| SYSTEMATIC BIOLOGY |
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| SUPERCLASS OSTEOSTRACOMORPHI+ |
Osteostracomorphi (O-ste-o-strak-o-MORF-i) is formed from three Greek roots that mean bone [osteon (οστούν)] shell [ostraco (όστρακo)] forms [morphi (μορφή)]. The name is a reference to the bony shell in which the head is encased.
These jawless armored fish were characterized by having bony head shields like the Anaspidomorphi. However, they had cellular bone and some had paired fins in addition to in-line fins. The heavy armor and dorsoventral flattening of the fish suggest that the animals were bottom-dwellers. These taxa were generally recognized as the agnathans that are closest to the gnathostomes; however, they did not have gnathostome-like gill arches or sensory line system. They appeared in the Ordovician period and disappeared in the Devonian.
The pectoral fins of the Osteostraci bear important similarities to the paired pectoral fins of the gnathostomes. Thus, Wilson et al. (2007) consider the paired fins of the Osteostraci to be homologous to those of the gnathostomes. They also suggest that the galeaspids lost their paired fins. The homology also seems to extend to other agnathans like the Thelodontomorphi.
| HIERARCHICAL TAXONOMY OF THE SUPERCLASS OSTEOSTRACOMORPHI+. The following descriptions come from Benton (2005) and Nelson (2006). The structure of the following system is also based, in part, on the phylogenetic treatment by Mikko's Phylogeny Web (from the Finnish Museum of Natural History). Taxa in red are extinct. |
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Drawing of a cephalaspid from: http://www.copernico.bo.it/subwww/lavoro%20evoluzione/agnati/Immagini/image07.jpg SUPERCLASS OSTEOSTRACOMORPHI+ The superclass has a single class with three orders. CLASS CEPHALASPIDOMORPHI (MONORHINA)+ The three orders of this class are united in having large bony head shields that cover the gills, which exit to the ventral side. They had two semicircular canals, a pineal eye, and a single nostril (monorhine condition). There is evidence the the bone was cellular. The Cephaloaspidiformes had true paired pectoral fins. Ordovician to Devonian ORDER CEPHALASPIDIFORMES (OSTEOSTRACI) Elaborate head shields. Most with paired pectoral fins. Ordovician to Devonian. NON-CORNUATE TAXA Ateleaspis, Hirella, Hemicyclaspis CORNUATA
(CORNUATE TAXA-8 FAMILIES) FAMILY
BENNEVIASPIDAE FAMILY
CEPHALASPIDAE FAMILY
DARTMUTHIIDAE FAMILY
KIAERASPIDAE FAMILY
SCLERODIDAE (=SCLERODONTIDAE) FAMILY
THYESTIIDAE FAMILY
TREMATASPIDAE FAMILY
ZENASPIDAE GENERA
INCLUDE: Trematapsis,
Boreaspis, Benneviaspis, Thyestes, Sclerodus ORDER
GALEASPIDIFORMES (GALEASPIDA) Head
shields, many with elaborate processes. No
pectoral fins. Up to 45 gill
pouches. Silurian
to Devonian. Unassigned
Genera: Macrothyraspis, Pentathyraspis. FAMILY
HANYANGASPIDAE Hanyangaspis,
Latirostraspis, Xiushuiaspis. FAMILY
DAYONGASPIDAE Dayongaspis. FAMILY
DUYUNOLEOIDAE Duyunolepis,
Neoduyunaspis, Paraduyunaspis. FAMILY
LUNGMENSHANASPIDAE Lungmenshanaspis,
Neoduyunaspis, Qingmenaspis. FAMILY
TRIDENSASPIDAE Tridentaspis. FAMILY
NANPANASPIDAE Nanpanaspis. FAMILY
GALEASPIDAE Galeaspis. FAMILY
EUGALEASPIDAE Eugaleaspis,
Sinogaleaspis, Yunnanogaleaspis. FAMILY
POLYBRANCHIASPIDAE Cyclodiscaspis,
Damaspis, Diandongaspis, Dongfangaspis, Kwangnanaspis, Laxaspis,
Polybranchiaspis, Siyingia. FAMILY
HUNANASPIDAE Antquisagittaspis,
Asiaspis, Hunanaspis, Sanchaspis, Sangiaspis, Szechuanaspis. ORDER
PITURIASPIDIFORMES (PITURIASPIDA) Known
only from fragmentary fossils from Lower
Devonian. Neeyambaspis, Pituriaspis. |
