Galeaspida

Their mouth and gill openings are situated on the ventral surface of the head (top right). In the most primitive forms, such as the Silurian genus Hanyangaspis (top), the median dorsal inhalent opening is broad and situated anteriorly. In other galeaspids, it is more posterior in position and can be oval, rounded, heart-shaped or slit-shaped. In some Devonian galeaspids, such as the hunanaspidiforms Lungmenshanaspis (middle) and Sanchaspis (bottom right), the headshield is produced laterally and anteriorly into slender processes. The eugaleaspidiforms, such as Eugaleaspis (bottom left) have a horseshoe-shaped headshield and a slit-shaped median dorsal opening, which imitates the aspect of the headshield of osteostracans.

Galeaspida (from Latin, "Helmet shields") is an extinct taxon of jawless marine and freshwater fish. The name is derived from galea, the Latin word for helmet, and refers to their massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, there being currently no evidence that the galeaspids had paired fins. However, Galeaspida are in fact regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase.

单词	Galeaspids
释义	Galeaspids 英语百科 Galeaspida Galeaspida (from Latin, "Helmet shields") is an extinct taxon of jawless marine and freshwater fish. The name is derived from galea, the Latin word for helmet, and refers to their massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, there being currently no evidence that the galeaspids had paired fins. However, Galeaspida are in fact regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase.
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Galeaspids

Galeaspida