5
1.2.1.2 Order Anaspida
The anaspida were a now extinct order of fi sh-like vertebrates. In contrast to the
Osteostraci, their body, was less fl attened and covered with dermal scales including
the head region. These animals possess hypocercal (tilting downwards) tail. Usually,
Anaspids were up to 15 cm in length. The Anaspids, which ranged from the Late
Silurian to the Late Devonian, included: Jaymoytius, Pharyngolepis, and Pterygolepis
(Allaby and Allaby 1999 ).
1.2.1.3 Order Heterostraci
The fossil group of heterostracans (Heterostraci) represents a large clade of the
Pteraspidomorphi. These armored, but jawless, vertebrates lived about 430–370
million years ago (from the Early Silurian to the Late Devonian). Their armored
head was generally fusiform and a tail fan-shaped. Both, the large dorsal and a large
ventral shield shields were formed by two plates. The exact morphological traits
with respect to scales are differing from one group of Heterostraci to another. For
example, such primitive forms as Lepidaspis, possess dorsal and ventral shields
which are composed of a mosaic of tiny scales.
From histological point of view, the scales observed in representatives of this
group, are distinct from other vertebrates. Their scales have dentine- and aspidinebased layers as well as an acellular bony tissue that is known to be unique to this
class (Halstead Tarlo 1963 ).
The so called “cancella” represents the honeycombed middle layer. In pteraspidomorphs, the unique biological material aspidin is present both in the attachment
of bone associated with the superfi cial dentine–enameloid tubercles, and in the
dermoskeleton which contains characteristic “spongy” and basal “lamellar” layers.
Aspidin is dominated by a collagenous fi brillar organic matrix and is acellular
(Donoghue and Sansom 2002 ).
There are about 300 species related to Heterostracans. They habituates were
sandy lagoons or deltas including marine environments with exception of some
fresh water species. They are known to occur in Europe, Siberia as well as in North
America. It is suggested that these animals were poor swimmers and probably
bottom- dwellers, and fed by scraping the bottom with their fan-shaped oral
plates that armed their lower lip. The representatives of Psammosteidae, developed
steer- like branchial plates and could grow up to 1.5 m in length and, however, most
heterostracans were relatively small (5–30 cm in total length). Their internal
anatomy is only known from the impressions of the internal organs on the internal
surface of the dermal armor because heterostracans have no calcifi ed endoskeleton.
One may trace the impressions of two distinct vertical semicircular canals of the
labyrinth as well as of the gills, eyeballs, paired olfactory organ, and brain. Similar
to extant hagfi sh, their olfactory organs seem to have opened ventrally into a large,
median inhalant duct (see for review Halstead 1973 ; Janvier and Blieck 1979 ;
Janvier 1997a ).
1.2 Part I: Biomaterials of Vertebrate Origin. An Overview
1.2.1.2 Order Anaspida
The anaspida were a now extinct order of fi sh-like vertebrates. In contrast to the
Osteostraci, their body, was less fl attened and covered with dermal scales including
the head region. These animals possess hypocercal (tilting downwards) tail. Usually,
Anaspids were up to 15 cm in length. The Anaspids, which ranged from the Late
Silurian to the Late Devonian, included: Jaymoytius, Pharyngolepis, and Pterygolepis
(Allaby and Allaby 1999 ).
1.2.1.3 Order Heterostraci
The fossil group of heterostracans (Heterostraci) represents a large clade of the
Pteraspidomorphi. These armored, but jawless, vertebrates lived about 430–370
million years ago (from the Early Silurian to the Late Devonian). Their armored
head was generally fusiform and a tail fan-shaped. Both, the large dorsal and a large
ventral shield shields were formed by two plates. The exact morphological traits
with respect to scales are differing from one group of Heterostraci to another. For
example, such primitive forms as Lepidaspis, possess dorsal and ventral shields
which are composed of a mosaic of tiny scales.
From histological point of view, the scales observed in representatives of this
group, are distinct from other vertebrates. Their scales have dentine- and aspidinebased layers as well as an acellular bony tissue that is known to be unique to this
class (Halstead Tarlo 1963 ).
The so called “cancella” represents the honeycombed middle layer. In pteraspidomorphs, the unique biological material aspidin is present both in the attachment
of bone associated with the superfi cial dentine–enameloid tubercles, and in the
dermoskeleton which contains characteristic “spongy” and basal “lamellar” layers.
Aspidin is dominated by a collagenous fi brillar organic matrix and is acellular
(Donoghue and Sansom 2002 ).
There are about 300 species related to Heterostracans. They habituates were
sandy lagoons or deltas including marine environments with exception of some
fresh water species. They are known to occur in Europe, Siberia as well as in North
America. It is suggested that these animals were poor swimmers and probably
bottom- dwellers, and fed by scraping the bottom with their fan-shaped oral
plates that armed their lower lip. The representatives of Psammosteidae, developed
steer- like branchial plates and could grow up to 1.5 m in length and, however, most
heterostracans were relatively small (5–30 cm in total length). Their internal
anatomy is only known from the impressions of the internal organs on the internal
surface of the dermal armor because heterostracans have no calcifi ed endoskeleton.
One may trace the impressions of two distinct vertical semicircular canals of the
labyrinth as well as of the gills, eyeballs, paired olfactory organ, and brain. Similar
to extant hagfi sh, their olfactory organs seem to have opened ventrally into a large,
median inhalant duct (see for review Halstead 1973 ; Janvier and Blieck 1979 ;
Janvier 1997a ).
1.2 Part I: Biomaterials of Vertebrate Origin. An Overview