| SYSTEMATIC BIOLOGY |
|
| CLASS PLACODERMI+ |
The Placodermi (PLAK-o-DER-me) is derived from two Greek roots which means "plated skin" [plate- plaka (πλάκα), and skin- derma (δέρμα)]. The name is in reference to the plated armor bone that covered the head and much of the thorax.
The placoderms were heavily-armored gnathostomes with the heads covered by fused bony plates that were articulated to allow for movement of the jaws. They seem to have evolved in two directions. Most of them were bottom dwellers with dorso-ventrally flattened bodies and subterminal mouths. However, the arthrodires were adapted to open water with laterally-compressed bodies, large eyes, and terminal mouths of shearing bony plates.
According to Benton (2005), the placoderms were the first vertebrates to possess jaws, but where did jaws come from? The state of answers to this question can be summed up well by Kimmel et al. (2001) as "a wonderful problem, long debated in the literature". The classical answer is that a jaw is a modified gill arch that lost its function as gill support and assumed the role as an articulated lower jaw. Janvier (2007), who had investigated this problem for many years summarized more recent possible answers. In short, the main problem is that there are no ostracoderms with adequate structure to suggest the transition of a gill to a jaw. The homology between the visceral arches of living lampreys and bony fishes also is in question (Janvier 2007; and Kimmel et al. 2001). Clearly, the function of the structure(s) that evolved into jaws must have had something to do with the oral opening like the velum of the lamprey ammocoetes larva. The velum forms a fleshy wall that covers the esophagus while the animal is ventilating water through its gill pouches. Janvier (2007) suggests that such a structure, if hardened and then covered with denticles (such as the scales of Chondrichthyes), would become a functional jaw. Still, the mystery remains. What is known is that jaws appeared relatively suddenly with the appearance of the placoderms and acanthodians in the Silurian.
| HIERARCHICAL TAXONOMY OF THE CLASS PLACODERMI+. The following descriptions come from Benton (2005) and Nelson (2006). Taxa in red are extinct. |
|
Drawings of Dinichthys (an arthrodire) and Bothriolepis ( a dorso-ventrally flattened bottom-dweller). Drawing of two placoderms from: http://bill.srnr.arizona.edu/classes/182/Placoderm.htm CLASS PLACODERMI+ Heavily armored fish with fused bony head shields that were articulated to allow the movements of the jaws and to allow the head to be lifted relative to the rest of the body. The dermal bony plates included bone cells. Fish with paired pectoral and pelvic appendages. Silurian to Devonian. ORDER
ACANTHOTHORACIFORMES (ACANTHOTHORACI) Head
shield plates separate in young and then fuse later. This is the basal placoderm
group. Upper
Silurian (?) to Lower Devonian. ONE
FAMILY? Brindabellaspis,
Murrindalaspis, Palaeacanthaspis, Radotina, Romundina ORDER RHENANIFORMES (RHENANIDA) Bony
plates reduced to regions on the head and around the eyes.
Body covered by small interlocking plates called tesserae.
Flattened bottom-dwellers; superficially resembled a ray. Lower
Devonian FAMILY
ASTEROSTEIDAE Gemuendina,
Jagorina ORDER
ANTIARCHIFORMES (ANTIARCHI) Bony
shields; pectoral fins encased in bone, some not moveable.
Some like Bothriolepis may have
had lungs. Bottom-dwellers; evidence
that they consumed mud and extracted food from it. Middle
to upper Devonian. FAMILY
CHUCHINOLEPIDAE Chuchinolepis FAMILY
YUNNANOLEPIDAE Phymolepis,
Yunnanolepis UNNAMED
FAMILY Heteroyunnanolepis,
Shimenolepis, Zhanjilepis FAMILY
SINOLEPIDAE Grenfellaspis,
Sinolepis FAMILY
MICROBRACHIIDAE Microbrachius FAMILY
BOTHRIOLEPIDAE Bothriolepis FAMILY
GERDALEPIDAE Gerdalepis FAMILY
ASTEROLEPIDAE Asterolepis,
Remigolepis, Stegolepis, unassigned
genera: Pterichthyodes, Dianolepis,
Minicrania ORDER
PETALLICHTHYIFORMES (PETALICHTHYIDA) Bottom-dwellers
with broad horn-like plates on the sides of their head shields.
Front of head shield with numerous small scales. Lower
to upper Devonian. ONE
FAMILY? Lunaspis,
Eurycaraspis, Macropetalichthys ORDER
PTYCTODONTIFORMES (PTYCTODONTIDA) Bottom-dwellers
with reduced plates and whip-like tails. May
have had claspers. Lower
Devonian to Lower Mississippian? FAMILY
PTYCTODONTIDAE Ctenurella,
Rhamphodopsis ORDER
ARTHRODIRIFORMES (ARTHRODIRA) Best
known of the placoderms, The arthrodires had plates in the anterior jaws that
formed shearing beak-like cutting edges. True
teeth occurred inside the mouth. Lower
Devonian to Lower Mississippian FAMILY
ACTINOLEPIDAE The
most primitive of the arthrodires. Actinolepis,
Aethapsis, Bollandapsis, Eskimapsis, Heightingtonaspis, Kudjanowiaspis FAMILY
PHYLLOLEPIDAE Middle
to upper Devonian Austlophyllolepis,
Placolepis, Phyllolepis FAMILY
BUCHANOSTEIDAE FAMILY
CAMUROPISCIDAE Campuropiscis FAMILY
COCCOSTEIDAE Coccosteus FAMILY DINICHTHYIDAE(?) Dinichthys FAMILY
DUNKLEOSTEIDAE Dunkleosteus,
Eastmanosteus, Hadrosteus FAMILY
MYLOSTOMATIDAE FAMILY
PANXIOSTEIDAE FAMILY
SELENOSTEIDAE FAMILY
TITANICHTHYIDAE FAMILY WUTTAGOONASPIDAE |
