Trimerocephalus chopini Kin & Żejowski, 2013, n. sp.
Creators: name: Kin, Adrian
name: Żejowski, Ej Bła
Publisher: name: Zenodo
Year: 2013
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Trilobita
Phacopida
Phacopidae
Trimerocephalus
Trimerocephalus chopini

description: Trimerocephalus chopini n. sp.
Figures 2–4
Material. Holotype, Tr.8/12.02.79, near complete exoskeleton (pygidium absent) of a late meraspid (M10) (Fig. 3 F), from Unit I (late Early Famennian, Palmatolepis marginifera Zone) in Kowala Quarry, 7 km southwest of Kielce, Holy Cross Mountains, central Poland, 50 ° 47' 46.21" N 20° 33' 57.52" E.
Etymology. In honour of Fryderyk Franciszek Chopin, the most famous Polish composer during the Romantic period of classical music, also called the ‘poet of the piano’.
Diagnosis. Facial suture running along marginal furrow and not cutting cheek; the latter showing a very slight recess in contact with suture. Border furrows generally indistinct, but very shallow and wide near truncated genal angle. Ocular protuberance absent. Preoccipital furrow continuous along its length. Preoccipital ring straight, with two small, median tubercles. Occipital ring without median node. Pygidium trapezoidal with posterior margin almost rectilinear, width to length ratio of 2.8:1. Glabella and cheeks covered by very large to massive granules, usually uniform in size. Carapace covered by fine and moderately dense granulation.
Description. Cephalon semicircular in outline, length to width ratio about 0.5. Glabella subpentagonal in outline, wide, rounded anteriorly with antero-lateral angles truncated (Figs 3 F and 4A–C), length to width ratio about 0.6. Glabellar furrows S2 and S3 usually poorly developed, only visible on partially exposed internal moulds of glabellar area on three individuals [specimens: Kow / TA 76 (Radwański et al., 2009, pl. 2, fig. 3b) and Kow / TA 89, 90]; S2 short, bent obliquely upward and positioned very close to preoccipital furrow; posterior ramus of S3 very short and bent obliquely downward (as in T. caecus). Preoccipital furrow straight, well developed and of equal depth. Preoccipital ring straight, slightly convex with two small median tubercles. Occipital furrow slightly bent medially. Occipital ring wide, convex and without median node. Facial suture runs within the antero-lateral border furrow and does not extend onto the cheeks; cheeks exhibit a small recess in contact with suture (e.g., Figs 3 F, 4A–C). In dorsal view, the margins of the glabella and cheeks are highly vaulted; cheek areas triangular in outline, with rounded to truncated posterolateral genal angles. Axial furrows deep and moderately wide, diverging forwards at an angle of about 55°. Lateral and posterior borders are widest at posterolateral corner of cephalon. Vincular furrow in holaspid moderately deep, straight medially, strongly directed transversely with smooth edges (Fig. 4 D–E). In the genal part it is directed posteriorly (parallel to the cheek), and is very slightly crenulated with small, shallow pits (Fig. 4 E).
Thorax of 11 segments. Axial rings straight (tr.). Pleurae with flat fulcrum (bent backward) that is covered in fine granules. Pleural furrows wide and distinct, subdividing two convex bands. The anterior band is narrower (Fig. 2 A–B).
Pygidium almost three times as wide as long. Axial furrows moderately deep. Pygidial axis long and wide (Fig. 2 C). Ring furrows (5 + 1) straight, with anteriormost furrows more distinct than weakly developed posterior furrows. Articulating half ring as wide as first axial ring. Interpleural and pleural furrows (4) rather indistinct, except first two pleural furrows (Fig. 2 C).
Morphological variation. Amongst studied representatives of Trimerocephalus chopini, the following morphological variations have been observed:
1) In most cephala examined, the genal angle is truncated, but in some it is rounded. Specimens that exhibit the latter condition are typically flattened, thus may represent taphonomic (as opposed to true morphological) variants.
2) Most studied specimens do not show clear lateral glabellar furrows S2 and S3, which are visible only on internal moulds of three specimens. However, in other internal moulds these furrows are completely absent. Similar variation in the development of the glabellar furrows has been documented in Trimerocephalus dianopsoides from the Holy Cross Mountains (Osmólska 1963).
3) Cephalic tuberculation varies throughout ontogenesis. Relatively small granules occur in earlier meraspides (e.g., Fig. 3 B, C) and become gradually larger on cephala assigned to subsequent meraspides (i.e., sub-adult representatives, e.g., Figs 2 A, B, 3E, F). Slight differences in cephalic tuberculation (i.e., dimension of granules) also occur between individuals representing the same ontogenetic stages (compare specimens in Fig. 3 E, F). The cephalic ornamentation in T. chopini closely resembles that of Trimerocephalus shotoriensis Feist in Feist, Yazdi and Becker, 2003 (see Table 1).
Comparison of Trimerocephalus chopini to other species. For the purposes of this comparison, we follow Crônier’s (2003) phylogenetic groupings for the better known representatives of Trimerocephalus. Accordingly, on the basis of 23 morphological characters, three clades were distinguished among early Famennian representatives of this genus: T. caecus, T. lelievrei, T. shotoriensis and T. tardispinosus [clade 1]; T. procurvus, T. interruptus, T. sponsor and T. polonicus [clade 2]; and T. dianopsoides, T. (Trifoliops) nigritus, T. (Trifoliops) trifolius, T.? steinachensis and T. mastophthalmus [clade 3] (cf. Crônier, 2003, fig. 8).
Key morphological features of Trimerocephalus chopini that have phylogenetic significance include (Table 1): (1) facial sutures that do not cut the cheeks; (2) a well developed, straight preoccipital furrow, S1; (3) a lack of ocular protuberances; (4) coarse cephalic tuberculation; (5) a trapezoidal pygidium; (6) a preoccipital ring with two median tubercles; (7) the absence of an occipital median node; (8) a slight recess in the cheeks in association with the facial suture; and (9) a truncated genal angle. These features suggest a close relationship to members of clade 1 (mentioned above), in addition to the recently erected T. mimbi from the Early Famennian (do II = rhomboidea Zone) of Australia (Feist et al., 2009) (see Table 1).
The following list outlines the main morphological differences of Early Famennian species of Trimerocephalus from Poland (mentioned above), Iran (T. shotoriensis; Feist et al., 2003) and Australia (T. mimbi; Feist et al., 2009), and the new species (T. chopini):
- Trimerocephalus caecus differs in its anteriorly curved preoccipital furrow (S1), rounded genal angle, and the presence of an occipital median node (Table 1);
- Trimerocephalus dianopsoides differs in the presence of ocular protuberances and the poor segmentation of the pygidium (Table 1);
- Trimerocephalus interruptus differs in the outline of the facial suture that cuts the cheek, the fine and condensed granulation, and poorly segmented lenticular pygidium;
- Trimerocephalus mastophthalmus differs in the outline of the facial suture that cuts the cheek, the presence of ocular protuberances, and much finer granulation of the cephalon (Table 1);
- Trimerocephalus mimbi differs in having a medially ill-defined preoccipital furrow (S1), rounded genal angle, a deep lateral border furrow and much deeper vincular furrow (Table 1);
- Trimerocephalus polonicus is represented only by cephala of early instars, but differences include a subsemicircular outline (but strongly pointed anteriorly), a facial suture that cuts the cheeks, and very sparse tuberculation;
- Trimerocephalus (Trifoliops) trifolius differs in the trilobed outline of the cephalon, a facial suture that cuts the cheeks, and the presence of ocular protuberances;
- Trimerocephalus shotoriensis differs in exhibiting a rounded genal angle and a subtrapezoidal outline of the pygidium (Table 1).
Crônier (2003) emphasised the acquisition of a marginal facial suture that does not cut the cheeks and the presence of an occipital median node as important morphological features in tracing the evolutionary trends in Trimerocephalus, despite being homoplastic. Trimerocephalus chopini conforms to most character acquisitions in the T. caecus plexus, but lacks the occipital median node, and appears to be most closely related to T. shotoriensis (see Table 1 for comparison). Interestingly, specimens of T. caecus described by Osmólska (1958) from the Holy Cross Mountains also lack an occipital median node (as in T. chopini), but do show all the other characteristic features for this widely distributed species (for details, see Crônier, 2003). It also should be clearly emphasized here that all representatives of T. caecus known from outside of Poland possess the occipital median node (Osmólska, 1958; Crônier, 2003). It is therefore possible that specimens of T. caecus from Poland represent the most ancestral populations (lacking the occipital median node) or, more probably, specific ecophenotypic variants of that species. Consequently, the absence of the occipital median node in T. chopini may be due to atavism. The only other species that seems to lack the occipital median node is T. mimbi (see Table 1), but this taxon is known only from the holotype cephalon (specimen WAM. 07.284; Feist et al., 2009, p. 23), hence additional specimens are needed to confirm if this is a true absence in this species.
description: Published as part of Kin, Adrian & Żejowski, Ej Bła, 2013, A new Trimerocephalus species (Trilobita, Phacopidae) from the Late Devonian (Early Famennian) of Poland, pp. 345-355 in Zootaxa 3626 (3) on pages 347-352, DOI: 10.11646/zootaxa.3626.3.3, http://zenodo.org/record/218660