18
the inner dentine layer, are the consequence of variability in the growth rate between
summer and winter. The rings are associated with metabolic variability, which is
effected by fl uctuating water temperatures and food availability. Slow winter growth
leads to the accumulation of continually deposited dentine in the inner dentine, visible
as dark or opaque zones of deposition, whereas fast summer growth is manifested
by light, translucent zones (Calis et al. 2005 ).
Chimeroids have internal fertilization. Their eggs are usually large and enclosed
in brownish colored horny capsules. There is broad variety of their forms in different
species: from spindle-shaped or tadpole-shaped elliptical. Chimeroids eggcases are
called mermaid’s purses or devil’s purses, and vary in size and shape. Some look
much like a mermaid’s purse might: a pouch with four long tendrils on each corner
to anchor the egg somewhere safe. These biopolymer-containing structures are the
reason for bioinspiration by biological materials scientists (see Chap. 12 ).
Class Acanthodii
Acanthodii is derived from the Greek root acantha (Ακανθα), which refers to a
spine (Owen 1846 ). The reference is to the spines at the anterior of each of the body
fi ns. This group include mostly shark-like, small, early jawed fi shes. With exception
of the tail fi n, they possess sharp spines along the leading edges of all of their fi ns.
It is established that the earliest acanthodians were marine. Freshwater species
became predominant only during the Devonian (see for review Janvier 1996 ). Most
of them had large eyes, which suggests that they lived at great depth. Both, the
evolutionary signifi cance of acanthodians and their relationships to modern jawed
vertebrates have been poorly understood until now. For example, it was thought that
the Acanthodii and Osteichthyes (bony fi sh) share a common ancestor separately
from other groups like sharks. The clade based on this common ancestor has been
called Teleostomi (see for review Zhu et al. 1999 ). However, the acanthodian fi sh
Ptomacanthus anglicus was reported as an earlier and more basal form than such
representatives as Acanthodes (Brazeau 2009 ).
The braincase (the internal head skeleton) of P. anglicus more closely resembles
that of early shark-like fi sh, and shares very few features in common with Acanthodes
and the bony vertebrates. Therefore, it is established that Ptomacanthus species
were either a very early relative of sharks, or close to the common ancestry of all
modern jawed vertebrates.
By and large, the acanthodians were lightly armored with scales of bone and
dentine. The scales covered the body of the animal and showed evidence of
concentric growth rings. Also had growing scales that resembled an onion-like
structure, as well as streamlined bodies, which enabled them to be fast swimmers
(Janvier 1996 ). Following orders of Acanthodii have been described: Climatiiformes,
Ischnacanthiformes and Acanthodiformes. Climatiiforma had shoulder armor and
many small sharp spines, while Ischnacanthiforma had teeth fused to the jaw. The
Acanthodiforma possess long gill rakers and were fi lter feeders, with no teeth in
the jaw. It is suggested that jaws of acanthodians evolved from the fi rst gill arch of
1 Introduction
the inner dentine layer, are the consequence of variability in the growth rate between
summer and winter. The rings are associated with metabolic variability, which is
effected by fl uctuating water temperatures and food availability. Slow winter growth
leads to the accumulation of continually deposited dentine in the inner dentine, visible
as dark or opaque zones of deposition, whereas fast summer growth is manifested
by light, translucent zones (Calis et al. 2005 ).
Chimeroids have internal fertilization. Their eggs are usually large and enclosed
in brownish colored horny capsules. There is broad variety of their forms in different
species: from spindle-shaped or tadpole-shaped elliptical. Chimeroids eggcases are
called mermaid’s purses or devil’s purses, and vary in size and shape. Some look
much like a mermaid’s purse might: a pouch with four long tendrils on each corner
to anchor the egg somewhere safe. These biopolymer-containing structures are the
reason for bioinspiration by biological materials scientists (see Chap. 12 ).
Class Acanthodii
Acanthodii is derived from the Greek root acantha (Ακανθα), which refers to a
spine (Owen 1846 ). The reference is to the spines at the anterior of each of the body
fi ns. This group include mostly shark-like, small, early jawed fi shes. With exception
of the tail fi n, they possess sharp spines along the leading edges of all of their fi ns.
It is established that the earliest acanthodians were marine. Freshwater species
became predominant only during the Devonian (see for review Janvier 1996 ). Most
of them had large eyes, which suggests that they lived at great depth. Both, the
evolutionary signifi cance of acanthodians and their relationships to modern jawed
vertebrates have been poorly understood until now. For example, it was thought that
the Acanthodii and Osteichthyes (bony fi sh) share a common ancestor separately
from other groups like sharks. The clade based on this common ancestor has been
called Teleostomi (see for review Zhu et al. 1999 ). However, the acanthodian fi sh
Ptomacanthus anglicus was reported as an earlier and more basal form than such
representatives as Acanthodes (Brazeau 2009 ).
The braincase (the internal head skeleton) of P. anglicus more closely resembles
that of early shark-like fi sh, and shares very few features in common with Acanthodes
and the bony vertebrates. Therefore, it is established that Ptomacanthus species
were either a very early relative of sharks, or close to the common ancestry of all
modern jawed vertebrates.
By and large, the acanthodians were lightly armored with scales of bone and
dentine. The scales covered the body of the animal and showed evidence of
concentric growth rings. Also had growing scales that resembled an onion-like
structure, as well as streamlined bodies, which enabled them to be fast swimmers
(Janvier 1996 ). Following orders of Acanthodii have been described: Climatiiformes,
Ischnacanthiformes and Acanthodiformes. Climatiiforma had shoulder armor and
many small sharp spines, while Ischnacanthiforma had teeth fused to the jaw. The
Acanthodiforma possess long gill rakers and were fi lter feeders, with no teeth in
the jaw. It is suggested that jaws of acanthodians evolved from the fi rst gill arch of
1 Introduction