FOOTNOTES:
[151] Bulletin Geol. Soc. America, 1892.
[CHAPTER XXXII]
THE CLASS OSTRACOPHORI[152]
Ostracophores.—Among the earliest vertebrates actually recognized as fossils belongs the group known as Ostracophori (ὄστρακος, a box; φορέω, 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."
Fig. 353.—Odontotodus schrencki (Pander) (Tremataspis), ventral side. Island of Oesel. (After Patten.)
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.
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.
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 (ἕτερος], different; ὀστράκος, 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.
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.
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.
Fig. 359.—Pteraspis rostrata Agassiz. Devonian. Family Pteraspidæ. (After Nicholson.)
Order Osteostraci.—The Osteostraci (ὄστεον, bone; οστρακος, 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.
Fig. 360.—Cephalaspis lyelli Agassiz, restored. (After Agassiz.)
Order Antiarcha.—The Antiarcha (ἀντί, opposite; ἀρχός, 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.
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.
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.
Fig. 363.—Pterichthyodes testudinarius Agassiz, side view. (After Zittel, etc.)
Fig. 364.—Birkenia elegans Traquair. Upper Silurian. (After Traquair.)
Order Anaspida.—Recently a fourth order, Anaspida (ἄ, without; ἀσπίς, 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."
Fig. 365.—Lasianius problematicus Traquair. Upper Silurian. (After Traquair.)