Preodontochile springfieldensis, 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 : 243

Preodontochile springfieldensis sp. nov.

Figures 15 , 16

Type material. Holotype NMV P312070 (cephalon with broken and displaced genal field) . Paratypes NMV P312071 (incomplete cephalothorax showing damage to genal field) , NMV P139350 (incomplete and partly disarticulated cephalothorax showing cephalic doublure and hypostome) , NMV P312074 (incomplete pygidium) , NMV P312075 (incomplete and partly disarticulated thoracopygon). All from PL1369 , Springfield .

Other material. NMV P139350–P139352, NMV P312072– P312073, from PL1369, Springfield.

Diagnosis. Preodontochile with eye oriented slightly obliquely to exsagittal line and visual surface having approximately 40 lenses arranged in about 15 files of up to 4 lenses each. Posterior cephalic border markedly expanding (exsag.) abaxially, genal spine very broad proximally and tapering strongly distally. Pygidial axis about 25% maximum pygidial width anteriorly, with 10 axial rings, first 5 well defined by inter-ring furrows that contain apodemal pits laterally and are shallow medially, posterior rings poorly defined by shallow inter-ring furrows that are not continuous medially. Pleurae with about 7 shallow pleural furrows and very weakly impressed interpleural furrows. Mucro with length (measured from terminus of axis) comprising about 25% sagittal pygidial length. Pygidial doublure wide.

Description. Cephalon (excluding genal spines) semi-circular in outline, anterior margin with very weak convex-downward medial flexure in anterior profile. Glabella weakly convex, slightly narrower across L1than across occipital ring,thereafter expanding moderately forward, a little more strongly in front of S3 than behind, width across frontal lobe 170–180% occipital width and about 80% sagittal length of glabella. Occipital ring raised slightly higher than remainder of glabella, width (tr.) about 30% cephalic width across same transverse line, length about 12% cephalic length sagittally, shorter (exsag.) laterally. Median section of occipital furrow gently convex forward and very shallow, lateral section (approximately 30% total width) with arcuate, slit-like apodemal pit not quite reaching axial furrow distally. S1 and S2 defined only by slit-like apodemal pits that are equal in width (tr.) to occipital apodemal pit and placed directly in front of it; S1 gently arcuate, equidistant from occipital furrow and S2; S2 transverse, situated at 33% cephalic length from posterior. L3 as long (exsag.) as L1 and L2 adaxially, 160% as long abaxially. Adaxial portion of S3 oriented at about 20° to transverse, abaxial portion very wide, lenticular in outline, oriented at about 35° to transverse. Frontal lobe comprising half sagittal length of glabella, elliptical in outline, with short, shallow sagittal depression in posterior half. Axial furrow moderately impressed, preglabellar furrow shallow. Anterior cephalic border of uniform length (sag., exsag.), comprising about 8% sagittal cephalic length, subhorizontal, weakly concave. Lateral border furrow defined mainly by change in slope, lateral border very wide (tr.), twice width of anterior border, gently sloping, weakly concave. Posterior border furrow deep, adaxial portion transverse, abaxial portion directed posterolaterally, terminating before reaching lateral border furrow. Genal spine long, with longitudinal furrow dividing horizontal lateral portion from steeply sloping adaxial portion. Eye with length about 20% sagittal cephalic length, anterior edge placed opposite front of L3 and posterior edge just in front of level of S2. Palpebral lobe not greatly raised above palpebral area, crescentic in outline, narrow (tr.). Palpebral furrow weak. Visual surface with lower margin subtending an angle of about 80° in dorsal view. Anterior branch of facial suture diverging gently forward towards widest part of frontal lobe, posterior branch weakly sinusoidal, meeting lateral cephalic margin more or less opposite posterior edge of eye. Librigenal field weakly convex, about 66% width of lateral border.

Cephalic doublure flat anteromedially, approximately 33% sagittal cephalic length, crossed by weak arcuate furrow defining crescentic area adjacent to hypostomal suture. Lateral to hypostomal suture, doublure with upturned inner flange that is continuous posteriorly with adaxial surface of genal spine. Only available hypostome is incomplete posteriorly. Anterior wing small, middle body with parabolic posterior outline, middle furrow indistinct, posterior portion of lateral border furrow and posterior border furrow shallow.

Anterior part of thoracic axis subparallel-sided and comprising about 33% segmental width (tr.) (fig. 15A–B, D, F), posterior part of axis relatively narrower (fig. 16G). Axial rings weakly convex (sag.), with indistinct lateral lobes that lack nodes. Axial furrow moderately impressed. Pleural furrows sigmoidal, short (exsag.) and sharply impressed, at fulcrum dividing segment into anterior and posterior bands of equal length (exsag.) and height. Distal ends of pleurae rounded anteriorly and angular posteriorly.

Pygidium moderately convex transversely, subtriangular in outline with weakly convex sides converging posteriorly at about 60°. Axis tapering uniformly backward, 2nd segment with broad, subrectangular pseudo-articulating half ring, much smaller and weaker pseudo-articulating half rings possibly present on next two segments. Mucro triangular, postaxial ridge absent.

Exoskeleton finely granulate, lacking tubercles.

Remarks. Thepoorpreservationoftheonlyavailablecranidium of P. camprodonensis does not permit detailed comparison with P. springfieldensis , but the latter differs in the less obliquely oriented eye and the greater abaxial expansion of the posterior border. Differences in the form of S1 and S2, which in springfieldensis are isolated and very narrow (exsag.) but in camprodonensis appear wider (exsag.) and deeper, with S1 said to be connected medially ( Degardin and Pillet, 1984, p. 87, fig. 5), can be attributed to crushing in the latter. The most striking difference between P. springfieldensis and P. camprodonensis is in pygidial segmentation. Pygidia of the type species have about 20 axial rings, at least 15 deep and medially continuous inter-ring furrows, at least 11 wide and deep pleural furrows, and very strongly incised interpleural furrows. In contrast, pygidia of springfieldensis have only ten axial rings, five continuous inter-ring furrows that are shallow medially, seven narrow (exsag.), shallow to moderately impressed pleural furrows, and very weak interpleural furrows.

The occurrence of P. springfieldensis at PL1369 together with another relatively small-eyed form ( Phacopidae gen. indet. 3) is significant. Eye reduction in phacopids and other trilobites has been widely interpreted as an adaptation to deep water, subphotic environments, although Álvaro and Vizcaïno (2003) noted that high turbidity may create subphotic environments in shallower settings. The taphonomy of the Deep Creek trilobite population is indicative of a deep water facies rather than turbid conditions. The degree of articulation is high (isolated tergites 40%), and although there are no fully articulated exoskeletons, specimens have been found with the hypostome only slightly displaced along the hypostomal suture (fig. 15F), and with incomplete thoraces attached to the cephalon or only slightly detached from it or from the pygidium (figs 15A–B, D, G, 16G). This evidence suggests deposition at depths below normal wave base where bottom current activity is negligible, preventing displacement, winnowing and concentration or reworking of the exoskeletal elements. These deep water beds correlate in age with the late Telychian (mid Monoclimacis crenulata Biozone ) eustatic highstand (event 4) documented from six palaeocontinents by Johnson (1996). The abrupt transition from these beds to the shallower water facies of the overlying Chintin Formation may reflect the influence of an extensive latest Telychian Gondwanan glaciation event documented from Brazil ( Grahn and Caputo, 1992). Otherwise, the Chintin Formation appears to correspond closely in age to the subsequent regressive phase that culminated in an earliest Wenlock lowstand.

Figures

Figure 15. Preodontochile springfieldensis sp. nov., from PL1369, Springfield. A–B, G, paratype NMV P312071, incomplete cephalothorax with damaged genal field; A–B, × 2.2; G, × 2.7. C, E (and figs 16A, E–F), holotype NMV P312070, cephalon with left cheek broken and displaced; C, × 2; E, × 1.8. D, F (and fig. 16H), paratype NMV P139350, cephalothorax; D, × 1.8, with glabella showing tool marks from preparation; F, × 2.0, showing cephalic doublure and hypostome. (A, C, E, G are internal moulds).

Figure 16. Preodontochile springfieldensis sp. nov., from PL1369, Springfield.A, E–F, holotype NMV P312070, × 2. B, paratype NMV P312074, pygidium, × 2. C, G, paratype NMV P312075, thoracopygon; C, × 3; G, × 2.7. D, paratype NMV P312072, cranidium and displaced cheek, × 5. H, paratype NMV P139350, enlargement of eye, × 8. I, NMV P139351, three thoracic segments, × 1.5. (B–C, E are internal moulds).