4
1.2 Part I: Biomaterials of Vertebrate Origin. An Overview
1.2.1 Supraclass Agnatha (Jawless Fishes)
Fossil evidence indicates that the group of agnathan (jawless fi shes) species was
once highly successful and extremely varied. The oldest fossil remnants of agnathans
were found in Cambrian rocks. Only two groups, the lampreys and the hagfi sh,
with about 100 species in total, still survive today. The relationship between the
two clades, however, has not been resolved. There are two competing views: the
cyclostome (circular mouth) hypothesis and the vertebrate hypothesis. In the fi rst,
hagfi sh together with lampreys form a monophyletic group, the Cyclostomata. In
the second, lampreys are sister to jawed fi shes and all other jawed animals
(Gnathostomata) and together form the clade Vertebrata. The hagfi shes, which lack
vertebrae, are the sister–group to the Vertebrata. The data in support of the cyclostome
hypothesis are mostly molecular, whereas those in support of the vertebrate hypothesis
are mostly morphological.
The problem of establishing homologies within and among the ingroups and
outgroups remains a challenge. It is of interest to note that Linnaeus ( 1758 ) classifi ed
hagfi shes in the class Vermes and the order Intestina (intestinal worms) and lampreys
in the class Amphibia and the order Nantes (swimming amphibians), erroneous
placements that nevertheless refl ect their great divergence. Both, identifi able stomach
or any appendages have not been identifi ed in all living and most extinct Agnatha.
These animals possess fertilization and development in external form without any
parental care. The Agnatha are cold blooded (ectothermic), and have a cartilaginous
skeleton. Extensively developed bony plates of many extinct agnathans are localized
directly under the skin. These served as protective armor and can be most often found
in the region of the skull. The extant agnathan species possess no bony plates (see
for review Xian–Guang et al. 2002 ; Janvier 2010 ; Renaud 2011 ).
1.2.1.1 Order Osteostraci
The Osteostraci are jawless and represent the sister taxon to jawed vertebrates.
Principally, they are integral to understanding the evolution of gnathostomes from a
jawless ancestor (Sansom 2009 ). The Osteostraci as a relatively compact group,
first appeared in the Late Silurian, fl ourished in the Early Devonian, but were
represented by only a few survivors by the Middle and Late Devonian (Robertson
1935 ). The surface of the exoskeleton of these animals was smooth, however
possess some small dorsal tubercles. Also the pores and grooves of the sensory
canal system and the related lateral lines were to detect. The thin layer of enamel,
underlain by a much thicker layer of dentine-like tissue, was located externally.
Together, the two formed the superfi cial layer. The dentine was perforated with
tubules that arose from a network of small vascular canals at the very top of the
middle layer (Denison 1947 ).
1 Introduction
1.2 Part I: Biomaterials of Vertebrate Origin. An Overview
1.2.1 Supraclass Agnatha (Jawless Fishes)
Fossil evidence indicates that the group of agnathan (jawless fi shes) species was
once highly successful and extremely varied. The oldest fossil remnants of agnathans
were found in Cambrian rocks. Only two groups, the lampreys and the hagfi sh,
with about 100 species in total, still survive today. The relationship between the
two clades, however, has not been resolved. There are two competing views: the
cyclostome (circular mouth) hypothesis and the vertebrate hypothesis. In the fi rst,
hagfi sh together with lampreys form a monophyletic group, the Cyclostomata. In
the second, lampreys are sister to jawed fi shes and all other jawed animals
(Gnathostomata) and together form the clade Vertebrata. The hagfi shes, which lack
vertebrae, are the sister–group to the Vertebrata. The data in support of the cyclostome
hypothesis are mostly molecular, whereas those in support of the vertebrate hypothesis
are mostly morphological.
The problem of establishing homologies within and among the ingroups and
outgroups remains a challenge. It is of interest to note that Linnaeus ( 1758 ) classifi ed
hagfi shes in the class Vermes and the order Intestina (intestinal worms) and lampreys
in the class Amphibia and the order Nantes (swimming amphibians), erroneous
placements that nevertheless refl ect their great divergence. Both, identifi able stomach
or any appendages have not been identifi ed in all living and most extinct Agnatha.
These animals possess fertilization and development in external form without any
parental care. The Agnatha are cold blooded (ectothermic), and have a cartilaginous
skeleton. Extensively developed bony plates of many extinct agnathans are localized
directly under the skin. These served as protective armor and can be most often found
in the region of the skull. The extant agnathan species possess no bony plates (see
for review Xian–Guang et al. 2002 ; Janvier 2010 ; Renaud 2011 ).
1.2.1.1 Order Osteostraci
The Osteostraci are jawless and represent the sister taxon to jawed vertebrates.
Principally, they are integral to understanding the evolution of gnathostomes from a
jawless ancestor (Sansom 2009 ). The Osteostraci as a relatively compact group,
first appeared in the Late Silurian, fl ourished in the Early Devonian, but were
represented by only a few survivors by the Middle and Late Devonian (Robertson
1935 ). The surface of the exoskeleton of these animals was smooth, however
possess some small dorsal tubercles. Also the pores and grooves of the sensory
canal system and the related lateral lines were to detect. The thin layer of enamel,
underlain by a much thicker layer of dentine-like tissue, was located externally.
Together, the two formed the superfi cial layer. The dentine was perforated with
tubules that arose from a network of small vascular canals at the very top of the
middle layer (Denison 1947 ).
1 Introduction