Wenndorfia expansa ( Hector, 1876 )

Sandford, Andrew C., 2005, Homalonotid trilobites from the Silurian and Lower Devonian of south-eastern Australia and New Zealand (Arthropoda: Trilobita: Homalonotidae), Memoirs of Museum Victoria 62 (1), pp. 1-66 : 56-60

Wenndorfia expansa ( Hector, 1876)

Homalonotus expansus Hector, 1876: 602 , pl. 27 fig. 2. Homalonotus sp. — Hutton, 1888: 257. Homalonotus (Burmeisteria) huttoni .— Allan, 1935: 28, pl. 1 figs 4–5. Homalonotus (Digonus) expansus .— Allan, 1935: 29, pl. 1 fig. 1. Homalonotus (Digonus) cf. expansus .— Allan, 1935: 29, pl. 1 fig. 3. Burmeisteria (Digonus) expansus .— Saul, 1965: 271. Burmeisteria huttoni . — Saul, 1965: 271.— Tomczykowa, 1975: 11. Digonus expansu s.— Tomczykowa, 1975: 11.— Wenndorf, 1990: 16. Burmeisteria (Digonus) cf. expansus .— Speden and Keyes, 1981: pl. 4 fig.I. Burmeisteria expansa .— Cooper, 1982: 27. Burmeisteria huttoni .— Cooper, 1982: 27.— Wenndorf, 1990: 16. Digonus margaritifer — Wenndorf, 1990: 77, pl. 11 figs 8–9.

Type material. Homalonotus expansus . Lectotype (pygidium, figured Hector, 1876, Allan, 1935: pl. 1 fig. 1, Fig. 22.10 herein, in the Geological Survey of New Zealand, cast in Museum Victoria registered NMV P16844) from McKay locality 129, Rainy Creek, Reefton, south island, New Zealand. Paralectotype (pygidium, also the holotype of Digonus margaritifer , figured Allan, 1935: pl. 1 fig. 3, Wenndorf, 1990: pl. 11 fig. 8, in the Geological Survey of New Zealand, cast in Museum Victoria registered NMV P16846) from McKay locality 130, LankeyGully, Reefton. Paralectotype (pygidium, figured Hector, 1876, in the Geological Survey of New Zealand, cast in Museum Victoria registered NMV P16847) from locality 129.

Homalonotus (Burmeisteria) huttoni . Holotype ZFc33 (dorsal exoskeleton, figured Allan, 1935: pl. 1 figs 4, 5) from “near Reefton”. Allan (1935) suggested the specimen is from locality 129.

Digonus margaritifer . Holotype (pygidium, and paralectotype of Homalonotus expansus , see above). Paratype (pygidium, figured Wenndorf, 1990: pl. 11 fig. 9, in the Paläontologisches Museum der Humboldt-Universität Berlin, Germany), documented as coming from Kilmore, Victoria, acquired through the fossil dealer Kranz in 1885. However as the paratype is undoubtedly conspecific with the New Zealand material, as no homalonotids other than Trimerus (Trimerus) vomer , Trimerus (Ramiotis) otisi and Dipleura garratti are yet known in the well-collected Wenlock-Ludlow trilobite faunas from the Kilmore area, and as Wenndorfia would be most unexpected in strata of this age, I conclude that the Berlin specimen is from the Reefton area and its documentation confused, possibly by Kranz, with a specimen of T. (T.) vomer . Kranz may have acquired the Berlin specimen from Theodore Ranft who was the first to collect fossils in Lankey Gully at Reefton in 1872. Part of Ranft’s collection was purchased by the Melbourne Museum, who also had dealings with Kranz.

Previously figured material. NZGS AR 676 (thoracopygon, figured Speden and Keyes, 1981: pl. 4 fig. I) from locality GS3737, S38/f523, Rainy Creek, Reefton.

Registered material. 12 specimens: 1 complete exoskeleton, 2 thoracopygons, 1 incomplete thorax, 8 pygidia,. NMV P16844 , NMV P16847 from locality 129. P16846 from locality 130. NZGS AR676 from locality GS3737. NMV P64120 – P64122 from Lankey Gully (Ranft collection). ZFc33, ZFc58, ZFc309, ZFc350 from “Reefton”. One pygidium in the Paläontologisches Museum der Humboldt-Universität Berlin .

Stratigraphic distribution. Lankey Limestone,? Boucotia loyolensis Assemblage Zone, Emsian.

Diagnosis. Glabella extremely low, sides parallel, without distinct lobation, anterior margin broadly rounded, length 1.1 times width. Palpebral lobes placed opposite ~0.55 glabellar length. Axial furrows very poorly defined. Pygidium short, with length 0.7 times width, weakly concave sides, converging posteriorly at about 100˚ to an obtusely angular tip. Pygidial axis wide, about 0.4 times pygidial width and very poorly defined anteriorly, tapering moderately with sides straight and converging at about 30˚, length about 0.85 times pygidial length, posterior margin parabolic, 12-13 rings, semicircular terminal piece with distinct posterior margin. Axial furrows poorly defined, indicated by flexure of pleurae, almost indistinct anteriorly. 9 pleural ribs. Pleural furrows not reaching margin. Pleural and ring furrows deep and of equal depth. Anteriorly ribs and rings continuous, pleural offset at seventh rib. Row of equidimensional, regularly spaced tubercles on axial area of thoracic and pygidial segments, with two or three supplementary rows of smaller tubercles. Tubercles large on larger specimens, small to fine on smaller specimens. Similar, but less regular tuberculation on posterior border of fixigenae and thoracic and pygidial pleurae. Genal field with densely spaced, coarse granules.

Discussion. Three homalonotids have been named from the Lower Devonian at Reefton. The best known of these is Homalonotus (Burmeisteria) huttoni , based on a single, nearcomplete, enrolled exoskeleton ( Fig. 22.7). The specimen was described in detail in open nomenclature by Hutton (1888), and later named by Allan (1935). Several points in Hutton’s original description of the specimen cannot be substantiated, in particular those concerning the cephalic shape and proportions, and the presence of distinct glabellar lobation. Although only the first six segments of the pygidium are preserved in the holotype, the ornament of tubercles on the pleural ribs and axial rings are distinctive and shared by several more completely preserved pygidia in the Museum of Victoria ( NMV P64120–2, Figs 22.8, 22.12–22.13) and in the New Zealand Geological Survey ( NZGS AR 676, Fig. 22.14). These pygidia also share very deep ring and pleural furrows that are continuous to the seventh segment. The axial furrow is not impressed on the anterior half of the pygidium, but its position is indicated by a posteriorly increasing flexure at the junction of the ring and pleural furrows. These specimens are considered to be conspecific with the holotype of huttoni .

The other two species, Homalonotus expansus and Digonus margaritifer , were based only on pygidia. Wenndorf’s (1990) description of margaritifer includes features identical with those of H. (Burmeisteria) huttoni , such as the ornament, the deep axial ring and pleural furrows continuous to the seventh rib, the very shallow axial furrows, and the wide roundedtriangular outline. Wenndorf noted expansus and margaritifer shared pleural offsetting at the seventh-eighth rib, but distinguished the former by the smooth surface texture (absence of tubercules) and the weak definition of the axis of the former species. Close inspection of the external mould of the lectotype of expansus reveals, however, a row of small tubercles on the first four axial rings and pleurae (see Fig. 22.10b). Corresponding ornament on the anteriormost rings on the internal mould were not recorded either by Hector (1876) or by Allan (1935) as this area of the pygidium is damaged. The spacing of these tubercles on the lectotype is similar to that of huttoni . The differences in the coarseness of tuberculation between the type specimens of expansus and specimens of huttoni are attributed here to the size of the pygidia. The lectotype of expansus , with the finest tubercles, is the smallest specimen. Moderately sized specimens including the holotype of margaritifer have slightly larger tubercles (see Fig. 22.9), whereas the largest of specimens including the holotype of huttoni have much larger tubercles. This interpretation is in accord with Cooper’s (1982) suggestion that expansus and huttoni might be synonyms. The differences in definition of the axis between margaritifer and expansus noted by Wenndorf (1990) can be attributed to the flattening of the lectotype of expansus, axial convexity obscured by crushing and longti-tudinal folding of the tergite. Taking this deformation into account, whatever differences may have existed between the specimens do not appear to be significant. In the absence of significant differences in ornament or other features between the types of expansus and pygidia of margaritifer and huttoni , the species are regarded as conspecific.

There is little to support the assignment of Homalonotus expansus to Burmeisteria . The most reliable character of Burmeisteria recognised in this work is the exceptionally high number of pygidial segments, with B. herschelii having up to 17 axial rings and 11 pleural furrows. H. expansus has only 12 axial rings and 9 pleural ribs. Reed (1918) emphasised the biconcave course of the rostral suture in herschelii as a generic character. Cooper (1982) suggested the course of the rostral suture of herschelii to be variable and probably ontogenetically related, supporting earlier doubts on its significance ( Sdzuy, 1957, Saul, 1965). Notwithstanding this debate, the anterior portion of the cephalon (and the rostral suture) of the holotype of huttoni is not preserved. B. herscheli i is strongly polymorphic, with variable glabellar morphology, but the glabella of expansus differs in having no trace of lobation or spines, and in that the axial furrows are subparallel and much shallower. In the absence of rostral morphology, the assignment of huttoni to Burmeisteria by various workers ( Allan, 1935, Wenndorf, 1990) was presumably based on the similarity of the tuberculate ornament to that of herschelii . The arrangement of these tubercles suggests, however, they are not homologous, but independently derived. As noted by Cooper (1982), the tubercules of herschelii are arranged longitudinally. This arrangement differs from the transversely arranged tubercles on expansus .

Tuberculation similar to that exhibited by Homalonotus expansus is known amongst members of Digonus . Wenndorf (1990) noted a close similarity in pygidial ornament between the paratypes of margaritifer and the ornatus group. Wenndorf added that margaritifer shared few other ornatus -group characters. The narrow pygidial axis is incompatible with assignment of expansa to Digonus .

Generic assignment of Homalonotus expansus to Wenndorfia emphasises its low glabellar height, rounded anterior glabellar margin, posteriorly placed eyes, indistinct pygidial axial furrows and narrow axial width, and the continuity of the ring and pleural furrows, the termination of the axis at about 0.9 pygidial length, and the absence of a postaxial ridge. Its relationships to other species of Wenndorfia are difficult to assess, but it differs from all other species in exhibiting a coarsely tuberculate thoracic and pygidial ornament. In overall appearance, pygidia assigned to W. expansa most closely resemble the deeply furrowed pygidia of W. multicostata . The triangular pygidial outline, however, distinguishes it from multicostata and most other Wenndorfia , as discussed above. As the cephalic morphology of multicostata is poorly known, it is difficult to make a detailed comparison with expansa . The very low height and poor definition of the glabella of expansa is approached only by W. plana plana , although the latter differs in that the glabellar axial furrows are quite distinct posteriorly.

Shirley (1938) recorded Homalonotus sp. from the Baton Formation at Baton River, east of Nelson, North Island, New Zealand. He considered ten crushed and deformed pygidia to be identical with pygidia of Wenndorfia expansa . Shirley’s description of the number of axial rings and pleural ribs and the continuity of the rings and pleurae are in accord with expansa , but as I have not examined these specimens I can make no judgement on their affinities. The Baton Formation is generally considered younger than the Lankey Limestone at Reefton, although its age is variably cited as Pragian ( Boucot et al., 1969) or Lochkovian ( Wright, 1990).

The newly documented material warrants a revised diagnosis of the species, but otherwise is too limited to warrant a complete description to replace those of Hutton (1888), Allan (1935) and Wenndorf (1990).

Environmental notes. The taphonomy of the population of Wenndorfia expansa (as a whole) are attributable to Speyer and Brett’s taphofacies 4B, with Ar=33% and the relative abundance of completely enrolled specimens (8%). W. expansa is interpreted to inhabit deep, outer-shelf environments. This suggested habitat is comparable to the deepest water facies associated with populations of many other species of Wenndorfia (see Wenndorf, 1990)