ARTHRODIRES
=The Arthrodires.=--Another large group of extinct fishes mailed and
helmeted is included under the general name of _Arthrodira_[157]
(~arthros~, joint; ~deira~, neck), or _Arthrognathi_ (~arthros~,
~gnathos~, jaw), the latter term recently framed by Dr. Dean with a
somewhat broader application than the former.
These fishes differ from the Ostracophores, on the one hand, in the
possession of jaws and in the nature of their armored covering. On the
other hand, the nature of these jaws, the lack of differentiation of
the skeleton, and the uncertain character of the limbs separate them
still more widely from the true fishes. Their place in the system is
still unknown, but their origin seems as likely to be traceable to
Ostracophores as to any other group.
The head in all the species is covered with a great bony helmet.
Behind this on the nape is another large shield, and between the two
is usually a huge joint which Dr. Dean compares to the hinge of a
spring-beetle (_Elater_).
As to the presence of limbs, no trace of pectoral fin or anterior
limb has been found. Dean denies the existence of any structures
corresponding to either limb, but Woodward figures a supposititious
posterior limb in _Coccosteus_, finding traces of basal bones which may
belong to it.
These monstrous creatures have been considered by Woodward and others
as mailed Dipnoans, but their singular jaws are quite unlike those
of the _Dipneusti_, and very remote from any structures seen in the
ordinary fish. The turtle-like mandibles seem to be formed of dermal
elements, in which there lies little homology to the jaws of a fish and
not much more with the jaws of Dipnoan or shark.
The relations with the Ostracophores are certainly remote, though
nothing else seems to be any nearer. They have no affinity with the
true Ganoids, to which vaguely limited group many writers have attached
them. Nor is there any sure foundation to the view adopted by Woodward,
that they are to be considered as armored offshoots of the Dipnoans.
According to Dean we might as well refer the Arthrodires to the sharks
as to the Dipnoans. Dean further observes ("Fishes Living and Fossil"):
[Illustration: FIG. 366.--_Coccosteus cuspidatus_ Agassiz, restored.
Lower Devonian. (After Traquair, per Woodward.)]
"The puzzling characters of the Arthrodirans do not seem to be lessened
by a more definite knowledge of their different forms. The tendency, as
already noted, seems to be at present to regard the group provisionally
as a widely modified offshoot of the primitive Dipnoans, basing
this view upon their general structural characters, dermal plates,
dentition, autostylism. But only in the latter regard could they have
differed more widely from the primitive Elasmobranch or Teleostome,
if it be admitted that in the matter of dermal structures they may
be clearly separated from the Chimæroid. It certainly is difficult
to believe that the articulation of the head of Arthrodirans could
have been evolved after dermal bones had come to be formed, or that
a Dipnoan could become so metamorphosed as to lose not only its body
armoring, but its pectoral appendages as well. The size of the pectoral
girdle is, of course, little proof that an anterior pair of fins must
have existed, since this may well have been evolved in relation to
the muscular supports of plastron, carapace, trunk, and head. The
intermovement of the dental plates, seen especially in _Dinichthys_,
is a further difficulty in accepting their direct descent from the
Dipnoans."
[Illustration: FIG. 367.--Jaws of _Dinichthys hertzeri_ Newberry. Upper
Devonian. Ohio. (After Newberry.)]
=Occurrence of Arthrodires.=--These fishes occur in abundance from the
Silurian times to the Mesozoic. In the Devonian their gigantic size and
thick armor gave them the leading position among the hosts of the sea.
Among the genera there occurred "series of forms most interesting as to
their evolution." "It is found more and more evident," says Dr. Dean
("Fishes, Living and Fossil," pp. 135, 136) "that the Arthrodirans may
have represented the dominant group in the Devonian period, as were the
sharks in the Carboniferous, or as are the Teleosts in modern times.
There were forms which, like _Coccosteus_, had eyes at the notches of
the head-buckler; others, like _Macropetalichthys_, in which orbits
were well centralized; some, like _Dinichthys_ and _Titanichthys_,
with the pineal foramen present; some with pectoral spines(?); some
with elaborately sculptured dermal plates. Among their forms appear
to have been those whose shape was apparently subcylindrical, adapted
for swift swimming; others (_Mylostoma_) whose trunk was depressed to
almost ray-like proportions. In size they varied from that of the perch
to that of a basking shark. In dentition they presented the widest
range in variation, from the formidable shear-like jaws of _Dinichthys_
to the lip-like mandibles of _Titanichthys_, the tearing teeth of
_Trachosteus_, the wonderfully forked tooth-bearing jaw-tips of
_Diplognathus_, to the Cestraciont type, _Mylostoma_. The latter form
has hitherto been known only from its dentition, but now proves to be,
as Newberry and Smith Woodward suggested, a typical Arthrodiran."
=Classification of Arthrodira.=--Our knowledge of the systematic
relations of the Arthrodira is mostly of recent origin. Woodward
refers most of the remains to the best known genus _Coccosteus_,
and recognizes as families the _Coccosteidæ_, _Mylostomidæ_,
_Asterosteidæ_, and _Phyllolepidæ_.
[Illustration: FIG. 368.--An Arthrodire, _Dinichthys intermedius_
Newberry, restored. Devonian, Ohio. (After Dean.)]
Dr. Bashford Dean in different papers has treated these fishes
in great detail. In a recent paper on the "Relationships of the
_Arthrognathi_"[158] he recognizes the group as a class coordinate
with _Cyclostomi_ and _Elasmobranchii_. This class, which he calls
_Arthrognathi_, is first divided into two suborders, _Anarthrodira_,
without joint at the neck, and _Arthrodira_, with such a joint. The
former comprises one order, _Stegothalami_, and the latter two orders,
_Temnothoraci_ and _Arthrothoraci_. The following is Dr. Dean's
definition of these orders and their component families:
=Arthrognathi.=--"Chordates whose anterior body region is encased in
dermal elements, and divisible by a more or less definite partition
into head and trunk. Dermal plates which surround the mouth function
as jaws. No evidence of branchial arches. Column notochordal, showing
no traces of centra; well-marked neural and hæmal elements. Paired
limbs [absent or uncertain]. Dermal plates consisting typically of two
layers, the superficial tuberculate, the inner bony with radiating
lamellæ. Orbits situated near or at the margin of the head-shield
and separated from one another by fixed integumental plates. A pineal
funnel present situated in a fixed plate. A mucous system whose canals
radiate from the preoccipital region."
=Anarthrodira.=--"Arthrognaths in which the cranial and dorsal regions
are separated by a fixed partition whose dorsal rim is overlapped and
concealed by superficial plates. Of these a large median dorsal element
is present which extends backward superficially from the region near
the pineal funnel. Also a pair of elements which overlie the position
of the external occipital joint. Suborbital plates apparently absent.
Jaw elements undescribed."
=Stegothalami= (~stegos~, roof; ~thalamos~, chamber).--"Anarthrodires
in which the cranio-dorsal septum is vertical and deep, its height
equal apparently to that of the arch of the head-shield. By this deep
partition the latter appears to inclose two chambers (whence the
ordinal name). Orbits inclosed by pre- and postorbital plates. Mucous
system lacks a postorbital canal."
One family, the _Macropetalichthyidæ_, thus defined:
"Stegothalami with large orbits and well-arched cranio-dorsal
shield. Dorso-central shield long, wide, gomphoidal, extending
backward to the hinder margin of the shield and bordered by all
plates save the postorbitals and marginals. Pineal funnel small and
obscure." _Macropetalichthys sullivanti_ from Ohio Devonian rocks,
and _Macropetalichthys agassizi_ from the Devonian of Germany, are
important species of this group.
The _Asterosteidæ_ perhaps constitute a second family in this order.
The single species _Asterosteus stenocephalus_ is from the Devonian of
Ohio.
=Arthrodira.=--"Arthrognaths in which the dorsal armoring is separated
into dorsal and cranial elements, the latter attached to the former
movably by means of a pair of peg-and-socket joints. The interval
lying between cranial and dorsal armoring does not appear to have
been protected by plates, and in the median line, instead of the
cranio-central of the Anarthrodires, there are separate elements,
median occipital, median dorsal, and perhaps others. Suborbital plates
present. Jaws of three pairs of elements. Ventral armoring of two
pairs of lateral and two median elements."
=Temnothoraci= (~temnô~, to cut; ~thôrax~, thorax).--"Arthrodires
whose cranial and dorsal shields are closely apposed, separated only
by a transverse fissure-like interval (whence the ordinal name);
interarticulation of cranial and dorsal shields little developed.
Head-shield elliptical in outline as far as the line of the transverse
division. The anterior rim of the shoulder-shield flattened at
its sides, suggesting a rudiment of the vertical partition of the
Anarthrodira. Suborbital plate is present, but takes no part,
apparently, in the ventral boundary of the orbit, this being formed,
as in the Anarthrodira, by the pre- and postorbital elements. Jaws,
ventral armoring, and endoskeleton not definitely known."
One family, _Chelonichthyidæ_, thus defined:
"Temnothoraci with orbits relatively small in size and situated
well forward in the head-shield. Occipital elements produced
antero-posteriorly, the external occipital forming the posterior
lateral angle of the head, no projection of the head occurring in the
region of the marginal plate. Median occipital trapezoidal. Centrals
take part in the median boundary of the orbits, and embrace the pineal
plate. Median dorsal with poorly developed keel and terminal process."
_Heterosteus asmussi_ (perhaps to be called _Ichthyosauroides
spinosus_) is a gigantic species from the Lower Devonian of Livonia.
Allied to this species is _Homostius milleri_ from Scotland, celebrated
as the "Asterolepis of Stromness" in Hugh Miller's "Footsteps of the
Creator." Another notable species is _Homostius formosissimus_ from the
Lower Devonian of Russia.
=Arthrothoraci.=--"Arthrodires whose dorsal shield articulates with
the head-roof by a conspicuous and movable peg-and-socket joint, and
leaves a definite interval (unprotected?) between the two armorings.
Orbits marginal, bounded inferiorly not by the suborbital element.
In the head-shield the postero-lateral angles formed by the marginal
plate (_Phlyctænaspis?_), the occipital border concave. A dorsal fin is
present, supported by endoskeletal elements." Five families, the most
important being the _Coccosteidæ_, thus defined:
"Arthrothoraci with head-shield hexagonal in outline. Median occipital
trapezoidal, margins underlapped conspicuously by the external
occipitals. Prefrontals meet below pineal plates, thus occluding this
element from contact with centrals. The median dorsal plate elongated,
terminating in an acute heavy point; no definite ventral keel; its
anterior border approaches the head-shield more closely than in related
families. Cranio-dorsal joint relatively small. Postero-dorso-lateral
large." (?A pair of spines occurs in the pectoral region.) The
best-known species is _Coccosteus cuspidatus_ (_decipiens_) of the
Lower Red Sandstone or Devonian of Scotland.
The family of _Dinichthyidæ_ consists of "Arthrothoraci with stout
trenchant jaws, whose cutting surfaces have worn away marginal teeth.
Plates heavy. Head-shield with conspicuous lateral indentation to form
dorsal border of orbit. Preorbitals separated by rostral and pineal
elements, the latter passing backward between the anterior ends of the
centrals. Cranio-dorsal joint conspicuous. Median dorsal shovel-shaped,
nearing a stout keel with a large neck and with heavy gouge-shaped
terminal. Postero-dorso-lateral relatively small in size." _Dinichthys
hertzeri_ and numerous other species are described from the Devonian
and Carboniferous rocks of Ohio.
The _Titanichthyidæ_ are "Arthrothoraci with slender edentulous jaws
bearing a longitudinal sulcus. Plates squamous. Head-shield wide,
with indentations to form dorsal border of orbit. Cranio-dorsal joint
complete, but of relatively small size. Median dorsal with lateral
border indented with rudimentary keel and with flat and rounded
terminal. Antero-dorso-lateral with an area of overlap on median
border." _Titanichthys agassizi_ is a gigantic mailed fish from the
Lower Carboniferous of Cleveland, Ohio.
The _Mylostomidæ_ are "Arthrothoraci with dental elements in the
character of crushing plates. Cranial shield wide, rounded anteriorly,
deeply indented in nuchal margin; orbital rim not apparent in dorsal
aspect. Central separated from marginal." _Mylostoma terrelli_ is based
on jaws from Cleveland, Ohio.
The _Selenosteidæ_ are "Arthrothoraci with jaws studded with cuspidate
teeth; the mandibular rami rounding out anteriorly or presenting
diverging tips, bearing teeth in the symphysis. Cranial shield deeply
concave on lateral margins, no orbital rim here apparent. Nuchal border
deeply indented. (Centrals separate from marginals.) Cranio-dorsal
hinges large in size. Dorsal armoring reduced antero-posteriorly,
giving an almost zone-like appearance. Dorso-median crescent-shaped,
with feeble keel and knob." _Selenosteus glaber_ is described by Dean
from the Cleveland shales.
=Relations of Arthrodires.=--To complete our account of the Arthrodira
we may here summarize Dr. Dean's reasons for separating its members
from true fishes on the one hand and from the Ostracophores on the
other.
"FIRST. The Arthrodira cannot be strictly included among the Pisces.
According to the definition of the latter class its members are
Craniotes possessing the following characters: _a_, dermal defenses
which in their simplest terms can be reduced to the shagreen
denticles of the Elasmobranch; _b_, a series of definite gill-arches
whose foremost elements are metamorphosed into hyoid and mandibular
apparatus; _c_, paired fins, or their equivalents. In the first of
these regards I think it can be shown that the remarkable character
of the dermal plates in the Arthrognaths approaches rather that
of the Ostracophores than that of the Pisces. In certain of these
forms, _Trachosteus_, for example, the tuberculated plates are made
up of inner and outer elements, each with tubercles, which denote a
distinctly different mode of origin from that of any known type of
fish. The absence of remains of gill-arches in the Arthrognaths would
be not a serious objection to including these forms among Pisces,
especially in view of the fact that cartilaginous gill-arches are
rarely preserved even in favorable fossils. But that their presence
is more than doubtful is indicated by the peculiar character of the
'jaws' in these forms. For the character of these structures is such
as to suggest that they are not homologous with the branchial arch
jaws of the true fishes, but are rather parallel structures which
owe their origin to distinctly exoskeletal elements, i.e., that they
were derived from dermal plates surrounding the mouth, which became
mobile, and whose edges became apposed as sectorial structures. I would
in this connection call attention to the fact that the 'mandibles,'
'premaxillary,' and 'maxillary' dental plates[159] were not fixed
in the sense in which these elements are in the true Pisces. On the
evidence of several types, _Dinichthys_, _Titanichthys_, _Mylostoma_,
_Trachosteus_, _Diplognathus_, and other of the American forms,
_Macropetalichthys_[160] excepted, there is the clearest proof that
each element of the jaws had a considerable amount of independent
movement. On account of the mobility of these elements the name
Arthrognathi is suggested. Thus the mandibular rami could change the
angle of inclination towards each other, as well as their plane with
reference to the vertical axis. So, too, could the 'premaxillæ' be
protracted like a pair of bent fingers, and it is more than probable
that the 'maxillæ' had a considerable amount of independent movement.
In connection with these characters it is also important to note
that the blades of the 'mandible' show nowhere the faintest trace
of an articular facet for attachment to the cranium. In short, the
entire plan of the mandibular apparatus in these forms is strikingly
unfish-like, although one will frankly confess that it is remarkable
that these forms should have paralleled so strikingly the piscine
conditions, to the extent of producing mandibular rami margined with
teeth, and an arrangement of toothed elements on the 'upper jaw' which
resembles superficially the premaxillary and maxillary structures of
teleostomes, or the vomero-palatine structures of lung-fishes and
chimæras.
"In the matter of paired fins there seems little evidence to conclude
that either pectoral or pelvic fins were present. In spite of the
researches upon these forms during the past half-century, no definite
remains of pectoral fins have been described. The so-called pectoral
spines described for _Dinichthys_ by Newberry, whatever they may be,
certainly are not, as far as the present evidence goes, pterygial, nor
are the similar structures in _Brachydirus_.[161] The sigmoid element,
described as a 'pelvic girdle' by Smith Woodward, in Coccosteus, a
structure which appears to occur in a small species of _Dinichthys_(?),
may as reasonably be interpreted as a displaced element of the
armor-plates of the trunk. In _Coccosteus_, as far as I am aware, it
occurs in well-preserved condition in but a single specimen.
"In referring to the singular joint between the shoulder-plates and the
hinder margin of the cranium Smith Woodward has called attention to
one of the striking features of the group. It is one, however, which,
as a functional structure, i.e., a joint, characterizes only a portion
of its members; and in these the region in which vestiges of the joint
are sought is overlaid and concealed by dermal plates. Such are the
conditions in _Macropetalichthys_ (with transitional characters in
_Trachosteus_ and in _Mylostoma_). For this form a special subclass (or
order) may be created which we may term Anarthrodira.
"SECOND. The _Arthrognathi_ cannot well be included in any other class.
It would certainly be more convenient to retain the Arthrognaths
among the Ostracophores, regarding them as a fourth subclass, were it
not that they differ from them in so marked a way in the presence of
well-marked vertebral arches, of supports for the unpaired fin, and in
the possession of 'jaws.' In these regards--add to them the (probable
if not certain) absence of the paired paddle-like 'spines'--they stand
certainly further from the Antiarcha than these from the Osteostraci,
or than the latter from the Heterostraci. It appears to me desirable,
therefore, that the Arthrodira and the Anarthrodira be brought together
as a separate class. Should subsequent researches demonstrate a closer
affinity with the Ostracophores, the Arthrognathi can be regarded as of
rank as a subclass, with the orders Anarthrodira and Arthrodira."[162]
In a recent paper Dr. Otto Jaekel unites Arthrodires and Ostracophores
under the name _Placodermi_. He regards _Pteraspis_ as a larval type,
_Asterolepis_ as one more specialized. In _Coccosteus_ he claims to
find a pelvic girdle as well as a more segmented skeleton. He regards
all of these as true fishes, the _Coccosteidæ_ as ancestral, related
on the one hand to the _Crossopterygians_, and on the other to the
_Stegocephali_ and other ancestral Amphibians.
=Suborder Cycliæ.=--We may append to the _Arthrodira_ as a possible
suborder the group called _Cycliæ_ by Dr. Gill, based on a single
imperfectly known species. Few organisms discovered in recent times
have excited as much interest as this minute fish-like creature, called
_Palæospondylus gunni_, discovered in 1890 by Dr. R. H. Traquair in the
flagstones of Caithness in Scotland. Many specimens have been obtained,
none more than an inch and a half long. Its structure and systematic
position have been discussed by Dr. R. H. Traquair, by Woodward, Gill,
Gegenbaur, and recently by Dean, from whose valuable memoir on "The
Devonian Lamprey" we make several quotations.
[Illustration: FIG. 369.--_Palæospondylus gunni_ Traquair. Devonian.
(After Traquair and Dean.)]
=Palæospondylus.=--According to Dr. Traquair: "The _Palæospondylus
gunni_ is a very small organism, usually under one inch in length,
though exceptionally large specimens occasionally measure one inch and
a half.... It has a head and vertebral column, but no trace of jaws or
limbs; and, strange to say, all the specimens are seen only from the
ventral aspect, as is shown by the relation of the neural arches to the
vertebral centra.
"The head is in most cases much eroded.... It is divided by a
notch ... into two parts.... The _anterior part_ shows a groove the
edges of which are elevated, while the surface on each side shows two
depressions, like fenestræ, though perhaps they are not completely
perforated, and also a groove partially divided off, posteriorly
and externally, a small lobe. In front there is a ring-like
opening ... surrounded by small pointed cirri, four ventrally, at least
five dorsally, and two long lateral ones which seem to arise inside
the margin of the ring instead of from its rim like the others. The
_posterior part_ of the cranium is flattened, but the median groove is
still observable. Connected with the posterior or occipital aspect of
the skull are two small narrow plates which lie closely alongside the
first half-dozen vertebræ.
"The bodies of the vertebræ are hollow or ring-like, and those
immediately in front are separated from each other by perceptible
intervals; their surfaces are marked with a few little longitudinal
grooves, of which one is median. They are provided with neural arches,
which are at first short and quadrate, but towards the caudal extremity
lengthen out into slender neural spines, which form the dorsal
expansion of a caudal fin, while shorter hæmal ones are also developed
on the ventral aspect."
Dr. Traquair concludes that "there seems to be no escape from
the conclusion that the little creature must be classed as a
Marsipobranch." "If _Palæospondylus_ is not a Marsipobranch, it
is quite impossible to refer it to any other existing group of
vertebrates."
=Gill on Palæospondylus.=--In 1896 Dr. Gill proposed to regard
_Palæospondylus_ provisionally as the type of a distinct order of
Cyclostomes to be called _Cycliæ_ (~kyklos~, circle), from the median
ring on the head, whether nostril or mouth. Dr. Gill observes:
"Assuming the correctness of Dr. Traquair's description and figures, we
certainly have a remarkable combination of characters. On the one hand,
if the 'median opening or rim' is indeed nasal, the animal certainly
cannot be referred to the class of Selachians or of Teleostomes.
On the other hand, the cranium and the segmental vertebral column
indicate a more advanced stage of development of the vertebrate line
than that from the living Marsipobranchs must have originated. We
may, therefore, with propriety isolate it as the representative not
only of a peculiar family (_Palæospondylidæ_), but of an order or even
subclass (Cycliæ) of vertebrates which may provisionally (and only
provisionally) be retained in the class of Marsipobranchs.
"The group may be defined as Monorrhines with a continuous (?) cranium,
a median nasal (?) ring, and a segmented vertebral column.
"The differences between the _Hyperoartia_ and _Hyperotreta_ are very
great, and Prof. Lankester did not go much too far when he elevated
those groups to class rank. Among the numerous distinctive characters
are the great differences in the auditory organs. Perhaps the organs
of _Palæospondylus_ might be worked out in some specimen and throw
light on the subject of affinities. At present even the region of
the auditory organs is not exactly known and we are now at a loss to
orient the several parts of the cranium. In fact, the question of the
relations of _Palæospondylus_ is a very open one."
=Views as to the Relationships of Palæospondylus.=--Dr. Dean thus
summarizes in a convenient and interesting fashion the views of
different students of fossil fishes in regard to _Palæospondylus_:
=Huxley.=--A "baby _Coccosteus_."
=Traquair, 1890.=--"Certainly not a Placoderm, its resemblance to a
supposed 'baby Coccosteus' being entirely deceptive. The appearance
of the head does remind us in a strange way of the primitive skull
of _Myxine_, a resemblance which is rendered still more suggestive
by the apparent complete absence of the lower jaw, or of limbs or
limb-girdles."
=Traquair, 1893.=--"It seems, indeed, impossible to refer the organism
to any existing vertebrate class, unless it be the Marsipobranchs or
Cyclostomata." Does not believe it a larval form, because the possible
adult is unknown, and because of the highly differentiated vertebræ.
Granting his interpretation of the parts of the fossil, "there seems no
escape from the conclusion that the little creature must be classed as
a Marsipobranch."
=Traquair, 1897.=--"The question of the affinities of _Palæospondylus_
is left precisely where it was after I had written my last paper on the
subject."
=Smith Woodward, 1892.=--"It seems to possess an unpaired nose, lip
cartilages in place of functional jaws, and no paired limbs; thus
agreeing precisely with the lampreys and hagfishes, of which the fossil
representatives have long been sought. It is extremely probable,
therefore, that _Palæospondylus_ belongs to this interesting category."
=Dawson, 1893.=--_Palæospondylus_ suggests "the smaller snake-like
Batrachians of the Carboniferous and Permian; and I should not be
surprised if it should come to be regarded as either a forerunner of
the Batrachians or as a primitive tadpole."
=Gill, 1896.=--"The group to which _Palæospondylus_ belongs may be
defined as Monorrhines with a continuous (?) cranium, a median nasal
(?) ring, and a segmented vertebral column." "The cranium and segmented
vertebral column indicate a more advanced stage of development of the
vertebrate line than that from which the living Marsipobranchs must
have originated. We may, therefore, with propriety isolate it as the
representative not only of a peculiar family (_Palæospondylidæ_),
but of an order or even subclass (_Cycliæ_) of vertebrates which may
provisionally (and only provisionally) be retained in the class of
Marsipobranchs."
=Dean, 1896.=--"Place it with the Ostracoderms among the curiously
specialized offshoots of the early Chordates, but this position would
be at the best unsatisfactory."
=Dean, 1898.=--"_Palæospondylus_ should not be given a place--even a
provisional one--among the Marsipobranchs." To be accepted "as the
representative of the new subclass (or class) Cycliæ constituted for it
by Professor Gill."
=Parker & Haswell, 1897.=--"There is some reason to regard that
_Palæospondylus_ is referable to the Cyclostomes." "A distinctly higher
type than recent forms."
=Gegenbaur, 1898.=--"Discovery of _Palæospondylus_ one of the highest
importance. If this organism stands in no way near the Cyclostomes, the
tentacles lose their higher importance, since they also occur in other
groups." "Through _Palæospondylus_ came also the attempt (Pollard) to
deduce the presence of the tentacular condition in the higher forms."
(_Mem._--In this Gegenbaur has not consulted the literature accurately.
At the time of founding his "Cirrhostomal Theory" Pollard was unaware
of the discovery of _Palæospondylus_). "Ich muss sagen, das die
positive Behauptung der einen wie der anderen Deutung mir sehr unsicher
scheint, da auch an den übrigen Resten des Kopfskelets keine bestimmten
Uebereinstimmungen mit anderen Organismen erweisbar sind. Es ist daher
auch nicht zu vermuthen, dass sogar an Beziehung zu Froschlarven
gedacht ward. Unter diesen Umständen möchte ich jene im Verhältniss
zum Kopfe wie zum gesammten Körper bedeutende, von Cirren umstellte
Eingangsöffnung als nicht einer Nase, sondern einem Munde oder beiden
zugleich angehörig betrachten. Zu einem dem Cyclostomenriechorgan
vergleichbaren Verhalten fehlen alle Bedingungen."
=Relationships of Palæospondylus.=--The arguments for and against the
supposition that _Palæospondylus_ is a Cyclostome may be here summed up
after Professor Dean.
The vertebral column agrees with that of the lamprey in having the
notochord in part persistent. On the other hand, the vertebræ have
continuous centra, showing definite processes. Those of the different
regions are differentiated. These conditions are quite unlike those
seen in the lamprey.
The cranium is massive, over twice as large proportionally as that
of the lamprey. In the latter type the cranium forms but a small
portion of the bulk of the head; in _Palæospondylus_, on the other
hand, the cranium bears every sign of having filled the contour of the
head. Moreover, if the region adjacent to the structure is admitted
to be that of the eye, and few, I believe, will doubt it, then the
brain-cavity must, by many analogies, have been much larger than that
of a Marsipobranch. Also the auditory capsules must have been of
extraordinary size. In short, there is very little about the cranium to
suggest the structures of Cyclostomes.
The "oral cirri" suggest somewhat the barbels of the nose and mouth
of a hagfish. They, however, resemble even as much in arrangement
and greater number the buccal cirri of _Amphioxus_. On the other
hand, similar mouth-surrounding tentacles are evolved independently
in many groups of fishes, siluroids, sharks, forms like _Pogonias_,
_Hemitripterus_. A possibility further exists that the "cirri" may
turn out to be remnants of cranial or facial structures of an entirely
different nature. In fact the very uncertain preservation of these
parts renders their evidence of little definite value. In but one
specimen, as far as I am aware, is there any evidence of the presence
of ventral cirri.
The jaw parts in _Palæospondylus_ are unknown. It is possible that the
ventral rim of the "nasal ring" may prove to be the remains of the
Meckelian cartilage (the cartilaginous core of the lower jaw).
It is possible that certain very faint ray-like markings noted by
Professor Dean may be the basalia of paired fins. In such case
_Palæospondylus_ can have no affinity with the lampreys. Dr. Dean
asserts that the presence of these, in view of the wide dissimilarity
in other and important structures, is sufficient to remove
_Palæospondylus_ from its provisional position among the Cyclostomes.
The postoccipital plates may represent a pectoral arch. It is, however,
much more likely, as Dr. Traquair has insisted, that the supposed rays
are due to the reflection of light from striations on the stone, and
that the creature had no pectoral limbs.
The caudal fin, with its dichotomous rays, is essentially like the
tail of a lamprey. This condition is, however, found in other groups
of fishes, as among sharks and lung-fishes. It is, moreover, doubtful
whether the rays are really dichotomous.
It is possible that _Palæospondylus_ may be, as Huxley suggests,
a larval Arthrodire. It is not probable that this is the case,
but, on the other hand, _Palæospondylus_ seems to be an immature
form. According to Dr. Dean, it is more likely to prove a larval
_Coccosteus_, or the young of some other Arthrodire, than a
lamprey. Against this view must be urged the fact that the tail of
_Palæospondylus_ is not heterocercal, a fact verified by Dr. Traquair
on all of his many specimens. It is more like the tail of a lamprey
than that of _Coccosteus_. It is, however, certain that it cannot be
placed in the same class with the living _Cyclostomes_, and that it is
far more highly specialized than any of them. In a still later paper
(1904) Dr. Dean shows that the fossil might as easily be considered a
Chimæra as a lamprey, and repeats his conviction that it is a larval
form of which the adult is still unrecognized.
We cannot go much farther than Dr. Dean's statement in 1896, that
it belongs "among the curiously specialized offshoots of the early
Chordates."
FOOTNOTES:
[157] "The name _Arthrodira_ as given to Coccosteans, as distinguished
from the _Antiarcha_, is not altogether a satisfactory one, since at
least from the time of Pander the head of Pterichthys (_Asterolepis_)
is known to be articulated with the armoring of the trunk in a way
closely resembling that of _Coccosteus_. This term may, however,
be retained as a convenient one for the order of Coccosteans, in
which, together with other differentiating features, this structure
is prominently evolved. A renewed examination of the subject has
caused me to incline strongly to the belief, as above expressed, that
_Pterichthys_ and Coccosteans are not as widely separated in phylogeny
as Smith Woodward, for example, has maintained. But, as far as present
evidence goes, they appear to me certainly as distinct as fishes are
from amphibia, or as reptiles are from birds or from mammals." (DEAN.)
The name _Placodermi_ used by McCoy in 1848 was applied to the
_Ostracophores_ as well as to the _Arthrodires_. Hay revives it as the
name of a superorder to include the _Antiarcha_ and the _Arthrodira_,
the former being detached from the _Ostracophores_. This superorder is
equivalent to the subclass _Azygostei_ of Hay.
[158] Memoirs New York Academy of Sciences, 1901.
[159] It will be recalled that there is no ground for concluding
that the "mandibular rami" possessed an endoskeletal core, and were
comparable, therefore, to the somewhat mobile jaws of Elasmobranchs.
On the other hand, there is the strongest evidence that they are
entirely comparable to adjacent dermal plates. Histologically they are
identical, and in certain cases their exposed surfaces bear the same
tuberculation.
[160] The similarity of _Macropetalichthys_ to Dinichthyids in the
general matter of the dermal plates is so complete that I have had no
hesitation in associating it with the Arthrognaths. (_Cf._ Eastman.)
The circumstance that its "jaws" have not yet been found has to a large
degree been due to the lack of energy on the part of local collectors.
In the corniferous quarries near Delaware, Ohio, this fossil is stated
to be relatively abundant.
[161] It is by no means impossible that there may ultimately be found
pectoral elements to correspond in a general way with the paddle-like
"spines" of the Antiarcha.
[162] The group Placodermi, created by McCoy (1848) as a "family"
for the reception of Coccosteus and Pterichthys might then be justly
elevated to rank as a class, superseding the Ostracophori of Cope
(1891). The latter group might, however, be retained as a subclass, and
include the Heterostraci and Osteostraci as ordinal divisions.