Page:A Revision of the Families and Genera of the Stylonuracea (Eurypterida).djvu/21

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185
KJELLESVIG-WAERING: STYLONURACEA

Type genus.—Drepanopterus Laurie, 1892.

The family Drepanopteridae differs from the Stylonuridae, with which it has always been associated, in the important feature of the entirely different-shaped metastoma. In contrast to the elongated, narrow metastoma of the Stylonuridae, deeply notched anteriorly and truncated at the posterior end, that of the Drepanopteridae comprises an unusual, "Eurypteridae-like" metastoma, namely, ovate and anteriorly cordate.

At present the family remains monotypic, and definitely known only from Scotland in Silurian beds, and possibly in the Devonian. A questionable occurrence has been recorded from the Ordovician of New York.


Genus Drepanopterus Laurie, 1892

Diagnosis.—Drepanopteridae of small size; prosoma horseshoe shaped, narrower at base than at the rounded lateral margins; wide marginal rim; eyes arcuate, located at midsection; prosomal appendages stout, increasing gradually in length posteriorly and devoid of spines except for possible spurs (?) on the ends of central joints of the posterior (VI) leg; this leg (VI) terminates in a flattened, slightly expanded, falcate joint, and reaches only to the anterior part of the postabdomen; metastoma ovoid and cordate at the anterior end; opisthosoma completely undifferentiated, wide and tapering into a clavate or wedge-shaped telson; opercula not well known; ornamentation comprises scale-like markings and pustules.

Type species.—Drepanopterus pentlandicus Laurie, 1892.

Distribution.—Scotland and New York (?).

Stratigraphic range.—Ordovician ?; Silurian; Devonian ?

Remarks.—Drepanopterus reveals characteristics which recall the Eurypteridae in many respects. This was first suggested by Peach to Laurie (1892, p. 161). The Drepanopteridae reveal characteristics, such as the ovoid metastoma, which distinguish them from the rest of the Stylonuracea, but show affinities to the Eurypteridae through the Dolichopteridae, and such genera as Onychopterella of the Eurypteridae. However, the development could just as well have been reversed. These evolutionary trends between the two groups are important to keep in mind when future discoveries in Ordovician and Cambrian rocks may possibly resolve the problem of the early phylogeny of this group.