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.