10. Brief Notes on the Eodiscids 11, Phylogeny of the Dawsonidea

Home , Calodiscus, Pagetia, Weymouthia (trilobite)

No. 1.] 43

10. Brief Notes on the Eodiscids 11, Phylogeny of the Dawsonidea.

By Teiichi KoBAYASHI. (Comm. by H. YABE,M.I.A., Jan. 12, 1943.) Microdiscus has long been in common use for eodiscids, but as pointed out by Barrande in 1881, Microdiscus quadricostatus Emmons which is the type species of the genus, was a larval form of Trinucleus which was 'found in association with Ordovician graptolites and pele- cypods. Because Microdiscus Emmons, 1885, and the Microdiscidae Koken or Coquin, 196, lost their standing, Raymond adopted Eodiscus Hartt in place of Microdiscus and established Eodiscidae to include, beside the genus, two new ones, Goniodiscus and Weymouthia. While the three genera were all blind, Delgado described eodiscids with eyes in 1904 and Vogdes founded Delgadoia on Microdiscus caudatus Delgado in 1917. One year prior to this, Walcott had described proparian Pagetia as a member of the Eodiscidae, which became a subject of debate among trilobite students. Poulsen mentioned in 1927 that the sole reason for the resemblance of Pagetia to hypoparian eodiscids is adaptative convergence. This opinion is intenable because it is now known in Pagetia, Delgadoia or Al emtejoia that the two forms of each genus which are almost identical except for the presence or the absence of eyes and facial sutures, are found at the same locality. Supporting Beecher's view on the migration of the facial suture, Raymond emphasized in 1917, that Pagetia shows the stage in which the suture just came up to the dorsal side from the ventral in the agnostidian stage, notwithstanding the fact that Dindstrom and others have proven that the agnostid has no suture on the ventral side of its cephalon. Richter on the contrary suggested that Pagetia is a transi- tional form from the proparian to the hypoparian trilobite, but this suggestion cannot be upheld because .Pagetia s. str, is the latest in a continuous series comprising the Pagetidae and the Spinod~scinae and the latter subfamily is older and generally hypoparian. The oldest eodiscid so far known is proparian Hebediscus which is however aberrant and bears many features showing its high specialization. After looking over all of the known eodiscids I am lead to the conclusion that proparian eodiscids were derived from different hypoparian ones and Pagetia or any other proparian eodiscid can never be the ancestor of later proparian polymeric trilobites. This can readily be understood by their marked differences from the polymeric proparians in most other features. The presence of proparian eodiscids leads me to conclude that Hypoparia and Proparia are not the two natural groups of Trilobita. In Trilobita the eodiscid is most closely allied to the agnostid, both being miomeric and isopygous. Furthermore Mallagnestus desideratus, Brezidiscus (?) troyensis and Brevidiscus (?) agnostoides bear certain 44 T. KOBAYASHI. [Vol. 19,

Pagetidae 1. Pageti z 2. Mesopagetia 3. Eopagetia Dawsoniidae 4. Dawsonia 5. Deltadiscus Delgadoiinae 6. Delgadoia 7. Alemtejoia Spinodiscinae Calodiscinae 8. Metadiscus 1~. Calodiscus 9. Spinodiscus 14. Cobboldites Weymouthiinae Brevidiscinae 10. Weymouthia 15. Brevidiscus Eodiscinae Dipharidae 11. Eodiscus 16. Hebediscus 12. Paradiscus 17. Aipharus No. 1.] Brief Notes on the Eodiscids II. 45

Table showing the Phylogenetical Relation among the Eodiscidian Genera and their Geological Range. characteristics which show that they are intermediate between the agnostid and eodiscid. Therefore I, have proposed the Agnostida to include the agnostidian and eodiscidian groups which I denominate here the Agnostidea and Dawsonidea respectively. At a glance they look alike, but there are many differences. None of the Dawsoniideahas a cephalon or pygidium of a quadrate outline or expanded distally as. frequently seen in the Agnostidea; the pygidia of the Dawsoniidea are sometimes very broad. The thorax consists of two or three segments in the Dawsoniideabut is always bisegmented in the Agnostidea. In some old species of the Dawsoniideathe glabella is so long that it reaches the frontal border,but there is always a preglabellar area in the Agnostidea. The glabella is divided into lobes of unequal size in most species of the Agnostidea but only in a few old species of the Dawsoniidea. The median tubercle which frequently exists in the Agnostideais generallyabsent in the Dawsoniidea. While the nuchal spine generally absent in the Agnostidea and also in Lower Cambrian ones of the Dawsoniidea,it is well developedamong Middle Cambrian ones of the Dawsoniidea. Basal side-lobesof glabella never exist in the Dawsoniidea. The cheeks have sometimes radial or ramiform furrows but the border is always smooth in the Agnostidea. In the Dawsoniidea the cheeks are unfurrowed but the border is tuberculated in many Lower Cambrian speciesand crenulated in a few Lower Cambrian and many Middle Cambrian ones. Genal spines frequently present in the 46 T. KOBAYASHI. (Vol. 19, Agnostidea are absent in most o1' the 1)awsoniidea. 1'roparian facial sutures or eyes which a,•( nc,ver found in the former stilxr•family are som(~times present in the latter. In the 1)awsoniidea, axial rings of the thorax are not very broad and do not have three bosses as are commonly seen in the Agnostidea. In the Dawsoniidea the axial lobe of the pygidium is generally long and regularly divided into rings ; its pleural lobes frequently ribbed. When a spine is present on the pygidium it issues from the rear part, while the Agnostidea frequently bear two lateral spines. The Dawsoniidea are restricted to the Lower and Middle Cambrian formations, whereas the Agnostidea lived from the Lower Cambrian to the Ordovician, thriving especially in the Middle and Upper Cambrian periods. Thus there are several differences between the two superf amilies, but nevertheless they form a solid group of miomeric and isopygous microtrilobites. Remarkably enough, the thorax has two or three segments in the Agnostida but five or more in polymeric trilobites, and thus there is a gap between 3 and 5 in the number of thoracic segments of Trilobita. The facts that the Dawsoniidea are not essentially different from polymeric trilobites in the configuration of the carapace and that larval forms of the Agnostida are allied to those of the Ptychoparida can hardly be overlooked. Accordingly the conclusion follows that Agnostida must be a suborder of Trrlobita instead of being, as Resser and Howell thought, an independent subclass of Trilobita, and the stock of the Agnostida is probably closer to that of the Ptychoparida than to that of the Redlichida. In spite of the fact that the Agnostida form a group of microtrilobites as old as Cambro-Ordovician, it shows high specializations in the reduction of the thoracic segments, enlargement of the pygidium and many other accounts. Therefore in agreement with Jaekel and Whitehouse I think it is one of the most specialized branches of Trilobita. The phylogenetical relationship among fifteen genera in addition to three subgenera in the proposed classification of the Dawsoniidea is shown in a table and on a plate inserted here. Because Brevidiscus bears some resemblance to a larval form of Calodiscus, it may be the closest to an unknown common ancestor of the superf amily, while highly specialized Dipharidae form an old isolated shoot from the common stock. It appears probable that the Calodiscinae having long glabellae form a branch of the main stem. From the Calodiscinae the Daw- soniidae were introduced by crenulation on the cephalic border while the Weymouthiidae were derived from Calodiscus through Cobboldites by reducing the relief. The Fodiscinae undoubtedly form an important branch of the main stem which came into being when a row of tubercles grew on the cephalic border, while the Spinagnostinae indicate anotherr branch which was introduced when a long nuchal spine and crenulation on the cephalic border came into existence. That the Pagetidae developed from Metadiscus by the acquisition of proparian facial sutures and by the loss of one of its thoracic segments is quite certain. Because the gcn ra of the Dawsoniidea are mostly short ranged as No. 1.] Brief Notes on the Eodiscids II. 47 seen in the table and bear distinguishing characteristics as illustrated on the plate, they are quite important for stratigraphic correlation, and in fact six stages can be distinguished in the development of the Dawsoniidea.. It is quite interesting to see that certain morphic features are found in common among two or more of these genera which are distributed' in either a limited age or a definite province. For example, 1. eodiscids having tuberculated cephalic borders are confined in their distribution, to the Olenellian, 2. eodiscids having eyes and tri-segmented thoraces are also restricted in their oc- currences to the Olenellian, 3. except one, all eodiscids having crenulated cephalic borders are Middle Cambrian ones, and 4, among these crenulated forms the eye-bearing ones are known only from the Indo- Pacific Province. I propose here the term, morphic index, to indicate the morpho- logical characteristic which makes it possible to use a given fossil as an index fossil. Although the morphic index does not always serve as a stratigraphic key of an accuracy attained by a good index fossil, it has a practical advantage Over the other in that it can be used by field geologists with little training in palaeontology.