THE CLASS OSTRACOPHORI[152]
=Ostracophores.=--Among the earliest vertebrates actually recognized
as fossils belongs the group known as _Ostracophori_ (~ostrakos~,
a box; ~phoreô~, to bear). These are most extraordinary creatures,
jawless, apparently limbless, and enveloped in most cases anteriorly
in a coat of mail. In typical forms the head is very broad, bony, and
horseshoe-shaped, attached to a slender body, often scaly, with small
fins and ending in a heterocercal tail. What the mouth was like can
only be guessed, but no trace of jaws has yet been found in connection
with it. The most remarkable distinctive character is found in the
absence of jaws and limbs in connection with the bony armature. The
latter is, however, sometimes obsolete. The back-bone, as usual in
primitive fishes, is developed as a persistent notochord imperfectly
segmented. The entire absence of jaw structures, as well as the
character of the armature, at once separates them widely from the
mailed _Arthrodires_ of a later period. But it is by no means certain
that these structures were not represented by soft cartilage, of which
no traces have been preserved in the specimens known.
=Nature of the Ostracophores.=--The Ostracophores are found in the
Ordovician or Lower Silurian rocks, in the Upper Silurian, and in
the Devonian. After the latter period they disappear. The species
are very numerous and varied. Their real affinities have been much
disputed. Zittel leaves them with the Ganoids, where Agassiz early
placed them, but they show little homology in structure with the true
Ganoids. Some have regarded them as aberrant Teleosts, possibly as
freakish catfishes. Cope saw in them a huge mailed group of archaic
Tunicates, while Patten has soberly and with considerable plausibility
urged their affinity[153] to the group of spiders, especially to
the horseshoe-crabs (_Limulus_) and their palæozoic ancestors, the
_Eurypteridæ_ and _Merostomata_.
The best guess as to the affinities of the Ostracophores is perhaps
that given by Dr. Ramsey H. Traquair ("Fossil Fishes of the Silurian
Rocks of the South of Scotland," 1899). Traquair regards them as highly
aberrant sharks, or, more exactly, as being derived, like the Chimæras,
from a primitive Elasmobranch stock. In favor of this view is the
character of their armature, the bony plates themselves to be regarded
as formed by the fusion of shagreen grains or scales. According to
Traquair: "Specialization from the most specialized form, _Lanarkia_,
has been accompanied by (1) fusion of the spinelets (_Lanarkia_) or
shagreen grains (_Thelodus_) into plates, scutes, and rhombic scales,
supported by hard matter developed in a deeper layer of skin, and (2)
alterations in the pectoral fin-flaps, which, becoming covered up by
the postero-lateral plates in _Drepanaspis_, are finally no longer
recognizable in the _Pteraspidæ_."
[Illustration: FIG. 353.--_Odontotodus schrencki_ (Pander)
(_Tremataspis_), ventral side. Island of Oesel. (After Patten.)]
[Illustration: FIG. 354.--_Odontotodus schrencki_ (Pander)
(_Tremataspis_), dorsal side. Island of Oesel. (After Patten.)]
Woodward leaves their exact relationship undefined, while others have
regarded them as mailed lampreys, at any rate to be excluded from
the _Gnathostomi_, or jaw-bearing series. The apparent absence of
true jaws, true limbs, and limb-girdles certainly seems to separate
them widely from true fishes, but these characters are negative
only, perhaps due to degeneration, and at any rate they are not yet
absolutely determined. Certainly they offer no positive proof of
affinity with the modern Cyclostomes.
Dr. Traquair regards the _Heterostraci_ or most primitive
_Ostracophores_ as most certainly derived from the Elasmobranchs. Other
writers have attacked the integrity of the group of Ostracophores,
questioning the mutual relationship of its component parts. Reiss,
for example, regards the association of the _Osteostraci_ with the
_Heterostraci_ as "unbegründet" and "unheilvoll," while Ray Lankester,
as quoted by Traquair, affirms that "there is absolutely no reason for
regarding _Cephalaspis_ as allied to _Pteraspis_ beyond that the two
genera occur in the same rocks, and still less for concluding that
either has any connection with _Pterichthys_." Elsewhere Lankester
states that the _Heterostraci_ are associated at present with the
_Osteostraci_, "because they have, like _Cephalaspis_, a large
head-shield, and because there is nothing else with which to associate
them." Patten, on the other hand, seems inclined to deny the rank of
_Heterostraci_ and _Osteostraci_ as even separate orders, regarding
them as very closely related to each other as also to their supposed
spider-like ancestors.
[Illustration: FIG. 355.--Head of _Odontotodus schrencki_ Pander, from
the side. (After Patten.)]
But the consensus of opinion favors the belief that the four orders
usually included under this head are distinct and at the same time are
really related one to another. For our purposes, then, we may regard
the _Ostracophori_ as a distinct class of vertebrates. By placing it
after the Elasmobranchs we may indicate its probable descent from a
primitive shark-like stock.
[Illustration: FIG. 356.--The Horseshoe Crab or King-crab, _Limulus
polyphemus_ Linnæus. Supposed by Professor Patten to be an ally of the
Ostracophores; usually regarded as related to the Spiders.]
On this subject Dr. Dean remarks: "The entire problem of the homology
of the dermal plates and 'scales' in the Ostracophores and Arthrognaths
is to the writer by no means as clear as previous writers have
conceded. From the histological standpoint, admitting the craniote
nature of the vasodentine and cancellous layers in the dermal plates,
it nevertheless does not follow that they have been derived from the
actual conditions of the dermal denticles of the ancestral Gnathostome,
as were unquestionably the dermal plates of Teleostomes and Dipnoans.
It seems equally if not more probable, on the other hand, that the
dermal armoring of the distinct groups may have had an altogether
different mode of origin, the product of a crude evolution which aimed
to strengthen the skin by a general deposition of calcareous matter
throughout its entire thickness. The tuberculation of plates thus
acquired might have become an important step in the development of a
more superficial type of armoring which is most preferably represented
by the dermal denticles of Selachians. Nor, in passing, need the
presence of a mucus-canal system in the early plated forms be of
greater morphological importance than a foreshadowing of the conditions
of Gnathostomes, for this system of organs might serve as well as
evidence, in a general way, of relationship with Marsipobranchs. Nor
is this evidence the more conclusive when we reflect that _no known
type of Gnathostome, recent or fossil, possesses open sensory grooves
in distinct dermal plates_. The presence, furthermore, of a dorsal
fin and a 'truly piscine heterocercal tail,' as noted by Traquair, is
by no means as Gnathostome-like as these structures at first glimpse
appear. For they lack not merely the characteristic radial supports of
fishes, but even actinotrichia. Their mode of support, on the other
hand, as Smith Woodward points out, is of a more generalized nature,
bent scales, homologous with those of the adjacent body region, taking
the place of the piscine external supports." The actual position in the
system to be finally assigned to the Ostracophores is therefore still
uncertain.
=Orders of Ostracophores.=--Four orders of _Ostracophori_ are now
usually recognized, known in the systems of Woodward and Traquair as
_Heterostraci_, _Osteostraci_, _Antiarcha_, and _Anaspida_. The former
is the most primitive and perhaps the most nearly allied to the sharks,
the second is not very remote from it, the last two aberrant in very
different directions. Hay places the _Antiarcha_ with the _Arthrodira_
under the superorder of _Placodermi_.
=Order Heterostraci.=--The _Heterostraci_ (~heteros~], different;
~ostrakos~, box) have no bone-corpuscles in the coat of mail. This
typically consists of a few pieces above, firmly united and traversed
by dermal sense-organs or "lateral lines." The ventral shield is
simple. Four families are recognized by Traquair as constituting the
_Heterostraci_, these forming a continuous series from shark-like forms
to the carapace-covered _Pteraspis_. In the most primitive family,
the _Thelodontidæ_,[154] the head and trunk are covered with small
scales or tubercles of dentine and not fused into large plates. The
tail is slender and heterocercal, the caudal fin deeply forked. Until
lately these tubercles were regarded as belonging to sharks, and they
are still regarded by Traquair as evidence of the affinity of the
_Heterostraci_ with the _Acanthodei_. Dr. Traquair thinks that a flap
or lappet-like projection behind the head may be a pectoral fin.
The three known genera are _Thelodus_, _Lanarkia_, and _Ateleaspis_.
In _Thelodus_ the scales consist of a base and a crown separated by
a constriction or neck. _Thelodus scoticus_, _Thelodus pagei_, and
_Thelodus planus_ are found in the Silurian rocks of Scotland. Other
species, as _Thelodus tulensis_ of Russia, extend to the Upper Devonian.
In _Lanarkia_ the large sharp scales have an expanded base like the
mouth of a trumpet. _Lanarkia horrida_ and _L. spinulosa_ are found
in the shire of Lanark in Scotland. In _Ateleaspis_ (_tesselatus_)
the skin is covered with small polygonal plates. The lateral flaps or
possibly fins take the form of flat rhombic sculptured scales. In this
genus the eyes seem to be on the top of the head.
[Illustration: FIG. 357.--_Lanarkia spinosa_ Traquair. Upper Silurian.
Family _Thelodontidæ_. (After Traquair.)]
In the _Psammosteidæ_ of the Devonian the head is covered with large
plates which are not penetrated by the sense-organs. These plates are
covered with minute, close-set tubercles, covered with brilliant ganoid
enamel and with finely crimped edges. According to Dr. Traquair, these
tubercles are shagreen granules which have coalesced and become united
to plates formed in a deeper layer of the skin, as in _Ateleaspis_ the
minute scales have run together into polygonal plates. These creatures
have been considered as "armored sharks," and Dr. Traquair regards them
as really related to the acanthodean sharks. Nevertheless they are not
really sharks at all, and they find their place with the _Pteraspis_
and other longer known Heterostracans.
The family of _Drepanaspidæ_ consists of a single recently known
species, _Drepanaspis gmundenensis_, found in a pyritized condition
in purple roofing-slate in Gmünden, Germany. This fish, which reaches
a length of about two feet, has a broad head, with eyes on its outer
margin, with a slender body and heterocercal tail. The head has a
broad median plate and smaller polygonal ones. The flaps, supposed
to represent the pectoral fins, are here cased in immovable bone. No
trace of internal skeleton is found by Traquair, who has given the
restoration of this species, but the mouth has been outlined.
[Illustration: FIG. 358.--_Drepanaspis gmundenensis_ Schlüter. Upper
Silurian, Gmünden, Germany. (After Traquair.)]
The best known of the Heterostracan families is that of _Pteraspidæ_.
In this family the plates of the head are coalesced in a large carpace,
the upper part originally formed of seven coalesced pieces. A stout
dorsal spine fits into a notch of the carapace. The slender body is
covered with small scales and ends in a heterocercal tail. The dermal
sense-organs are well developed. _Pteraspis rostrata_ occurs in the
Lower Devonian. Other genera are _Palæaspis_ and _Cyrthaspis_.
[Illustration: FIG. 359.--_Pteraspis rostrata_ Agassiz. Devonian.
Family _Pteraspidæ_. (After Nicholson.)]
=Order Osteostraci.=--The Osteostraci (~osteon~, bone; ~ostrakos~, box)
(called _Aspidocephali_ by Rohon) have bone-corpuscles in the shields,
and the shield of the back is in one piece without lateral-line
channels or sense-organs. Ventral shield single. The order includes
three families. The _Cephalaspidæ_ have the shields tuberculate,
the one between the eyes fixed, and the anterior body-shields are
not fused into a continuous plate. The best known of the numerous
species is _Cephalaspis lyelli_ from the Lower Devonian of England.
_Hemicyclaspis murchisoni_ occurs in the Upper Silurian of England, and
the extraordinary _Cephalaspis dawsoni_ in the Lower Devonian of Gaspé,
Canada. _Eukeraspis pustulifera_ has the head-shield very slender and
armed with prickles. In the _Thyestidæ_ the anterior body-scales are
fused into a continuous plate. _Thyestis_ and _Didymaspis_ are genera
of this type. The _Odontotodontidæ_ (_Tremataspidæ_) have the shield
truncate behind, its surface finely punctate, and the piece between
the eyes not fixed. _Odontotodus[155] schrenki_ is found in the Upper
Silurian of the Island of Oesel in company with species of _Thyestes_.
The _Euphaneropidæ_ are represented in the Devonian of Quebec.
[Illustration: FIG. 360.--_Cephalaspis lyelli_ Agassiz, restored.
(After Agassiz.)]
=Order Antiarcha.=--The Antiarcha (~anti~, opposite; ~archos~, anus)
have also bone-corpuscles in the plates, which are also enameled. The
sense-organs occupy open grooves, and the dorsal and ventral shields
are of many pieces. The head is jointed on the trunk, and jointed to
the head are paddle-like appendages, covered with bony plates and
resembling limbs. There is no evidence that these erectile plates
are real limbs. They seem to be rather jointed appendages of the
head-plate, erectile on a hinge like a pectoral spine. There are traces
of ear-cavities, gill-arches, and other fish-like structures, but
nothing suggestive of mouth or limbs.
This group contains one family, the _Asterolepidæ_, with numerous
species, mostly from Devonian rocks. The best known genus is
_Pterichthyodes_,[156] in which the anterior median plate of the back
is overlapped by the posterior dorso-lateral. _Pterichthyodes milleri_
from the Lower Devonian, named by Agassiz for Hugh Miller, is the best
known species, although numerous others, mostly from Scottish quarries,
are in the British Museum. _Asterolepis maximus_ is a very large
species from the same region, known from a single plate. _Bothriolepis
canadensis_ is from the Upper Devonian of Scaumenac Bay near Quebec,
numerous specimens and fragments finely preserved having been found.
[Illustration: FIG. 361.--_Cephalaspis dawsoni_ Lankester. Lower
Devonian of Canada. Family _Cephalaspidæ_. (After Woodward.) In the
square a portion of the tubercular surface is shown.]
_Microbrachium dicki_ with the pectoral appendages small occurs in the
Devonian of Scotland.
The earliest remains of _Ostracophori_ are found in Ordovician or
Lower Silurian rocks of the Trenton horizon at Cañon City, Colorado.
These consist of enormous numbers of small fragments of bones
mixed with sand. With these is a portion of the head carapace of a
small Ostracophore which has been named by Dr. Walcott _Asteraspis
desiderata_ and referred provisionally to the family of _Asterolepidæ_,
which belongs otherwise to the Lower Devonian.
[Illustration: FIG. 362.--_Pterichthyodes testudinarius_ (Agassiz),
restored. Lower Devonian Family _Asterolepidæ_. (After Traquair and
others.)]
With these remains are found also scales possibly belonging to a
Crossopterygian fish (_Eriptychius_). These remains make it evident
that the beginning of the fish series lies far earlier than the rocks
called Silurian, although fishes in numbers are not elsewhere known
from rocks earlier than the Ludlow shales of the Upper Silurian,
corresponding nearly to the Niagara period in America.
In the Ludlow shales we find the next appearance of the Ostracophores,
two families, _Thelodontidæ_ and _Birkeniidæ_, being there represented.
[Illustration: FIG. 363.--_Pterichthyodes testudinarius_ Agassiz, side
view. (After Zittel, etc.)]
[Illustration: FIG. 364.--_Birkenia elegans_ Traquair. Upper Silurian.
(After Traquair.)]
=Order Anaspida.=--Recently a fourth order, _Anaspida_ (~a~, without;
~aspis~, shield), has been added to the _Ostracophori_ through the
researches of Dr. Traquair. This group occurs in the Upper Silurian
in the south of Scotland. It includes the single family _Birkeniidæ_,
characterized by the fusiform body, bluntly rounded head, bilobate,
heterocercal tail, and a median row of hooked spinous scales along the
ventral margin. No trace of jaws, teeth, limbs, or internal skeleton
has been found. Unlike other Ostracophores, _Birkenia_ has no cranial
buckler with orbits on the top, nor have the scales and tubercles
the microscopic structure found in other Ostracophores. In the genus
_Birkenia_ the head and body are completely covered by tubercular
scutes. The gill-openings seem to be represented by a series of
small perforations on the sides. A dorsal fin is present. _Birkenia
elegans_ is from the Ludlow and Downstonian rocks of southern Scotland.
_Lasianius problematicus_ from the same rocks is very similar, but is
scaleless. It has a row of ventral plates like those of _Birkenia_, the
only other hard parts it possesses being a number of parallel rods
behind the head, homologous with the lateral series of _Birkenia_.
_Lasianius_ is therefore a specialized and degenerate representation
of _Birkenia_, differing somewhat as "the nearly naked _Phanerosteon_
differs from other _Palæoniscidæ_ whose bodies are covered with osseous
scales."
[Illustration: FIG. 365.--_Lasianius problematicus_ Traquair. Upper
Silurian. (After Traquair.)]
FOOTNOTES:
[152] This group was first called by Cope _Ostracodermi_--a name
preoccupied for the group of bony trunkfishes, _Ostracidæ_. The still
earlier name of _Placodermi_, chosen by McCoy (1848), was intended
to include Arthrodires as well as Ostracophores. Rohon (1892) calls
the group _Protocephali_, and to the two orders he assigns the
names _Aspidorhini_ and _Aspidocephali_. These groups correspond to
_Heterostraci_ and _Osteostraci_ of Woodward. Another name of early
date is that of _Aspidoganoidei_, given by Professor Gill in 1876, but
not defined until 1896. These fishes are, however, not "Ganoids" and
the name _Ostracophori_ seems to receive general preference. The group
_Peltacephalata_ of Patten corresponds essentially to _Ostracophori_,
as does also the order _Hypostomata_ of Gadow.
[153] According to Professor Patten's view, the close resemblance of
the shields of _Pteraspis_ to those of contemporaneous _Eurypterids_
indicates real affinity. But the _Eurypterids_ are related to the
spiders and to _Limulus_. The only reason for thinking that _Pteraspis_
is a fish at all lies in its resemblance to _Cephalaspis_, which is
in several ways fish-like, although its head shield is much like that
of _Limulus_. All these resemblances in Patten's view indicate real
affinity. Patten considers the _Pteraspids_ as derived from primitive
arachnid or spider-like forms having a bony carapace as _Limulus_ has.
From _Pteraspis_ he derives the other Ostracophores, and from these
the sharks and other vertebrates, all of which appear later in time
than the earliest Ostracophores. This view of the origin of vertebrates
is recently urged with much force by Professor Patten (Amer. Nat.,
1904, 1827). Most naturalists regard such resemblances in specialized
structures on the outside of an animal as parallelisms due to likeness
in conditions of life. The external structure in forms of really
different nature is often similarly modified. Thus certain catfishes,
pipefishes, sea-moths, and agonoid fishes are all provided with bony
plates not unlike those of ganoid fishes, although indicative of no
real affinity with them. Commonly the ancestry of vertebrates is traced
through enteropneustans to soft-bodied worms which have left no trace
in the rocks.
In the same connection, Professor Patten suggests that the lateral fold
from which many writers have supposed that the limbs or paired fins
of vertebrates is evolved is itself a resultant of the fusion of the
fringing appendages on the sides of the body. Such appendages are found
in the primitive mailed arachnoids and in _Limulus_. They are shown
very plainly in Patten's restoration of _Cephalaspis_. About thirty
of them of a bony nature and jointed to the body occur on either side
between the gill opening and the vent.
[154] Called _Coelolepidæ_ by Pander and Traquair, but _Coelolepis_ is
a later synonym of _Thelodus_.
[155] This name, inappropriate or meaningless, is older than
_Tremataspis_.
[156] The earlier name of _Pterichthys_ has been already used for a
genus of living fishes.