The latter is further divided into two infraorders: Lagniformes and Scutelliformes. The Clypeaster is the most specious and the most cosmopolitan genus within the Clypeasteroida, and it can be found in the marine settings ranging from tropical to sub-tropical climate zones of Asia, Africa, America and Australia. On the contrary, the rest of the genera in Clypeasteridae have relatively limited species diversity and some exhibit endemism. For example, Ammotrophus comprises three species and can only be found in southern and western Australia, and Fellaster is monospecific which is restricted to eastern Australia and New Zealand.

Currently, there are 11 living clypeasteroid species reported in Taiwan, including Arachnoides placenta (Linnaeus, 1758), Astriclypeus mannii Verrill, 1867, Clypeaster reticulatus (Linnaeus, 1758), Clypeaster virescens Döderlein, 1885, Echinocyamus megapetalus Clark, 1914, Sculpsitechinus auritus (Leske, 1778), Fibularia plateia Clark, 1928, Laganum fudsiyama Döderlein, 1885, Peronella lesueuri (Agassiz, 1841) and Sinaechinocyamus mai (Wang, 1984). Some of them, such as A. mannii, have good fossil records in Taiwan since Miocene, whereas A. placenta has no local fossil record. Current distributions show that A. placenta has originated from Australia, and Taiwan is one of the active migration fronts in the North Hemisphere. Two possible colonization pathways are proposed: Dispersal via the Kuroshio Current during the planktonic larval stage or moving along the continental margin of South Asia during the benthic adulthood.

Based on the fossil occurrences of Arachnoides and related taxa and the regional tectonic history, three major events of faunal expansion are evident: 1) Origination along the coast of Australia and New Zealand in the South Hemisphere since Eocene (56-33.9 Ma); 2) Expansion into the Malay Archipelago in the Tropics during the Pliocene (5.3-2.58 Ma); and 3) Young colonization of Taiwan in the North Hemisphere during the late Holocene (Meghalayan Stage; <4250 yr b2k). There is a major time gap between the events one and two. This is probably due to the fact that Australia was far from the Malay Archipelago during Paleogene. New dispersal mechanisms hypothesized here can be tested further with other Taiwanese species.