Adelogonus prichardi, Smith & Paterson & Brock, 2018

Smith, Patrick M., Paterson, John R. & Brock, Glenn A., 2018, Trilobites and agnostids from the Goyder Formation (Cambrian Series 3, Guzhangian; Mindyallan), Amadeus Basin, central Australia, Zootaxa 4396 (1), pp. 1-67 : 56-59

Adelogonus prichardi sp. nov.

Fig. 28

Etymology. In honour of C.E. Prichard, for his work on the Goyder Formation, including coining its name.

Holotype. CPC 42403, cranidium from the Goyder Formation at spot locality AS 273 ( Fig. 28A–D ).

Paratypes. Seven cranidia; CPC42404 ( Fig. 28E, F ); CPC42405 ( Fig. 28G, P ); CPC42406 ( Fig. 28H ); CPC42407 ( Fig. 28I, J ); CPC42409 ( Fig. 28L ); CPC42410 ( Fig. 28M ); and CPC42413 ( Fig. 28P ); one librigena, CPC42408 ( Fig. 28K ); and two pygidia; CPC42412 ( Fig. 28O ); and CPC42414 ( Fig. 28R ) from sample GOY/97.0, 73.2 m above the base of the formation. One librigena, CPC42411 ( Fig. 28N ) from sample GOY/109, 83.9 m above the base of the formation.

Other material. Eight cranidia, two librigenal, and two pygidia figured, CPC42403–CPC42414. Seven cranidia, one librigenal and nine pygidia not figured (mostly fragments).

Diagnosis. Cranidium up to 19 mm long sagittally. Anterior branches of facial suture diverge strongly at 136°– 156° (mean 148°; n = 4). Glabella anteriorly truncate, pyriform in outline; width:length ratio of 91% to 93%, occupying 77% to 81% of the cephalon length. S1 faintly defined, moderately deep and broad (exsag.), approximately level with the midpoint between γ–ε, not intersecting axial furrow. S2 fainter, not intersecting axial furrow; S3 either effaced or faint. SO moderately shallow, narrow, slightly bowed backwards medially; lateral extremities bifurcate at level of anterolateral corners of glabella. Anterior cranidial border moderately broad (sag.), occupying 12% to 13% sagittal cranidial length. Preocular and preglabellar short, 8% to 13% of sagittal cranidial length. Faint tropidium extends across cranidium in a broad arc from slightly behind the β point. Palpebral lobe about 24% to 27% cranidial length, reniform in shape. Palpebral area maximum width (tr.) 30% to 34% adjacent glabellar width. Prosopon varies over cephalon. Glabella, occipital lobe, eye ridges, palpebral lobes and palpebral areas with minute granules. Preocular, preglabellar and genal field with an anastomosing network of genal caeca. Anterior border covered with horizontal terrace ridges approaching Bertillon pattern.

Description. Cranidium up to 19 mm long (sag.) in complete specimens and potentially over 25 mm long (sag.) based on incomplete fragments. Maximum width possibly across posterolateral projections of fixigenae, narrowest point of cranidium forward of the palpebral lobes (γ–γ); moderately convex (sag., tr.). Anterior margin strongly rounded. Posterior margin incompletely preserved. Anterior branches diverge strongly at 136°–156° (mean 148°; n = 4), then curve abruptly towards the midline from β, approximately level with the border furrow before they reach the anterior margin. Glabella anteriorly truncate, pyriform in outline, moderately convex, with maximum convexity across midwidth, lateral slopes gently convex; width:length ratio of 91% to 93% (mean 92%; n = 3), occupying 77% to 81% of the cephalon length; axial furrow narrow (sag., exsag., tr.) and moderately deep. S1 faintly defined, moderately deep and broad (exsag.), approximately level with the midpoint between γ–ε, not intersecting axial furrow, directed slightly posteromedially for a short (tr.) distance, before becoming indistinct. S2 and S3 fainter, not intersecting axial furrow; S2 is directed slightly more transversely than S1, whilst S3 is either effaced or faint and directed slightly anteromedially. Occipital ring of moderate length (sag.), posterior margin slightly bowed backwards. SO moderately shallow, narrow, slightly bowed backwards medially; lateral extremities bifurcate at level of anterolateral corners of glabella; not intersecting axial furrow. Anterior cranidial border moderately broad (sag.) and slightly convex, occupying about 12% to 13% (mean 13%; n = 3) of sagittal cranidial length, narrowing abaxially; defined by a moderately deep and broad anterior border furrow. Preocular and preglabellar field slightly concave, downsloping toward the anterior border; short, 8% to 13% (mean 10%; n = 3) of sagittal cranidial length. Faint tropidium extends across cranidium in a broad arc from slightly behind the β point, passing in front of the axial furrow. Palpebral lobe about 24% to 27% (mean 26%; n = 3) cranidial length, reniform in shape, defined by shallow palpebral, anterior tip situated slightly anterior to S3, posterior tip opposite posterior portion of L1. Eye ridge strongly defined, extending posterolaterally from L 2 in a straight line diagonally towards the anterior tip of palpebral lobe. Palpebral area of fixigena flat, gently downsloping anteriorly towards eye ridge, maximum width (tr.) is 30% to 34% (n=2) adjacent glabellar width. Postocular field moderately short (exsag.) and sloping slightly posteriorly. Posterolateral projections of fixigena incompletely preserved. Posterior border narrow (exsag.), separated from fixigenal field by shallow, broad (exsag.) border furrow.

Librigena up to 7 mm in length in incomplete specimen. Lateral margin and posterior margins not preserved. Genal field subtrapeziform, moderately convex.

Pygidium large, up to 17 mm long in a single incomplete specimen (sag.), moderately convex, shape incompletely preserved. Axis, anterior margin and posterior margin incompletely preserved. Pleural regions gently convex, with broad (exsag.), shallow pleural furrows and narrower interpleural furrows dividing the field into a border (exsag.) anterior and a narrower (exsag.) posterior band. Border not clearly differentiated.

Prosopon varies over entire cephalon. Glabella, occipital lobe, eye ridges, palpebral lobes and palpebral areas covered with minute granules. Preocular, preglabellar and genal field with an anastomosing network of genal caeca. Anterior border covered with horizontal terrace ridges approaching Bertillon pattern. Pygidium prospone smooth.

Hypostome, rostral plate and thorax unknown.

Discussion. This species is the largest taxon in the Goyder Formation, making fragments easily identifiable. The large, anteriorly truncated glabella with a pyriform outline, bifurcated SO furrow that is not connected to the axial furrow, narrow (tr.) palpebral area, moderately long (exsag.) palpebral lobes, the presence of a faint tropidium (“preocular ridge” of Peng et al. 2004b), and minutely granular prosopon all support assignment to Adelogonus Öpik, 1967. The Goyder Formation material resembles two Chinese taxa, Adelogonus oblongus Lin, Peng Zhou & Yang, 2013 and the paratype (GPIN127899) of Sinocoosella typica Yuan & Yin, 1998 ; the latter most likely represents a new species (discussed above). These three species share a short preglabellar field that is convex rather than concave (cf. Yuan & Yin 1998, pl. 1, fig. 16), as seen in other species of Adelogonus discussed below. Adelogonus prichardi sp. nov. differs from these comparable species in having a more distinct bifurcation in the SO furrow, a slightly more anteriorly situated S1, a longer (sag.) anterior border and more strongly divergent anterior branches of the facial sutures. The presence of a faint tropidium (“preocular ridge” of Peng et al. 2004b) in many Goyder specimens near the anterior border furrow ( Fig. 28E–G, L, P ) is remarkably similar to Adelogonus hunanensis Peng, Babcock & Lin, 2004b from the Guzhangian Huaqiao Formation in South China (Peng et al. 2004b, pl. 9, figs 1–3). The tropidium of A. prichardi sp. nov. is located slightly more posteriorly and is not effaced near the axial furrow, continuing in a broad arc across the entire frontal area. The cranidia of Adelogonus hunanensis and the type species, Adelogonus solus Öpik, 1967 from the Mindyallan O’Hara Shale in the Georgina Basin , differ from A. prichardi sp. nov. in having much more convergent anterior branches of the facial suture and poorly defined S1 furrows. The only other material currently assigned to the genus is Adelogonus? sp., a pygidium from the Huaqiao Formation (Peng et al. 2004b, pl. 35, figs 15, 16). However, it is unlikely that this pygidium belongs to Adelogonus (see Peng et al. 2004b, p. 27, 28).

Occurrence. GOY section horizon 73.2 m ( Fig. 3 ). Also recovered from AS 273.

Distribution. Goyder Formation, Amadeus Basin, Northern Territory. Cambrian Series 3, Guzhangian (Mindyallan) in age.

Figures

FIGURE 3. Generalised lithostratigraphy and taxon ranges through the Goyder Formation at the GOY section. Sampled horizons are indicated on the right side of the section, with sample numbers representing tape meterage along the section line (see Fig.2). Measurements on the left side of the section are in meters (true thickness) from the base of the Goyder Formation. SF = Shannon Formation, PSS = Pacoota Sandstone. Solid black circles represent the occurrences of taxa present in the GOY section. Hollow circles represent the approximate stratigraphic occurrences of taxa found at nearby spot localities. Ranges of taxa found only atspot localities approximate, and has been inferred using lithological, faunal and geographical information.

FIGURE28. Adelogonus prichardi sp. nov. A–D come from from spot locality AS273; E–M, O–R from sampleGOY/97; and N comes from sample GOY/109. A–D, holotype, CPC42403, partial cranidium; A, dorsal view; B, lateral view; C, anterior view; D, oblique anterolateral view. E, F, paratype, CPC42404, partial cranidium; dorsal view; F, dorsal view of latex cast. G, Q, paratype, CPC42405, partial cranidium; G, dorsal view; Q, close up of healed region of anterior border. H, paratype, CPC42406, partialcranidium,latex cast.I, J, paratype, CPC42407, partialcranidium;I, dorsalview of latex cast; J,dorsal view. K, paratype, CPC42408, partial librigena. L, paratype, CPC42409, partial cranidium. M, paratype, CPC42410, partial cranidium.N, paratype, CPC42411,partial librigena.O, paratype, CPC42412,partial pygidium. P, paratype,CPC42413, partial cranidium.R, paratype, CPC42414, partial pygidium. All scale bars are 4 mm, except A, D, I, Q = 1 mm, B, C, J, P = 2 mm, K = 0.5 mm, and M = 5 mm.