Amazonaspis maecurua ( Clarke, 1890 )

Carvalho, Maria Da Gloria Pires De & Fonseca, Vera Maria Medina Da, 2007, The Trilobite ‘‘ Dalmanites’ ’ maecurua Clarke, 1890 (Middle Devonian, Amazon Basin, Brazil) and the New Genus Amazonaspis (Synphoriidae), American Museum Novitates 3591, pp. 1-16 : 6-10

Amazonaspis maecurua ( Clarke, 1890)

figure 3A–H

Dalmanites maecurua Clarke ; Clarke, 1890: 23–29, pl. II, figs. 1–3; 6–7; 10; 15.

Dalmanites (Synphoria) maecurua Clarke ; Clarke, 1900: 68.

Dalmanites maecurua Clarke ; Katzer, 1903: 212, pl. 15, fig.12.

Dalmanites maecurua Clarke ; Lima, 1989: 12.

Dalmanites maecurua Clarke ; Silva and Fonseca, 2005: 76, fig. 3C–D.

TYPE MATERIAL: From Clarke’s syntypes , MN 3383 -I, internal mold of glabella plus small part of left fixigena (figured by Clarke, 1890: pl. II, fig. 2; refigured here as fig. 3C–E) is here designated as lectotype ; this designation is made in order to establish a single name-bearing type for the species. Paralectotypes are: MN 3382 -I, internal mold of part of the left librigena including genal spine ; MN 3384 -I, incomplete internal mold of hypostome ; MN 3385 -I, internal mold of an incomplete thoracic segment ; MN 3386 -I and MN 3387 -I, external and internal molds respectively of an almost complete pygidium .

OTHER MATERIAL: UFRJ-DG 115-Tr, internal and external molds of a cranidium with cheeks missing.

DESCRIPTION: The cephalon is represented only by two very incomplete specimens (MN 3383-I, UFRJ 115-Tr). It is moderately convex (sag., tr.) with the highest point on the posterior part of the frontal glabellar lobe. The anterior cephalic border has a rounded and smooth margin. The axial furrow is broad and shallow. It diverges gently forward to about the middle (exsag.) of L3, thereafter converges to the outer end of S3, from where it becomes deeper and diverges again anteriorly. The lateral glabellar furrows are well developed. S3 is broad, shallow, slightly narrower and deeper proximally, lengthening (exsag.), and diverging forward abaxially. S2 is deeper than S3, directed slightly obliquely backward abaxially, shallowing as it reaches the axial furrow. S1 is situated very close to the occipital ring, curved in a gentle forwardly concave arc parallel to the occipital furrow and becomes shallower distally where it meets the axial furrow. The occipital furrow (S0) is bowed slightly forward medially, becoming narrower and deeper as it approaches the axial furrows. L3 and L2 are inflated above the median part of the glabella adjacent to them. L3 is subtriangular with rounded edges; L2 is roughly rectangular with the edges similarly rounded. L1 is shorter (sag., exsag.) than the occipital ring and extends across the entire width of the glabella, slightly curved forward medially. The occipital ring is longer than L1 (sag.) with a weakly concave anterior margin and convex posterior margin. The median area of the glabella is ornamented with tubercles. The frontal glabellar lobe is relatively large, rhombic in outline, with its anterior margin broad and rounded, and its posteromedian extremity forms a subacute and prominent angle. There is a very faint elongate posteromedian depression on the sagittal line and a circular pit near each of the lateral angles. The frontal glabellar lobe, L3, and L2 are all covered with low tubercles. Those on the frontal glabellar lobe are approx. 1–1.5 mm across and scattered unevenly (2–3 mm apart) over its surface. Tubercles on L3 and L2 are slightly smaller and closer together. A small portion of the left fixigena is preserved, with a small and elevated palpebral lobe situated opposite the anterior half of L2 and the posterior half of L3. The anterior branch of the facial suture delimits a very narrow strip of the gena between the facial suture and the axial furrow, along of the frontal lobe, running close to the glabella. The lateral border is beveled; the lateral and posterior furrows are broad and quite deep. The posterior furrow curves backwards and tapers distally toward the genal spine but does not meet the lateral cephalic furrow (which is wide and also tapers posteriorly). The furrows are separated by a small, narrow convex portion of the free gena. The genal spine is small, diverging laterally, its internal margin forms a smooth concave curve. The hypostome is elongatelinguiforme in outline, with a convex anterior margin. The lateral margins converge in a gently concave curve from the anterior wings to the shoulder, situated opposite the posterior lobe of the middle body; behind the shoulder the posterior margin is parabolic in outline. The anterior lobe is convex, separated from the posterior lobe by a well-developed furrow that is deep and narrower abaxially and very shallow medially. The macula is located approximately midway between the lateral margin and the midline. It is unknown whether the posterior margin was spinose, although Clarke (1890: pl. II, fig. 3) restored its outline with three small spines.

The thorax is represented only by an incomplete internal mold of a single segment lacking the pleural termination. The axial ring is broad (tr.), rounded and thickened near the axial furrow, which is shallow and moderately wide. The posterior margin of the axial ring is gently convex posteriorly while its anterior part is depressed medially and merges with the articulation furrow. The depression expands toward the sagittal line, where it is approximately equal in length to the remainder of the ring. The lateral part of the articulation furrow/apodemal pits has a markedly oblique orientation. The posterior edge of the articulating half ring is transverse and sharply defined over its medial two-thirds. The pleural furrow is well developed, deep, wide, and symmetrical in cross section. The anterior portion of the pleura adjacent to the axial ring is almost transverse, then deflected gently backward and outward.

The pygidium is triangular in outline, approximately 125% as wide as long (sag.) and moderately convex (tr.), with gently rounded margins and an obtuse posterior angle. The axial furrow is well developed, shallow, and narrow. The axis is gently convex in transverse profile, tapered evenly backward, and its anterior width is slightly less than 30% of the maximum pygidial width. The axis includes 16–17 axial rings plus a blunt terminal piece that does not reach the posterior margin of the pygidium. The first three axial rings are arched gently forward medially, but subsequent ones are transverse. The pleurae are gently arched (tr.) and almost flat proximally, but curve more conspicuously toward the margin. There are 11 well-developed pleural furrows plus an inconspicuous final one. The first three pleural furrows curve smoothly to the margin, but subsequent furrows are almost straight and are directed successively more strongly backward, with the last one running very close to the axis. The pleural furrows are wide and deep, shallow distally, and probably symmetrical in cross section, but this is not clear from the specimen. The pleural ribs become obsolete before reaching the margin. Interpleural furrows are narrow and weakly defined, and the subsequent ones are line-like. There are indistinct nodes scattered over some ribs in an apparently random pattern. The marginal area is smooth, scarcely developed into a faint border. Terminal spine or mucro is absent.

Comparison of A. maecurua with other North American synphoriinids:

Odontocephalus selenourus ( Eaton, 1832) , from the Lower Devonian of New York (Onandaga Limestone) is readily distinguished from A. maecurua by having an anterior cephalic border that bears a series of distally broadened and coalesced processes, and the pygidium bears a distinct pair of pointed or blunt spines posteriorly (see Whiteley et al., 2002: pl. 125).

Anchiopsis anchiopsis ( Green, 1832) , from Lower Devonian of New York (Schoharie Formation) differs from A. maecurua by having L2 and L3 highly elevated and more or less completely fused; S1 and S2 with adaxial pits connected by transglabellar furrows; the occipital ring bears a median spine; and the pygidium has a long posterior process (see Lespérance and Bourque, 1971: pl. 27, figs. 1–3, 10–11; Whiteley et al., 2002, pl. 120).

Synphoroides biardi ( Clarke, 1907) from the Lower Devonian of the Gaspé Peninsula, Canada, differs from Amazonaspis maecurua in possessing a trifid anterior cephalic process and crenulated lateral cephalic margin. In addition, its pygidium ends in a blunt, triangular, slightly upturned spine (see Clarke, 1907: pl. 6, figs. 1 –12).

Comparison with Malvinokaffric synphoriinids from South America:

These include Dalmanitoides drevermanni ( Thomas, 1906) from the Lower Devonian of the Talacasto Formation, Jáchal, Argentina and Fenestraspis amauta Braniša and Vaněk (1973) from the Lower Belén Formation (Lower Devonian), of Chacoma, Bolivia.

Dalmanitoides drevermanni (see Thomas, 1906: pl. 11, figs. 1–3) is readily distinguished from A. maecurua by having five very short frontal processes on the cephalon; S1–S3 connected by indistinct transglabellar furrows; well-developed longitudinal glabellar furrow; a slender pygidial axis reaching the posterior end and extending into a short, slightly upturned point, as well as a higher number of pygopleurae (14).

Branisa and Vanek (1973) suggested that Dalmanites maecurua may belong to their new genus Fenestraspis , but Eldredge and Ormiston (1979) considered the Brazilian species to be distinct and referred it to a new genus aff. Fenestraspis . We are in agreement with Eldredge and Ormiston (1979) that the Brazilian and Bolivian species deserve separation at genus level.

Fenestraspis amauta (interpreted here largely on the basis of specimens in the AMNH collections) differs from Amazonaspis maecurua in the presence of small crenulations on the anterior cephalic margin (the anterior margin is smooth in A. maecurua ); the shape of S1 and S2, which in F. amauta are deeper adaxially forming suboval impressions that do not reach the axial furrows (in A. maecurua S1 and S2 distinctly meet the axial furrow); the occipital ring bears three spines in F. amauta , but is smooth in A. maecurua ; finally the exoskeleton is densely and coarsely tuberculated in F. amauta (the tubercles are not as coarse in A. maecurua ). The pygidium of F. amauta has at least 20 rings and is extended posteriorly over the pygidial margin to form a stout, upwardly directed spine ( A. maecurua has 16–17 rings and the axis ends in front of the posterior margin, which is smooth and lacks a mucro or spine); there are stout, upwardly directed paired spines on several axial rings situated toward or at the distal ends of the pleural ribs (such spines are absent in A. maecurua ); and the pygidial apodemes are deeply impressed (exsag.) and connected with the axial furrow (they are comparatively shallow and are not connected to the axial furrow in A. maecurua .)

Lespérance (1975) excluded Fenestraspis from the Synphoriidae despite the fact that it possesses at least one of the characters he regarded as diagnostic of the family (fusion of L2 and L3). Significantly, it also has S2 reduced in width (tr.) and forming a suboval impression; the distance between cephalic apodemes S1 and S2 is more than 1.5 times that between the occipital and S1 apodemes; and the genal spine does not have a longitudinal furrow, all features now considered diagnostic of the Synphoriidae ( Campbell, 1977; Holloway, 1981).

Comparison with Silurian synphoriinids:

Additionally, two Silurian synphoriinid taxa were compared with A. maecurua , Lygdozoon collatum Holloway, 1981 , and Delops obtusicaudatus ( Salter, 1849) . Like A. maecurua , these forms have a rounded anteri- or cephalic margin, a smooth, entire pygidial margin, and the pygidium is not mucronate. However, both forms differ from A. maecurua in certain features.

L. collatum from the St. Clair Limestone (Wenlock) of Arkansas differs from A. maecurua in having a longer, more slender genal spine; an ornamented occipital ring (with a small tubercle medially and weak nodes distally); a smaller pygidium (9–11 axial rings and 9 pleural furrows); and the hypostome has a deeper posterior border furrow.

Delops Rickards 1965 was originally assigned to the subfamily Zeliskellinae within the Dalmanitidae , but Holloway (1981) considered it to be a synphoriinid. The type species Delops obtusicaudatus ( Salter, 1849) is readily distinguished from Amazonaspis maecurua by several features; the frontal glabellar lobe protrudes in front of the outline of the cheeks; the genal spine is slender; L2 and L3 are fused abaxially; S2 does not reach the axial furrows; the pygidium is smaller (11–13 axial rings and 9 pleural furrows), and the posterior extremity of the pygidial axis reaches the posterior margin of the pygidium.

Three other Devonian species, Gamonedaspis scutata , from the Devonian of Bolivia, Gamonedaspis accola ( Clarke, 1913) from the Ponta Grossa Formation of the Paraná Basin, Brazil, and Gamonedaspis boehmi ( Knod, 1908) from the Voorstehoek Formation of South Africa were previously assigned to the Subfamily Synphoriinae . However, Gamonedaspis Branisa and Vanek. 1963 , is now considered a dalmanitinid rather than a synphoriinid ( Edgecombe, 1993).