Berylacaste berylae, Sandford & Holloway, 2006

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 250-252

Berylacaste berylae gen. et sp. nov.

Figures 19–20

Acastidae n. gen. — Rickards and Sandford, 1998: 752.

Type material. Holotype NMV P138224 (dorsal exoskeleton) and paratypes NMV P138225 (dorsal exoskeleton), NMV P138226 (cephalon), NMV P139330 (dorsal exoskeleton), NMV P139331 (thoracopygon with displaced pygidium), NMV P139332 (cephalon with displaced librigena), NMV P139333 (cephalon), NMV P139334 . All from PL206 , Bylands Siltstone.

Other material. NMV P138227 from PL 206, Wallan. Bylands Siltstone.

Derivation of name and diagnosis. As for genus.

Description. Exoskeleton elliptical in outline, estimated maximum length 20 mm. Cephalon strongly convex (sag., tr.), about twice as wide as long (sag.), anterior margin with maximum curvature medially in dorsal view and with outline interrupted anteromedially by slightly protruding glabella. Glabella rounded-pentagonal in dorsal view, about as wide across frontal lobe as long (sag.), maximum width situated at approximately 66% sagittal glabellar length from posterior. Axial furrows deep, diverging forward at about 55°. Occipital ring comprising about 30% maximum glabellar width, approximately twice as long sagittally as distally, with median node situated close to posterior edge. Occipital furrow moderately impressed medially and forming convex forward arc, increasing in depth abaxially and curving slightly forward distally. L1 about as long (exsag.) distally as occipital ring, apparently with weakly defined lateral node. S1 very deeply impressed laterally and directed slightly obliquely inward and backward, shallowing abruptly adaxially and curving forward slightly subparallel to occipital furrow, apparently very weakly impressed medially. S2 and S3 more distinct on internal moulds than on exterior of exoskeleton, S2 transverse, reaching axial furrow, S3 directed obliquely inwards and backwards from axial furrow in concave-forward curve, inner end situated opposite glabellar midlength (sag.). Frontal lobe transversely elliptical or rhombic, lacking sagittal furrow. Anterior border subvertical. Genal field moderately convex (tr.), steeply sloping laterally and in front of palpebral lobe. Palpebral lobe slightly raised, placed opposite anterior half of L3, palpebral furrow shallow, oblique and weakly curved, continuous adaxially with weak furrow behind eye ridge. Anterior branch of facial suture diverging forward in gentle curve subparallel with axial furrow, not cutting across anterolateral corner of glabella, posterior branch directed posterolaterally across genal field and inner part of lateral border, deflected sharply backward across outer part of border to meet cephalic margin opposite L1. Librigena lenticular in outline with long posterior projection. Posterior border narrow proximally, gently widening abaxially to about midpoint (tr.), thereafter uniform in width and gently curving forward. Posterior border furrow deep proximally, moderately impressed distally and continuous with shallower lateral border furrow. Lateral border weakly convex, steep and poorly defined anteriorly. Genal spine diverging slightly from line of lateral margin. Cephalic doublure strongly convex laterally, widening and flattening medially, no vincular furrow.

Hypostome unknown.

Thorax of 11 segments. Axis weakly tapering backward, comprising about 30% segmental width at front of axis and about 25% toward the back. Axial rings strongly convex (sag.), flexed forward distally, lateral lobes not defined. In transverse profile pleurae convex adaxially, concave abaxially. In dorsal view pleurae convex forward adaxially, concave forward abaxially, posterior edge of segment curving slightly forward distally to spinose tip. Pleural furrows short (exsag.) and deep, extending onto articulating facet distally. Anterior pleural band approximately as long (exsag.) as posterior band at fulcrum.

Pygidium lenticular, about twice as wide as long (sag.). Axis as wide as pleura anteriorly, comprising about 75% sagittal pygidial length, weakly tapering backwards and broadly rounded posteriorly. 1st axial ring high, 2nd ring well defined, 3rd ring poorly defined. Pleurae moderately convex (tr., exsag.), comprising at least 5 segments, 1 st 4 four with posterior pleural bands slightly more raised than anterior bands on internal moulds and extending onto border distally. Only 2 pleural furrows distinct, 1st sharply incised, not extending onto articulating facet distally, 2nd reaching halfway to margin. 1st 2 interpleural furrows weaker than pleural ones. Pygidial margin smooth on exterior of exoskeleton, weakly scalloped on internal mould. Pygidial doublure narrow, convex.

Remarks. The specimens are rather poorly preserved and affected by compression, shearing and fracturing. As a result there is apparent variation in the depth of some furrows, such as the medial part of S1 (compare fig. 19A with 19B–C, G) and the lateral cephalic border furrow (compare figs 19B–C with 19H). Some other morphological features are unclear, such as the extent to which lateral nodes are defined on L1 by adaxial expansion and/or bifurcation of S1 (compare figs 19F–G with 19K).

The blind condition of Berylacaste reinforces other evidence for a deep-water, possibly sub-photic environmental setting for the trilobite fauna at PL 206, Wallan. Berylacaste berylae is well-represented, but the fauna is dominated by the blind, effaced styginid Thomastus aops Sandford and Holloway, 1998 . Together these blind trilobites represent 63% of the trilobite population and 22% of the faunal diversity.Furthermore Ivops wallanensis , representing 19% of the trilobite population, bears an eye markedly smaller than other Wenlock phacopids. The taphonomy of the fauna at PL 206 contains a high proportion of partly articulated exoskeletons (60%) and indicates assignment to trilobite taphofacies TIV (see Sandford, 2002), considered to represent depths below maximum storm wave base. Exoskeletons of T. aops showing partly displaced librigenae may be moult assemblages characteristic of taphofacies TIV, although as the amount of displacement is small it is more likely due to post-depositional compaction.

Four other acastids are known from central Victoria, all in the Lower Devonian. Acaste lokii Edgecombe, 1993 and Acastella frontosa Shergold, 1968 are from the Humevale Siltstone at Lilydale, and Acastella sp. was recorded by Holloway and Neil (1982) from the Mt Ida Formation at Heathcote. These species are easily distinguished from Berylacaste berylae by their large eyes and deeply impressed S2 and S3. A distinctive, as yet undescribed acastid with long occipital and thoracic axial spines occurs in the Humevale Siltstone at Yarra Junction and at Kinglake West.

Figures

Figure 19. Berylacaste berylae gen. et sp. nov., from PL206, Wallan. A, paratype NMV P138224, dorsal exoskeleton, × 4. B–C, paratype NMV P139332, cephalon with displaced librigena, × 5. D–E (and fig. 20D), paratype NMV P139331, thoracopygon; D, × 8; E, × 4. F–H, paratype NMV P139333, cephalon, × 4. I–J (and fig. 20C), paratype NMV P138225, dorsal exoskeleton with down-flexed pygidium, × 5. K, holotype NMV P138224, cephalothorax, × 5. (A, C, F, H, I, K are internal moulds).

Figure 20. Berylacaste berylae gen. et sp. nov., from PL206, Wallan. A–B, E, paratype NMV P138226, cephalon; A–B, × 4; E, enlargement of genal angle showing thorn-like genal spine. C, paratype NMV P138225, enlargement of pygidium, × 10. D, paratype NMV P139331, enlargement of pygidium, × 8.