Pterygotus

“ Pterygotus minor is a small form found associated with P. anglicus, but it is a unique form, and shows a marked divergence from congeneric forms throughout the world. The telson is elongate, spatulate, with a pronounced median keel, which is represented on the last three segments of the postabdomen as a long spine, rather than a ridge (Woodward, 312, 199, Pl. X, fig. 2; 195, p. 35, Pl. I, fig. 4) . There can be little doubt that this species, which is represented by a single, nearly entire individual, represents a neanic stage of some form, the adult of which probably is not known. ”

“ On the other hand they show extreme variation in the genera of the Pterygotus group, viz, Hughmilleria, Slimonia and Pterygotus. In Slimonia they were discovered by Laurie [1893, p. 511, pl. 1, fig. 3] and found to be very small and with strong curved pincers, but otherwise as in Limulus. Those of Hughmilleria have been fully described by Sarle. They are larger [pl. 61, fig. 6] than in any other genus except Pterygotus, projecting beyond the margin of the carapace when extended and thus forming a transition to their condition in the latter genus. ”

“ In H. shawangunk the eyes appear marginal in many specimens, because the marginal portion of the carapace was so impendent that in specimens vertically compressed it becomes folded upon itself and only a portion of the eye is shown [pl. 65, fig. 9] . Wherever the whole carapace is flattened out, the lateral eye is intramarginal by a considerable interspace [pl. 65, figs. 3, 4, 12] . In Pterygotus the lateral eye, at least in the mature stage, is always truly marginal.
Thus we infer that the eyes of Hughmilleria exhibit transitional or intermediate features between Eurypterus and Pterygotus. ”

“ The outline of the carapace is very much alike in all species, recalling Pterygotus banksii Salter. [1] The latter, however, is by no means a typical Pterygotus, but in various characters, as notably also the form of the carapace, is quite apart from the other Pterygoti. On the other hand, the carapace of H. socialis resembles most closely that of a species described by Salter and Woodward as an Eurypterus (E. lanceolatus) . [2] The latter is, in our opinion, either a true Hughmilleria or a transitional form between Hughmilleria and Eurypterus ”

“ These constitute a part of the famous Arbroath flags and because of their abundant eurypterid
remains are called the Pterygotus beds. In them are found the most perfect specimens of Pterygotus anglicus, though complete individuals are rare, and the rock often contains also an abundance of Parka decipiens, which has been variously identified as crustacean egg cases and as spores of plants. Above these beds follows the main mass of the lavas upon which rest the beds of Auchtertyre, which have yielded Cephalaspis lyelli, Pteraspis mitchelli, and certain of the Acanthodian fishes. ”

“ Laurie has regarded the simple character of the Pterygotus legs as an argument in favor of the primitive character of the genus. While we do not share this view we agree as to the manifest primitiveness of these four pairs of legs and therefore assign such appendages to the prototype. [1] In Eurypterus the fourth pair has also retained its primitive character; and while the legs are of uniform length in Pterygotus, in all other eurypterids, mature and adolescent [E. remipes, pl. 2] , they form a series that is longer backward. ”

“ Though the preoral appendages of Hughmilleria are larger than those of Eurypterus they differ still more widely from the very long appendages of Pterygotus with their dentate pincers, but they may well be considered as indicating a tendency to a development of Pterygotuslike chelicerae.
The difference between Pterygotus and Hughmilleria in the character of the spiniform legs, is clearly set forth by Sarle [4] and the slender lanceolate telson is distinctly a Eurypteruslike feature. ”

“ Nevertheless, there may here exist a difference from that of Pterygotus, for in the other eurypterids no facets have been observed. It is obvious that both Eurypterus and Eusarcus possessed an exteriorly smooth cornea just as Pterygotus; as to the interior of their compound eye, however, several possibilities present themselves: either the lenses were separate from the cornea and thereby lost in fossilization, not being united by the sclera as in the holochroal eyes of the trilobites; or the facets were so feebly
developed as to escape observation. ”

“ Characteristic telsons of Pterygotus [pl. 87, fig. 5, 7] are not infrequent at Schenectady and Duanesburg. A very different type is represented by plate 87, figure 8. It exhibits a peculiar ornamentation, consisting of small groups of tubercles on low nodes arranged in subconcentric lines; the other side was smooth. The serration of the margin is very distinct and shows by its direction that the small incision on the longer side is the middle of the posterior margin of the telson and is a faint beginning of the bilobation characteristic of Erettopterus. ”

“ But aside from this observation, the fact that the attachment of the chelicerae, in Slimonia, Eurypterus and Hughmilleria, has since been fully established to be directly in front of the mouth as in Limulus, leaves little doubt that the large pincers of Pterygotus, if they are at all homologous to the minute chelicerae of those three genera, must have had the same place of insertion, viz, at the posterior end of the epistoma. We may add that the homology of the large pincers of Pterygotus with the chelicerae of Limulus, Eurypterus etc. ”

“ Slimonia has this tendency still more developed, so that the telson appears as lanceolate with winglike lateral extensions of the anterior half. A reduction of the posterior spine may then be conceived to produce the telson of Pterygotus, and a further suppression of the axial lanceolate portion would lead to the bilobed telson of Erettopterus. The telson of Hughmilleria, Slimonia, Pterygotus and Erettopterus thus seems to present a continuous series of developmental stages, and the pointed or bluntly lanceolate-triangular form to be that of the prototype of the eurypterids. ”

“ The Caledonian Old Red, which is largely conglomeratic, has yielded comparatively few fossil remains, but in the Pterygotus-or Carmylie- sandstones of Forfar, Pterygotus anglicus has been found associated with Parka decipiens and at a higher horizon Cephalaspis and Pteraspis occur, and still higher the Acanthodian beds of Turin with a good fish fauna as well as Pterygotus anglicus and Stylonurus ensiformis. Thus, in the Caledonian Old Red, a series 12,500 feet or more in thickness, the fish and eurypterids are the only abundant organisms. ”

“ Pterygotus monroensis Sarle
Page 380
3 Fragment of free pincer or chela probably belonging to this species. Natural size 4 Metastoma, probably belonging to this species. Natural size 5 The type. Carapace. Natural size
Horizon and locality. Pittsford, Monroe co., N. Y.
Pterygotus? sp.
6 Fragment of carapace, probably belonging to Pterygotus. Natural size 7 Distorted carapace of a Dolichopterus or Pterygotus. Natural size 8 Telson of a Pterygotus. Natural size
Horizon and locality. McKenzie formation (Basal Cayugan period of Upper Siluric) . Near Hancock, Md. ”

“ It is a fauna in which the genera Eurypterus and Pterygotus prevail in number of species (Eurypterus with five, Pterygotus with three) and in the size of the creatures, which are there obviously near the climax of their development. It is also the horizon where individual development is greatest. [5]
One or two stragglers, notably Eurypterus remipes, return in the Rondout waterlime in greatly diminished number, and a new peculiar type of Eurypterus (E. microphthalmus) appears in waterlime intercalations of the Manlius limestone above. ”

“ The seventh and eighth segments are broad and relatively short, their longitudinal and transverse diameters being about equal. Their outer edges are obscurely serrate. The lobe of the seventh segment which is narrowly triangular in Eurypterus, is here produced into a leaflike process, which in length equals the segment. The eighth segment, which is the palette in Eurypterus and Pterygotus, is in this genus of similar form as the preceding segment. The ninth segment, the small "claw" in other eurypterids, has here assumed the bladelike shape of the eighth segments in Eurypterus and Pterygotus. ”

“ The carapace of the nepionic stage is also wider than any other part of the body, the latter tapering directly from the base of the carapace to the tail.
The imperfection of the body is very apparent in the first individual, which probably had no more than six or seven segments and the next but eight or nine without the telson. The segments are all of equal length, but diminish rapidly in width, while in the mature Pterygotus they gradually widen to the fourth and thence again very gradually contract to the telson. ”

“ The Devonic of America shows a great decline of the eurypterids, so far as we can judge from the fossil record, for, while in the Siluric there had been an ever-increasing number of species and of individuals, in the Devonic, on the other hand, there are no representatives in the Lower and Middle, and it is not until the very top of the Upper that a few stragglers are found. The first, a specifically undetermined Pterygotus mentioned by Billings, is from the Grand Greve limestone of Lower Devonic age. Remains of Pterygotus have also been found in the lower marine Devonic at Dalhousie. ”

“ Its relatively great width will be disregarded here as not characteristic of the prototype. It suggests that Strabops is already nearer to the Eurypterus than to the Pterygotus stock.
The Siluric and later genera of the eurypterids distinctly fall into two groups or stocks, namely, that of Pterygotus and that of Eurypterus. The former group (Hughmilleria, Slimonia, Pterygotus, Erettopterus) is principally distinguished by the marginal, faceted eyes; the latter, containing the genera Eurypterus, Dolichopterus, Echinognathus (?) ”

“ The fact of chief interest is that the Baltic form is more closely related to the Manlius species than to any other. There remain the specimens of Pterygotus osiliensis and its variety, laticauda.
In the lower beds of the Old Red sandstone are two species of Pterygotus, bilobus and anglicus to both of which P. osiliensis shows some similarity, though the stronger affinity is to the latter of the two from Great Britain. A comparison of Schmidt's restoration of P. osiliensis and of an actual specimen of P. bilobus var. ”

“ The proximal joint, or gnathobase, has the form of an upright retort. The seventh bears on the inner half of the distal end a subtriangular lobe demarked by a suture. Metastoma elongate, cordate. Type, H. socialis Sarle.
In the elaborate comparison with other genera which Mr Sarle adds to the generic diagnosis, the characters suggesting Pterygotus, as well as the resemblances to Eurypterus and Slimonia, are fully discussed. Since the date of his study the rocks of New York have afforded representations of this interesting genus in the Shawangunk grit and the Frankfort shale. ”

“ As the relative dimensions of the chelae to the other parts of the body are approximately uniform in the other species, we infer from their size (13 cm) that it also attained 5 feet in length.
Pterygotus buffaloensis and P. cobbi vastly surpassed all other arachnids or any organisms of our Upper Siluric era in size and armed with their powerful prehensile pincers, and being evidently active swimmers, as shown by their large swimming legs and telson, they must have been the terrors of the waterlime sea. ”

“ It is important to determine which of the two forms of eyes is the more primitive. Laurie has argued in favor of the eyes of Pterygotus, representing the first group, basing his view on a comparison with the trilobites (on the assumption that the eurypterids are derived from the trilobites) , the Scorpionidae and Thelyphonidae, "which must be derived from some way down the eurypterid stem." The Lower Siluric fauna is constituted of species with submarginal eyes and this fact seems to favor Laurie's view. ”

“ From present evidence it may be stated as a general proposition that there are two lines of development of the genital appendages of the operculum in the eurypterids, those of the Pterygotus group and those of the Eurypterus group. In the first, the appendage is composed of but one unpaired lobe, which may become more or less elaborated, as in Slimonia. In the second group two unpaired and two paired lobes are developed. ”

“ That advantage is not reserved for us, for the Silurian basin of Bohemia, so favored in all other respects, is relatively poor in fossils of the genus Pterygotus, not only because of their great rarity, but also because of the reduction of the specimens to little fragments. Since almost all of the remains are found in the large horizon of the Cephalopods, that is, in our limestone band e 2, it seems to us that one may attribute the almost total disappearance of these gigantic Crustacea to the voracity of these molluscs, against whom they were forced to maintain the struggle for existence. ”

“ Many of the growth stages measure but 2 millimeters or even less in length and hence so little surpass in size the eggs [1] of Limulus and probably those of Eurypterus lacustris and Pterygotus that they may be safely considered as representing the first larval or nepionic stage. Our large amount of material has allowed us to connect these larval stages with the adult forms by a fairly continuous series. ”