Early Cambrian – Late Permian (Tatarian)
Characteristics of the Order
- Highly diverse order
- Generally biconvex and subquadrate
- Strophic (elongated and well-developed hinge line)
- Unisulcate and radially ornamented
- The pedicle opening is generally unobstructed (unobstructed delthyria and notothyria)
- Simple teeth supported by dental plates
- Divergent brachiophore supported by variably disposed plates or secondary shell concretions
- In later stocks, the pedicle cellist is well developed
- Ridge like cardinal process
- Strophic, subquadrate, unisulcate, biconvex, radially ornamented rhynchonellates with open delthyria and notothyria, apsacline ventral and anacline dorsal interareas normally short, curved; teeth simple to mainly deltidiodont with crural fossettes and normally supported by dental plates; ventral muscle field variable, impressed on valve floor, pedicle cellist well developed in later stocks; elongate sockets may be delineated by fulcral plates, divergent prachiophores, as blades or rods, may be supported by variably disposed plates or accretions of secondary shell, notothyrial platform vestigial to well developed, cardinal process ridgelike becoming differentiated into myphore and shaft; ventral canal system typically saccate with divergent
- , dorsal canal system saccate, digitate or lemniscate.
- EXTENDED INFORMATION:
- The Orthida, are an assemblage of extinct Paleozoic stocks that include the oldest known rhynchonellate brachiopods. They were especially prolific during the Cambrian and Ordovician and became so diversified as to anticipate many of the morphological characteristics of other rhynchonellates. Many of these orthide stocks, however, were short lived deviations from a small number of persistent lineages that typify the 4 superfamilies of the order. Moreover, the dfferences between the orthoid
- and the dalmanelloid
- , which display the most commonly occurring characteristics of their respectivesuperfamilies, involve a minority of features so that a representative orthide can be defined.
- The exterior of a typical member of the Orthida is quite distinctive.The shell is biconvex and subquadrate in outline with a maximum width anterior of a well-developed hinge line and a curved apsacline ventral interarea that is longer than the anacline dorsal interarea. A strong, median dorsal sulcus is normally present and the shell surface is usually costellate. Variations include the convexoconcave profile of many plaesiomyids, the bilobed outline of
- , the reduced or mucronate hinge lines respectively of
- , the obsolescent interareas of
- , the dorsal median fold of both
- , the latter which its additional superimposed radial plications, and the occurrence of hollow ribs (aditicules) among dalmanellidines and plectorthoids. The most important external distinctions, however, are the strophic hinge line and the open delthyrium and notothyrium. Chilidial-like structures are known in some hexperonomiids, productorthids, and plaesiomyids; and deltidial plates were variably developed in a few independent stocks such as
- ; but the latter structures are not homologous with the strophomenate pseudodeltidium.
- Basic similarities are also evident in the impressions of muscle attachment areas and mantle canals. The orthide ventral muscle impressions are normally subtriangular to bilobed in outline and confined to the posterior half of the ventral valve. The umbonal chamber never accommodated the base of the pedicle as in other, later rhynchonellates. It was occupied solely by the muscle system up to a variably developed pedicle cellist, by which the cuticle of a flatbased pedicle was evidently attached to the ventral valve. The median adductor scars were normally not enclosed anteriorly and the adjustors and lateral diductor lobes were inserted on the inner surfaces of the dental plates. There is, however, great variation in the outline of the ventral muscle scars reflecting their differential development, especially the relationship between the adductor and adjacent diductor components. Moreover, outlines can vary with the age as well as the curvature of the valve. Even so, there are several kinds of outlines that are stable and distinctive enough to be used to discriminate genera.
- The adductor impressions on the floors the orthide dorsal valves are also variable in outline although they are essentially quadripartite and consist of a pair of posterior and anterior scars on either side of a variably developed median ridge. The ridge was build up as a low anterior extension of the notothyrial platform or convergent brachiophore plates. Sporadically it culminated in a high median septum, as in the phragmorthids, cremnorthids, and kayserellids. In such stocks as these, the median septum effectively divided the mantle cavity into two compartments, each of which presumably contained a brachium. Irrespective of the nature of the median partition the overall outline of the dorsal adductor field can vary significantly; especially in the relative sizes of individual scars, and he main changes are used to discriminate genera.
- Despite the variation that is known to have occurred in the mantle canal systems of the Orthida, one of the least specialized patterns, comprising saccate and digitate distributions in the ventral and dorsal valves respectively, is the most common among early orthoids. A lemniscate arrangement in both valves, on the other hand, is prevalent among the dalmanelloids and enteletoids. Yet these differences between superfamilies are not mutually exclusive. The ventral and dorsal patterns of the dalmanelloid paurorthids, for example, are saccate and digitate like most orthoids, while those of the finkelnburgiids (digitate and digitate) and some plectorthids (saccate and digitate but with the gonads pervading the entire mantles) approach the enteletoid condition.
- The most important changes affecting internal morphology were those that led to the diversification of the articulatory devices involving the ventral teeth and dorsal cardinalia. The teeth of Cambrian orthides are typically deltidiodont but are grooved by crural fossettes in later stocks. The brachiophores defining the sockets may be rods or blades proximally encased in a notothyrial platform as in most orthoids, or supported by plates reaching to the floor of the dorsal valve as in plectorthoids and most punctate groups. The brachiophore plates together with fulcral plates define sockets that are distally elevated above the floor of the dorsal valve. The disposition of the brachiophore plates can vary greatly even within a series of such closely related genera as those assigned to the Dalmanellidae. Using members of this family, it is possible to demonstrate a continuous gradient of change between arrangements in which the angle subtended by the brachiophore bases is significantly less of greater than that subtended by the brachiophore bases is significantly less or greater than that subtended by the tops of the brachiophores. Hence, although
- are each characterized by a distinctive attitude, the last two genera together include species showing every conceivable variation in the disposition of the bases relative to the top of the brachiophores. Generally, however, their attitude tends to be consistent in different stocks, so that they may converge onto a median septum to form a septalium, as in
- , or be widely divergent as in
- . The brachiophores may be prolonged distally as processes in attitudes suggesting their having supported the mouth segment of the lophophore. These elaborations are rare and are well exemplified by the brachiophore extensions of
- , which evolved independently of one another and of the crura of the later rhynchonellates.
- There are important differences in the origin and function of the diverse structures, collectively referred to as the cardinal process, that project posteriorly from the notothyrial platform. The lack of such outgrowths is relatively rare and presumably represents the ancestral condition. Yet it is characteristic of species of the Ordovician punctate
- , while well-developed processes are found in such Cambrian stocks as the eoorthid
- , and the orthoid bohemiellids. In most orthoids, the most common type of cardinal process consists of a median partition that effectively divides the posterior surface of the notothyrial platform into two halves, each of which reveived the dorsal ends of the diductor muscles. In a few stocks (e.g.,
- , the notothyrial areas flanking the median partitions developed into ridges for reception of the diductor bases, and in the orthidiellids these grew high and became fused with the median partition to form a trilobed process. In most plectortoids, however, further modification involved the migration of the diductor bases onto the posterolateral surfaces of the median partition which, in consequence, is either indented by muscle impressions, as in some plectorthids, or, more commonly, is differentiated into a bulbous crenulated myophore surmounting a thickened shaft, as in many plaesiomyids. This latter arrangement is prevalent among punctate orthides with the myophore mostly fashioned in a bilobed or trilobed manner, thereby separating the diductor bases from each other.
- The orthide shell succession appears to have been homologous with that of living rhynchonellides with a banded, calcitic primary layer (normally recrystallized) and a secondary layer composed of orthodoxly stacked fibers ensheathed in glycoproteinaceous membranes. In post-Cambrian times, however, orthides with punctate shells appeared and were the sole representatives of the order in the Late Paleozoic. Orthide punctae with their subconical distal ends are structurally similar to the endopunctae of the later rhynchonellates except that no perforated calcitic canopies to the canals have yet been found. Until the inferred homology of all punctuation has been ultrastructurally investigated it still seems better to assume that the punctuation of the orthide shell was a single transformation
- Indeed, preliminary study indicates that there is no significant difference in the punctation of the Early Ordovician dalmanelloid
- and the early Carboniferous dalmanelloid
- . The fibrous walls of punctae in the secondary shell of
- appear to be more steeply inclined externally than those of
- . This difference, however, could be related to the more lathlike fibers of the Ordovician genus because the canals of both are more or less equispaced at about 50 µm and range between 9 and 14 µm in diameter. As shown in
- , punctae are immediately distinguishable from the larger, horizontally disposed aditicules.
- Punctation of the enteletoid
- appears to be homologous with that of the dalmanelloids. The Carboniferous specimens studied were well enough preserved to retain a recrystallized patina on the exterior of the shells underlain by fibrous secondary layer. The patina is assumed to be recrystallized primary layer, through which the distal ends of punctual infills penetrate. This relationship suggests that, in life, the punctae were not covered by canopies of primary shell, only by an organic plug, possible little more than the periostracum. In some specimens, the distal ends of fibers partly cover punctual infills. It is, however, doubtful whether this arrangement is indicative of the closure of some punctae within the fibrous secondary layer. The outward deflection of fibers delineating punctae could simulate partial closure in transverse section.
- The erection of the 300 or so orthide genera currently in circulation has involved the diagnostic use of almost every aspect of the shell morphology and structure. In the classification adopted here, 42 character sets have been phylogenetically analyzed to establish an interaordinal hierarchy for all recognized genera. This was done in stages. First, ach traditional family grouping of genera was analyzed to determine their phylogenetic homogeneity. Excluded genera were then repeatedly analyzed with other families in order to identify the suprageneric taxa, with which they have closes morphological affinities. Finally, all such suprageneric taxa, represented by their internal nodes, were assembled into two cladograms that became the basis of the present infraordinal classification.
- All impunctate suprageneric units, including the Protorthidae and Skenidiidae, were analyzed using the nisusiids as the outgroup. Three clades resulted; one representing the Proteorthidae and Skenidiidae were analyzed using the nisusiids as the outgroup. Three clades resulted: one representing the Protorthida and the other two a dichotomy of the orthidines. This is more o r less consistent with a division of the orthidines into orthids and plectorthids, in the wide sense, groups that have long been distinguishable especially in the relative complexity of the plecctorhid cardinalia. Such a grouping can be accomated within a widely recognized hierarchy consisting of the suborder Porthidinia and two constituend superfamillies, the Orthoidea and the Plectoroidea.
- All punctate suprageneric units were analyzed using the plectorthid node as the outgroup. On moprphological grouds, this was an appropriate choice; in deed, Havelíčk identified the nanorthids as ancestral to the dalmanellids. Two well-defined clades emerged, with the paurorthids forming a possible sister group. The clades aggregate around the dalmanellids and the draboviids. The latter is distinguished by their poorly developed notothyrial platform, convergent brachiophore plates, and a relatively simple cardinal process. This grouping is compatiblde with a hierarchy within the suborder, Dalmanellidina, which is composed of the two superfamilies, Dalmanelloidea and Enteletoidea.
- The relationship among the orthide superfamilial units was assessed by comparing the nodal codes for the four superfamilies with the nisusiids and protothides as outgroups. The impunctate and punctate orthides formed two distinct clades which validated the assignment of these superfamilies to suborders.
- This classification is not truly phylogenetic as it has not been possible to identify and allow for all the homoplasy that must have contributed to the evolution of the orthide shell. The repeated lengthening of the ventral muscle scars is an obvious example of a convergence that cannot easily be categorized for phylogenetic analysis. It is also noteworthy that the sudden appearance of mature stocks in the geological record, as it is presently known, occurred during periods of active plate tectonics. Thus difficulties in assigning, for example, the portranellids and tyronellids within the dalmanelloid clade are more likely to reflect their cryptic appearance in the Late Ordovician than a close common ancestry; while the linoporellids and saukrodictyids were also probably enteletoid latecomers with articulatory devices and cardinalia that have been modified by the abnormal growth of the cardinal areas in the former and the development of spike like brachiophores and a septalium in the latter. Notwithstanding the complications such factors impose on attempts to trace orthide phylogeny, familial groupings have been improved by these analyses, which have also eliminated some of the more contrived aspects of previous classifications.