BRACHIOPODA
PART II
PALAEONTOLOGY OF THE BRACHIOPODA
BY
F. R. COWPER REED, B.A., F.G.S.
Trinity College, Cambridge
CHAPTER XVIII
PALAEONTOLOGY OF THE BRACHIOPODA
INTRODUCTION—DIVISION I. ECARDINES—EXTERNAL CHARACTERS—INTERNAL CHARACTERS—DIVISION II. TESTICARDINES—EXTERNAL CHARACTERS—INTERNAL CHARACTERS—SYNOPSIS OF FAMILIES—STRATIGRAPHICAL DISTRIBUTION—PHYLOGENY AND ONTOGENY
Introduction
The wide distribution and vast abundance of the Brachiopoda throughout the whole series of geological formations make this group of especial importance to the student of the past history of the earth; and the zoologist must always regard the fossil forms with peculiar interest, because they not only largely outnumber the living representatives, but comprise numerous extinct genera, and even families, exhibiting types of structure and characters entirely absent in the modern members of the group. It is a most fortunate circumstance that the excellent state of preservation in which we frequently find them, and the immense amount of material at our disposal, enable us to determine with accuracy and certainty the internal characters of the shells in the great majority of cases. But it is only since the beginning of the present century that our knowledge of the anatomy of the soft parts of the living animal has rendered any tracing of homologies possible. In the case of features in fossil extinct types the interpretation must be to some extent doubtful. Barrande, Clarke, Davidson, Hall, King, Oehlert, Waagen, de Verneuil, and a host of other workers have contributed to the information which we now possess; and their works must be consulted for details of the subject.[428]
Since all Brachiopods are inhabitants of the sea, the geologist at once recognises as a marine deposit any bed which contains their remains. Under favourable conditions they swarmed in the seas of Palaeozoic and Mesozoic times. Beds of limestone are frequently almost entirely composed of their shells, as, for instance, some of the Devonian limestones of Bohemia. Often they give the facies to the fauna and outnumber in species and individuals all the other organisms of the period. The Ungulite Sandstone (Cambrian) of Russia and the Productus Limestone of the Salt Range in India of Carboniferous and Permian age are well-known examples.
Many species seem to have been gregarious in habit; thus Productus giganteus of the Carboniferous Limestone may generally be found in crowded masses, as in some localities in Yorkshire.
The fact that certain species of Brachiopods characterise definite stratigraphical horizons or “zones” gives them occasionally an importance equal to that of Graptolites; for instance, the Ecardinate species Trematis corona marks a set of beds in the Ordovician, and the isolated Stringocephalus Burtini is restricted to the upper part of the Middle Devonian, giving to the limestone on that horizon its distinctive name. It is noteworthy also how certain species affect a sandy and others a calcareous sea-bottom, so that beds of the same age show differences in their Brachiopod fauna owing to a dissimilar lithological composition.
While few of the recent Brachiopods reach a large size, some of the extinct species measure several inches in breadth, but the great Productus giganteus attained the width of even a foot.
The bright colours of the shells of the living animals are not generally preserved amongst the fossil species from the older rocks; yet in a Carboniferous Terebratula we can even now detect the purple bands in some specimens, and a Cretaceous Rhynchonella similarly exhibits its original colour.
The Brachiopoda are evidently a group in its decline, as the geological record shows; but they date back from the earliest known fossiliferous rocks, in which the Ecardinate division is alone represented. As we ascend through the stratigraphical series the number and variety of genera and species belonging to both divisions rapidly increase until in the united Ordovician and Silurian there are nearly 2000 species and about 70 genera. From this point of maximum development down to the present day there is a gradual decrease in numbers.
According to Davidson, at least 17 Upper Tertiary species are still living on our sea-bottoms; and many recent Mediterranean forms occur in the Pliocene rocks of the islands and shores of that sea, and in the Crags of East Anglia.
A brief review of the chief characteristics of fossil Brachiopoda is given below. Those genera which have the greatest zoological or geological importance can alone be noticed owing to the exigencies of space.
I. ECARDINES
External Characters
A considerable diversity of external form is met with even in this division, from the limpet-like Discina to the flattened tongue-shaped Lingula. The valves have most commonly a smooth external surface with delicate growth-lines; but sometimes pittings (Trematis) or radiating ribs (Crania) are present, and in a few forms the shell is furnished with spines (Siphonotreta), which perhaps serve to anchor it in the soft mud of the sea-bottom. The usual mode of fixation was by means of the pedicle (= peduncle or stalk), which either (1) passed out simply between the posterior gaping portion of the valves (Lingula), or (2) lay in a slit in the ventral valve (Lingulella), or (3) pierced the substance of the latter valve by a definite foramen (Discina). The first-mentioned condition of the pedicle seems the most primitive. Rarely the pedicle was absent, and the shell was attached by the whole surface of the ventral valve (Crania, p. [467]).
The two valves in the fossil Ecardines were held together by muscular action, though in some families (Trimerellidae) we see traces of articulating processes. The “hinge line,” or line along which the valves worked as on a hinge, is in most forms more or less curved. A “hinge area” (i.e. that portion of the shell generally smoother than other parts of the valves, more or less triangular in form, and lying between the beaks on one or both sides of the hinge line), is usually absent in the Ecardines.
Fig. 322.—Muscle-scars of Lingula anatina. Inner surface of A, Pedicle-valve or ventral valve. B, Brachial or dorsal valve; p.s, parietal scar; u, umbonal muscle; t, transmedians; c, centrals; a.m.e, laterals (a, anteriors; m, middles; e, externals).
Fig. 323.—Trimerella. (After Davidson and King.) A, Inner surface of pedicle-valve or ventral valve: a, pseudo-deltidium; b, deltidial slope; c, deltidial ridges; d, areal borders; e, cardinal callosities; f, cardinal facet; g, lozenge; i, umbonal chambers separated by cardinal buttress; j, platform; k, platform vaults; l, median plate; m, median scars; n, anterior scars; o, lateral scars; p, post-median scars; q, crown crescent; r, side or lateral crescent; s, end or terminal crescent; t, transverse scars; u, archlet (vascular sinuses); w, sub-cardinal scars; x, umbo-lateral scars. B, Brachial or dorsal valve: e, cardinal sockets; j, platform; k, platform vaults; l, median plate; m, median scars; n, anterior scars; q, crown crescent; r, side or lateral crescent; s, end or terminal crescent; t, transverse scars; u, archlet (vascular sinuses); v, cardinal scars; w, sub-cardinal scars.
Internal Characters
Owing to the rarity of well-preserved interiors of valves in this division, our knowledge of their internal characters is still far from satisfactory. The arrangement of the muscular impressions varies greatly amongst extinct genera, but we are often able to interpret them with a considerable amount of certainty by a study of the scars and the muscles of the well-known recent Lingula (Fig. [322]). The extreme specialisation of the muscles in many of the earliest genera (e.g. Lingula) is remarkable, and points to a long but so far undiscovered ancestry in pre-Cambrian times.[429] In fossil species of Crania and Lingula the muscle-scars correspond closely with those in the living representatives of these genera. In the most highly specialised family of the Ecardines—the Trimerellidae—we meet with features of peculiar interest.[430] The muscle-scars in this family (Fig. [323], A, B) are most remarkable for the development of the so-called “crescent,” (q.r.s.) which skirts the posterior margin of both valves as a sub-cardinal impression. It is believed to be the trace of a strong post-parietal muscular wall, analogous in position to that of Lingula. The three pairs of “lateral” muscle-scars in the latter genus seem to be represented by the “terminal” (s) and “lateral” (r) scars on the crescent of the Trimerellidae. A pair of “transverse” scars (t) occurs in each valve between the “terminals” and the antero-lateral edge of the “platform” (j). “Cardinal” (v), “sub-cardinal” (w), and “umbo-lateral” (x) scars also occur. The median impression which covers the “platform” (j) consists of a central, lateral, and usually an anterior pair of scars; and the impressions of the genital organs, according to Davidson and King, lie medianly posterior to the “platform.” The “platform” itself is a more or less conspicuous central calcareous elevated area occurring in each valve, but most developed in the dorsal; in some cases it is double-chambered with tubular cavities (“platform vaults,” Fig. [323], A, B, k), in others it is more or less solid. It appears to have originated through a posterior shifting of the central muscular bands, that they might be inserted behind the liver; at the same time a deposition of shelly material, to form fulcra to work the heavy valves, took place at these points. The tunnelling-out of the platform was probably due to the continual pressure of the lobes of the liver. The division of the umbonal cavity into definite chambers in Monomerella, and to a less extent in other members of this family, appears, according to Davidson and King, to have been caused by pressure of the ovarian lobes.
In connexion with the foregoing remarks on the development of the “platform,” it may be mentioned that the paths along which the muscle-bands move, as the shell of Brachiopods increases in size, are marked by elongated scars, and often by shelly deposits; and when the members of a muscle-pair come into juxtaposition these shelly deposits (which act as fulcra for the muscles) combine, and by the growth of the shell form a septum, as in the case of the median septum of Lingulepis.
The Obolidae show some important features in the internal impressions. Obolella crassa (Hall) may be taken as a well-known type of the family. In this species a pair of small scars, one on each side of the pedicle-groove, lies close under the hinge line in the ventral valve. There is also a well-marked scar for the insertion of the pedicle-muscle at the end of the pedicle-groove. A pair of much elongated lateral impressions extending forward from the “cardinals” may be homologous with the “laterals” of Lingula; and the two small central scars between them may be compared with the “centrals” of Lingula which are in a somewhat similar position. In the dorsal valve of O. crassa a pair of “cardinals” is found, and on each side of a low median rounded ridge are two small “central” scars. Indistinct “lateral” scars arise close to or in the central area, and diverge anteriorly.
Sometimes a great concentration of muscle-scars occurs round the foramen in the ventral valve, as in Siphonotreta.
As regards the minute structure and composition of the shell in the Ecardines, we find that the Lingulidae and Discinidae have their shell composed of alternating layers of phosphate of lime and a corneous substance; the former layers are pierced by microscopic canals. The Craniidae have calcareous shells traversed by tubules, which divide into many fine branches near the external surface; a thin periostracum covers the exterior. The Trimerellidae have heavy thick calcareous shells, for which they required the previously-described elaborate arrangement of muscles to open and shut them.
II. TESTICARDINES
External Characters
It is to this division that the great majority of the Brachiopoda belong; and the diversity of form, of ornamentation, and of internal characters is correspondingly greater than in the Ecardines.
A transversely or longitudinally oval shape of shell is the commonest; but sometimes it is triangular, as in Rhynchonella (Fig. [327]), or bilobed, as in Pygope (= Terebratula diphya). The ventral valve is usually more convex than the dorsal, and the former may be prolonged into a tube by the accelerated growth and infolding of the anterior and lateral margins, producing a very abnormal form (Proboscidella). The external surface of the valves is frequently ornamented with more or less prominent radiating ribs; and fine concentric growth-lines are commonly shown, and may be developed into coarse ridges or wrinkles, particularly in old individuals. The members of the family Productidae are usually furnished with tubular spines, which are sometimes of great length, and served to anchor the free shells in the mud, or were twisted round Crinoid stems and similar objects.
In the ventral valve of many genera there is a median sinus, with a corresponding fold in the dorsal valve, and rarely vice versâ; sometimes the fold and sinus are double.
The hinge line is either curved or straight, and the valves are articulated by means of a pair of “hinge-teeth” (Fig. [329], t) in the ventral valve, which fit into corresponding sockets in the opposite valve. Some genera have the teeth very rudimentary, or have lost them altogether. The teeth are frequently supported by “dental plates,” and the sockets by “socket plates” (e.g. Conchidium, Figs. [324], [325]). A few genera with a long hinge line have the whole of it denticulated (Stropheodonta). In the dorsal valve medianly close under the hinge line is a shelly protuberance—the “cardinal process”—to which the diductor muscles are attached. It is sometimes of great length and forked (Stringocephalus, Fig. [326]), or tripartite, or even quadripartite; but in Rhynchonella and some other genera it is rudimentary.
Fig. 324.—Conchidium galeatum. Wenlock Limestone.
Fig. 325.—Conchidium galeatum. Transverse section. d, Dorsal valve; d.s, dorsal septum; s, socket plate; v, ventral valve; v.s, ventral septum; d.p, dental plate.
Fig. 326.—Stringocephalus Burtini. (Modified from Woodward.) Devonian. A, Interior of dorsal valve. B, Side view of interior of shell; a, adductor (= occlusor) scars; c, crura; c.p, cardinal process; d.s, dorsal septum; h.p, hinge plate; l, brachial loop; s.p, shelly processes; t.s, dental sockets; v.s, ventral septum.
A “hinge area” (Fig. [334], c.a) is often present on one or both valves, and may be of great size, as in Clitambonites, but in Productus it is wholly absent. In those genera that possess it a triangular fissure—the “deltidial fissure”—frequently traverses it on both valves; in the dorsal valve the fissure is merely the space between the dental sockets, and may be occupied by the cardinal- process (Fig. [334], C) or covered by a shelly plate—the “chilidium.” In the ventral valve it gives passage to the pedicle, and may be partly or entirely closed by a similar plate (Fig. [334], d) known as the “pseudo-deltidium,” especially large in Clitambonites, or remain open (Orthis). This pseudo-deltidium is a primitive character, and arises in an early stage of the development as a shell-growth on the dorsal side of the animal, becoming attached to the ventral valve subsequently. The pedicle in many genera passes out through a special foramen in the beak of the ventral valve; and its proximal portion is often embraced by a pair of small plates—the deltidial plates or “deltidium”—which are formed on lateral extensions of the ventral mantle lobe, according to Beecher. These plates lie on each side of the pedicle, or grow round and unite in front of it (Rhynchonella, Fig. [327]), or constitute merely its anterior border (Terebratula, Fig. [328]). In some cases this foramen becomes closed in old age.
Fig. 327.—Rhynchonella Boueti. (Cornbrash.) d, Deltidium; f, foramen.
Fig. 328.—Terebratula sella. (Lower Greensand.) d, Deltidium; f, foramen.
The dorsal valve in a few cases has its beak perforated by a foramen—the “visceral foramen.” This foramen is in no way connected with the pedicle foramen, but points perhaps to the existence in the early Testicardinate genera of an anal aperture. In Athyris concentrica (Devonian) this foramen is connected internally with a cylindrical tube, which extends longitudinally to about one-third the length of the valve. In Centronella the aperture in the cardinal plate is rounded and complete; and in Strophomena and its allies the opening lies between the cardinal processes. If this feature is correctly interpreted, it suggests a retrogression of the group since Palaeozoic times not only in numbers, but in structure; and other evidence points the same way.
Internal Characters
The interior of the shell is sometimes more or less divided up by septa. A median septum occurs in one or both valves of many genera as a low ridge or strongly developed partition (Waldheimia, Fig. [329], ss; and Stringocephalus, Fig. [326], B, v.s). Conchidium (Fig. [325]) has its dental plates of great size, and uniting to form a V-shaped chamber or “spondylium,” supported by a median double septum; and by means of these with a pair of septa and the large socket-plates in the dorsal valve the interior of the shell of this genus is divided up into several chambers.
The interiors of several other genera are somewhat similarly divided up.
Fig. 329.—Waldheimia (Magellania) flavescens. A, Interior of ventral valve: a, adductor scars; v.a, ventral adjustors; d, divaricators; a.d, accessory divaricators; p, peduncular muscle; dm, deltidium; f, foramen; t, teeth. B, Interior of dorsal valve: a.a, anterior adductor (occlusor) scars; a.p, posterior adductor (occlusor) scars; c.p, cardinal process; cr, crura; d.s, dental sockets; hp, hinge-plate; l, brachial loop; ss, septum. (After Davidson.)
In the Carboniferous genus Syringothyris two special plates, situated between the dental plates, are rolled into an incomplete tube, so as to enclose probably the anal extremity of the alimentary canal; and in several genera a sub-umbonal “cardinal plate” is present, which is perforated (Athyris) or slit in some cases for the passage of the anal tube.
For the support of the fleshy “spiral arms” the calcareous structures forming the “brachial apparatus” are of two main types—(1) the loop type; (2) the spiral-cone type. In the Strophomenidae no special calcareous support seems to have been usually present (Fig. [334]), though in some species of Leptaena spirally-grooved elevated areas supported the fleshy arms; in the Productidae it is probable that the ridges enclosing the “reniform impressions” (Fig. [333], i) served for a similar purpose.
The Terebratulidae show the “loop type” of brachial apparatus. In Waldheimia (Fig. [329]), which may be taken as an example, we notice first in the dorsal valve the “crura” (cr), from which arise the two “descending branches” which run forwards and then are bent back to form the “ascending branches” which are united by the “transverse band.” In some genera the “ascending branches” may be reduced to mere points, and the “transverse band” become a median vertical plate; the “crura,” too, may be fused so as to form a “crural band”; and the “descending branches” may be connected by a cross band—the “jugal band.” In Stringocephalus (Fig. [326], l, s.p) the loop is furnished on its inner edge with radiating processes; and in Argiope the loop is simple, not reflected, and fused with marginal septa; while in the Thecidiidae it is more or less fused with the shell itself, and with the mass of calcareous spicules secreted by the mantle.
The “spiral-cone type” of brachial apparatus is found in the Spiriferidae, Atrypidae, and Koninckinidae, and consists of two spirally-enrolled calcified lamellae, forming two cones with their apices directed laterally (Spirifera, Fig. [330]), or towards the interior of the dorsal valve (Atrypa, Fig. [332]), or towards each other (Glassia); or forming two flat spirals in the same plane (Koninckinidae). A “jugal band” is generally present, but varies much in position, and in some genera has complicated posterior processes.
The Rhynchonellidae have no loop or spiral cones, but merely a pair of short “crura.”
Fig. 330.—Spirifera striata. (Carboniferous Limestone.) Showing brachial spires.
The principal modifications in the attachments of the muscles in the Testicardines are illustrated by Productus giganteus (Fig. [333]), Leptaena rhomboidalis (Fig. [334]), and Waldheimia flavescens (Fig. [329]).
In Productus (Fig. [333]) we see in the ventral valve a pair of dendritic occlusor, often called adductor, impressions and a pair of large flabellate divaricator impressions. In the dorsal valve the large “cardinal process” served for the attachment of the divaricator, and a low median septum separated the dendritic occlusor scars, which are rarely divisible into anterior and posterior pairs.
Fig. 331.—Atrypa reticularis. (Wenlock Limestone.)
Fig. 332.—Interior of the same, seen from the dorsal side, showing brachial spires. (After Hall.)
In Leptaena (Fig. [334]) the occlusor scars (a) in the ventral valve are narrow and median, and are enclosed by a pair of flabelliform divaricator impressions (d.v); in the dorsal valve two pairs of occlusor scars (a.a, p.a) are well marked, and accessory posterior occlusor scars are traceable in some specimens. The vascular sinuses (v.s) and genital areas are conspicuous in many species of this and other genera.
Fig. 333.—Productus giganteus. (After Woodward.) Carboniferous Limestone. A, Interior of dorsal valve. B, Interior of ventral valve. C, Transverse section of valves. D, Hinge line of A: a, occlusor scars; d, divaricator scars; i, “reniform impressions”; ca, cardinal process; h, hinge line; p, brachial prominence; s, cavity for spiral arms; do, dorsal valve; ve, ventral valve.
In Waldheimia (Fig. [329]) a sub-umbonal “peduncular muscle” scar (p) in the ventral valve has before it a pair of “accessory divaricator” scars (a.d) flanked by a pair of “ventral adjustor” (v.a) and a pair of “divaricator” impressions (d), between which lie the two occlusor scars (a). In the dorsal valve anterior and posterior pairs of occlusor scars (a.a, a.p) are visible.
The minute structure of the calcareous shell of the Testicardines is of flattened fibrous prisms inclined at a very acute angle to the surfaces. In many forms minute tubes more or less closely arranged pierce through the fibrous shell-substance; but in some genera (Productus) they do not reach the outer surface (see p. [468]). Allied genera, however, differ much in the punctate or impunctate character of the shell.
Fig. 334.—Leptaena rhomboidalis. (Silurian.) A, External view of ventral valve. B, Interior of ventral valve: a, occlusor scars; d, pseudo-deltidium; d.v, divaricator scars; c.a, hinge area; t, teeth. C, Interior of dorsal valve: a.a, anterior occlusor scars; p.a, posterior occlusor scars; c.a, hinge area; c.p, cardinal process; d, chilidium; s, dental sockets; v.s, vascular sinuses.
Synopsis of Families
I. Ecardines
Family. Lingulidae
Shell elongated, composed of alternating chitinous and calcareous layers, the latter of which are perforated. Attached by a pedicle passing between apices of valves.
Arms have no calcified supports.
(For muscles see Fig. [322].)
Range.—Lower Cambrian to Recent.
Principal Genera.—Lingula, Lingulella, Lingulepis.
Family. Obolidae
Shell varies in shape. Ventral valve provided with pedicular groove or foramen. Cardinal border thickened. No brachial supports. Shell composed of alternating chitinous and calcareous layers.
(For muscles see p. [496].)
Range.—Lower Cambrian to Devonian.
Principal Genera.—Obolus, Obolella, Kutorgina, Linnarssonia, Siphonotreta, Acrotreta, Neobolus.
Family. Discinidae
Shell rounded, valves more or less conical, fixed by pedicle passing through slit or tubular foramen in ventral valve. No calcified brachial supports. Shell structure chitino-calcareous.
Range.—Ordovician to Recent.
Principal Genera.—Discina, Orbiculoidea, Trematis.
Family. Craniidae
Shell calcareous, subcircular; fixed by surface of ventral valve; dorsal valve the larger, depressed-conical. Shell structure punctate.
Four principal muscular scars in each valve, with central triangular protuberance in ventral valve (see p. [476]).
Range.—Ordovician to Recent.
Principal Genus.—Crania.
Family. Trimerellidae
Shell thick, calcareous, inequivalve; beak of ventral valve usually prominent; rudimentary teeth maybe present; hinge area well developed, with pseudo-deltidium. In interior of valves muscular platform, “crescent,” and sometimes sub-umbonal chambers (see p. 494, Fig. [323]).
Range.—Ordovician and Silurian; maximum in Wenlock.
Principal Genera.—Trimerella, Monomerella, Dinobolus, Rhinobolus.
II. Testicardines
Family. Productidae
Shell entirely free, or fixed by ventral valve or spines. Concavo-convex, more or less covered with tubular spines. Hinge line straight. Hinge-teeth absent or rudimentary.
Cardinal process prominent.
Reniform impressions in dorsal valve.
(For muscular impressions see p. 501, Fig. [333].)
Range.—Silurian to Permian. Genus Productus very characteristic of the Carboniferous.
Principal Genera.—Productus, Chonetes, Strophalosia, Proboscidella, Aulosteges.
Family. Strophomenidae
Shell very variable in shape; concavo-convex, plano-convex, or biconvex; hinge line usually straight; frequently with an area on each valve; foramen may or may not be present. Shell structure near always punctate. Ventral valve usually furnished with hinge-teeth; and dorsal valve with cardinal process.
Brachial supports completely absent or very rudimentary.
(For muscular impressions see p. 502, Fig. [334].)
Range.—Wholly Palaeozoic.
Principal Genera.—Orthis, with many sub-genera, Clitambonites, Skenidium, Strophomena, Orthothetes, Leptaena, Stropheodonta, Plectambonites.
Family. Koninckinidae
Shell plano-convex or concavo-convex. Brachial apparatus composed of two lamellae spirally enrolled in the same plane, or in the form of depressed cones, with the apices directed into the ventral valve.
Range.—Silurian to Lias.
Principal Genera.—Koninckina, Koninckella, Coelospira, Davidsonia.
Family. Spiriferidae
Shell biconvex. Brachial apparatus consisting essentially of two descending calcareous lamellae which by spiral enrolment form a pair of laterally-directed cones (Fig. [330]).
Range.—Chiefly Palaeozoic, but a few forms pass up into the Lias.
Principal Genera.—Spirifera, Cyrtia, Uncites, Athyris, Merista.
Family. Atrypidae
Brachial apparatus consists of two descending calcareous lamellae which bend outwards at the extremity of the crura and are coiled into two spiral cones, the apices of which either converge towards each other (Glassia) or towards the dorsal valve (Atrypa, Fig. [332]), or diverge towards the dorsal valve (Dayia); shell structure impunctate.
Range.—Ordovician to Trias.
Principal Genera.—Atrypa, Dayia, Glassia.
Family. Rhynchonellidae
Shell biconvex, hinge line usually curved.
Beak of ventral valve incurved, with foramen.
Calcareous brachial supports reduced to a pair of short curved crura.
The septa, dental and socket plates may be highly developed and divide up the cavity of the shell into chambers (Stenochisma, Conchidium).
Shell structure fibrous, rarely punctate; muscular impressions as in Terebratulidae.
Range.—Ordovician to Recent: majority of the genera are Palaeozoic.
Principal Genera.—Rhynchonella (Fig. [327]), Stenochisma, Stricklandia, Conchidium.
Family. Terebratulidae
Shell structure punctate.
Arms supported by a calcareous loop, usually bent back on itself.
(For muscular impressions see p. 502, Figs. [328], [329].)
Beak of ventral valve perforated by foramen, furnished with deltidium.
Range.—Devonian to Recent; maximum development in Mesozoic times.
Principal Genera.—Terebratula, Terebratulina, Waldheimia, Terebratella, Kingena, Magas, Centronella.
Family. Argiopidae
Large foramen for passage of pedicle. Marginal septa present in both valves. Calcareous brachial loop follows margin of shell and is more or less fused with the septa. Shell structure punctate.
Range.—Jurassic to Recent.
Principal Genera.—Argiope, Cistella.
Family. Stringocephalidae
Shell subcircular, punctate. Cardinal process highly developed, bifid. Brachial apparatus composed of two calcareous free lamellae, prolonged at first downwards, then bent back, upwards and outwards to run parallel to margin of shell and to unite in front, thus constituting a wide loop.
Range.—Silurian and Devonian.
Sole Genus.—Stringocephalus.
Family. Thecidiidae
Shell usually fixed by beak of ventral valve, plano-convex. Sub-cardinal apophysis in ventral valve for attachment of occlusors. Marginal septa in dorsal valve. Calcareous brachial loop more or less fused with shell, and with calcareous spicules of mantle. Shell structure: inner layer fibrous, outer layer tubulated.
Range.—Carboniferous to Recent.
Principal Genera.—Thecidium, Oldhamina.
Stratigraphical Distribution of Brachiopoda
It is remarkable that some of the earliest types of Brachiopoda exist generically unchanged at the present day. Such are Lingula, ranging from the Cambrian; Discina and Crania, ranging from the Ordovician; and amongst the hinged forms Terebratula from the Devonian, and Rhynchonella from the Ordovician.
In the lowest Cambrian (Olenellus beds) the most important genera are Linnarssonia and Kutorgina. The hinged forms appear in the Cambrian, being represented by Orthis; but the majority in this formation belong to the Ecardines. Lingula, Lingulella, and Obolella are characteristic.
In the Ordovician many new genera of the Testicardines make their appearance, such as Strophomena, Leptaena, Atrypa, Rhynchonella, Clitambonites, etc., but the extraordinary abundance and variety of Orthis is most remarkable. The Ecardines are reinforced by such forms as Trematis and Siphonotreta. It is, however, in the Silurian that the Testicardinate Brachiopoda attain their maximum, for in addition to a great development of species amongst the older forms, a host of new genera for the first time occur here (Spirifera, Athyris, Conchidium, Stricklandia, Chonetes, Cyrtia, etc.); and the Trimerellidae are especially characteristic of the Wenlock.
With the commencement of Devonian times many species and genera become extinct, but new forms come in (Terebratula, Orthothetes, Productus, etc.), and some genera are wholly confined to this formation (Uncites, Stringocephalus). The Carboniferous is marked by the maximum development of Productus and Spirifera; Orthothetes, Stenochisma, and Athyris are also abundant, but there is a considerable extinction of the older genera and species, and a great diminution in the number of individuals and species of those that persist.
A further reduction occurs in the Permian, where the most important genera are Productus, Strophalosia, and Stenochisma; but Aulosteges is a new form peculiar to this period. In the Trias a new era commences; the principal families and genera of the older rocks disappear entirely; a few spire-bearing genera persist (Spiriferina, Athyris), and the genus Koninckina is restricted to this formation.
The enormous development of species of the Terebratulidae and Rhynchonellidae is the most noticeable feature in Jurassic times; and a few ancient types linger on into the Lias (Spiriferina, Suessia, a sub-genus of Spirifera); Koninckella here occurs.
The Cretaceous Brachiopoda are closely allied to the Jurassic; Magas and Lyra are peculiar to the period, and the Terebratulidae and Rhynchonellidae are very abundant, together with the Ecardinate genus Crania.
With the commencement of Tertiary times the Brachiopoda have lost their geological importance, and have dwindled down into an insignificant proportion of the whole Invertebrate fauna.
The distribution of the Brachiopoda in past time is shown in the following table:—
| Palaeozoic | Mesozoic | |||||||||||
| C | ||||||||||||
| a | ||||||||||||
| r | ||||||||||||
| O | b | C | ||||||||||
| r | o | r | ||||||||||
| C | d | S | D | n | J | e | T | |||||
| a | o | i | e | i | P | u | t | e | ||||
| m | v | l | v | f | e | r | a | r | R | |||
| b | i | u | o | e | r | T | a | c | t | e | ||
| r | c | r | n | r | m | r | s | e | i | c | ||
| i | i | i | i | o | i | i | s | o | a | e | ||
| a | a | a | a | u | a | a | i | u | r | n | ||
| ECARDINES | n | n | n | n | s | n | s | c | s | y | t | |
| Lingulidae | Lingula | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ |
| Lingulella | ___ | |||||||||||
| Obolidae | Obolus | ___ | ___ | |||||||||
| Obolella | ___ | ___ | ||||||||||
| Kutorgina | ___ | ___ | ||||||||||
| Linnarssonia | ___ | |||||||||||
| Trematis | ___ | ___ | ||||||||||
| Siphonotreta | ___ | ___ | ||||||||||
| Acrotreta | ___ | |||||||||||
| Discinidae | Discina | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | |
| Craniidae | Crania | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | |
| Trimerellidae | Trimerella | ___ | ||||||||||
| Dinobolus | ___ | |||||||||||
| TESTICARDINES | ||||||||||||
| Productidae | Productus | ___ | ___ | ___ | ||||||||
| Chonetes | ___ | ___ | ___ | |||||||||
| Strophalosia | ___ | ___ | ___ | |||||||||
| Strophomenidae | Orthis | ___ | ___ | ___ | ___ | ___ | ||||||
| Skenidium | ___ | ___ | ||||||||||
| Clitambonites | ___ | |||||||||||
| Strophomena | ___ | ___ | ||||||||||
| Stropheodonta | ___ | ___ | ___ | |||||||||
| Leptaena | ___ | ___ | ___ | ___ | ||||||||
| Orthothetes | ___ | ___ | ___ | |||||||||
| Davidsonia | ___ | |||||||||||
| Koninckinidae | Koninckina | ___ | ||||||||||
| Koninckella | ___ | |||||||||||
| Spiriferidae | Spirifera | ___ | ___ | ___ | ___ | |||||||
| Spiriferina | ___ | ___ | ___ | ___ | ___ | |||||||
| Cyrtia | ___ | ___ | ___ | |||||||||
| Syringothyris | ___ | |||||||||||
| Uncites | ___ | |||||||||||
| Athyris | ___ | ___ | ___ | ___ | ___ | |||||||
| Merista | ___ | ___ | ||||||||||
| Retzia | ___ | ___ | ___ | ___ | ___ | |||||||
| Atrypidae | Atrypa | ___ | ___ | ___ | ___ | ___ | ___ | |||||
| Dayia | ___ | |||||||||||
| Coelospira | ___ | |||||||||||
| Rhynchonellidae | Rhynchonella | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | |
| Stenochisma | ___ | ___ | ___ | |||||||||
| Stricklandia | ___ | |||||||||||
| Conchidium | ___ | ___ | ||||||||||
| Terebratulidae | Terebratula | ___ | ___ | ___ | ___ | ___ | ___ | ___ | ___ | |||
| Terebratulina | ___ | ___ | ___ | ___ | ||||||||
| Waldheimia | ___ | ___ | ___ | ___ | ||||||||
| Terebratella | ___ | ___ | ___ | ___ | ||||||||
| Kingena | ___ | ___ | ||||||||||
| Magas | ___ | |||||||||||
| Centronella | ___ | ___ | ___ | |||||||||
| Argiopidae | Argiope | ___ | ___ | ___ | ___ | |||||||
| Cistella | ___ | ___ | ___ | ___ | ||||||||
| Stringocephalidae | Stringocephalus | ___ | ||||||||||
| Thecidiidae | Thecidium | ___ | ___ | ___ | ___ | ___ | ||||||
| Oldhamina | ___ | |||||||||||
PHYLOGENY AND ONTOGENY
Wherever successive stages in the life history of an individual resemble in important anatomical features the adult individuals of other species occurring in successive members of a stratigraphical series, the development of the individual may be regarded as an epitome of the development of the species; it also generally throws light on the origin and relationships of allied genera and families.
In the case of the fossil Brachiopoda comparatively little work has yet been done in tracing their ontogeny or phylogeny, though the abundance, variety, and excellent state of preservation of the extinct species offer a promising field for investigation. It is to Dr. C. E. Beecher and other recent American palaeontologists that we owe our advance in this branch of the subject.
In the first place, in about forty genera, representing nearly all the leading families of the group, the important fact has been established of the presence of a common form of embryonic shell, termed the “protegulum,” which is “semicircular or semielliptical in shape with a straight or arcuate hinge line and no hinge area” (Beecher).[431] Its minute size and delicate texture cause its preservation to be rare, but its impression is not uncommonly left on the beak of the adult shell.
The main features of this embryonic shell are exhibited in the adult Lower Cambrian Brachiopod Obolus (Kutorgina) labradoricus (Billings); the sub-equal semielliptical valves have lines of growth running concentrically and parallel to the margin of the shell, and ending abruptly against the straight hinge line; and this indicates that there has been no change in the outline and proportions of the shell during its stages of growth, but only a general increase in size. It is very significant that we have here a mature type possessing the common embryonic characters of a host of widely separated genera, and we may therefore regard it as the most primitive form known.
Many genera pass through this so-called “Paterina” stage either in the case of both their valves, or more generally in the case of the dorsal valve only; but modifications in the form of the protegulum arise, which are due to the influence of accelerated growth, by which features belonging to later stages become impressed on the early embryonic shell. The most variable and specialised valve—the ventral or pedicle valve naturally exhibits the effect of this influence first and to the greatest extent. The Palaeozoic adult forms of many species represent various pre-adult stages of the Mesozoic, Tertiary, and Recent species, as is especially well shown in the genera Orbiculoidea and Discinisca.
In the Strophomenoid shells the protegulum in the dorsal valve is usually normal, but in the ventral valve abbreviation of the hinge and curvature of the hinge line are produced by acceleration of the “Discinoid stage” in which a pedicle notch is present.
No marked variation has yet been noticed in the spire-bearing, or Terebratuloid, or Rhynchonelloid genera.
The form of the shell and the amount of difference in shape and size of the valves seem to be largely due to the length of the pedicle and its inclination to the axis of the body, as evidenced by the development of Terebratulina. A series showing progressive dissimilarity of the two valves arising from these causes can be traced from Lingula to Crania. The greater alteration that takes place in the ventral valve appears to be due to its position as lower and attached valve. If the pedicle is short a transversely-expanded shell with long hinge line results when the plane of the valves is vertical or ascending, but when the latter is horizontal a Discinoid form is found. This mode of attachment is often accompanied by a more or less plainly developed radial symmetry. Shells with long pedicles, on the other hand, are usually longer than wide.
The character of the pedicle-opening is of great significance from an evolutional and classificatory point of view, for the successive stages through which it passes in embryonic growth are chronologically paralleled by different genera, and are likewise accompanied by the successive acquisition of other important anatomical characters, as has been shown by Beecher and others. The first and simplest type of pedicle opening is in shells with a posterior gaping of the valves, where the pedicle protrudes freely between them in a line with the axis, and the opening is shared by both valves, though generally to a greater extent by the ventral valve. Paterina (= Obolus labradoricus) and Lingula furnish examples of this type. In the second type the pedicle opening is restricted to the ventral valve, and the direction of the pedicle makes a right angle with the plane of the valves; in the lower forms the pedicle lies in a slit or sinus (Trematidae), but by further specialisation it becomes enclosed by shell growth so as to lie within the periphery, and finally becomes sub-central in some genera (Discinidae). The third type shows the pedicle opening confined to the ventral valve and sub-marginal. A pseudo-deltidium may preserve the original opening (Clitambonites); or this shelly plate may become worn away or reabsorbed in the adult so that the deltidial fissure through which the pedicle passes remains quite open (Orthidae). In the fourth type the incipient stage marks a return to the simple conditions of the first type; but ultimately a pair of deltidial plates develop, and may completely limit the pedicle opening below. Examples of this type are Spirifera and Rhynchonella. By means of these four types the Brachiopods have been divided into four Orders: the Atremata (type i.); the Neotremata (type ii.); the Protremata (type iii.); and the Telotremata (type iv.).
The Telotremata were the last to appear, but the four types of pedicle-opening with the various forms of calcareous brachial apparatus were in existence in the Bala period of the Ordovician.
As Paterina is the most primitive form of all, we may place it at the root of the phylogenetic tree. From it sprang the Atremata, which gave off the Neotremata and Protremata; the most primitive Neotremata seem to be the Trematidae, while the connecting link between the Protremata and Atremata is furnished by the Kutorginidae. From the genus Conchidium and its allies we may see how the Rhynchonellidae ushered in the Telotremata as an offshoot from the Protremata. The Telotremata subsequently gave off two main branches, which became specialised with the loop-bearing and spire-bearing forms respectively.
The evolution and mutual relationships of genera have been indicated with much probability by Hall, Clarke, and others. The Obolelloid type may be connected with the Linguloid by means of Lingulella and Linyulepis, while in Lingula itself we find the point of divergence for the ancestors of Trimerella, and for a line of variation culminating in Dignomia. The Palaeozoic Rhynchonelloids branched off at an early period from the same stock as Orthis, and are connecting links between this genus and Mesozoic Rhynchonellae; and a whole series of genera exhibit intermediate stages of structure between the Rhynchonelloid and Pentameroid groups. The Terebratuloids can be traced back to the primitive type Renssoellaria; and amongst spire-bearing forms, the protean genus Spirifera can be split up into groups of species which diverge along lines tending to forms no longer congeneric. When we come to deal with specific differences we find frequently such a host of intermediate varieties that the separation of many species, as in the case of Mesozoic Terebratulae, is to a large extent arbitrary and artificial.