Machairagnostus, Harrington & Leanza, 1957

Taylor, John F., Loch, James D. & Repetski, John E., 2024, Taxonomy and stratigraphic distribution of Lotagnostus (Agnostida: Agnostidae) and associated trilobites and conodonts in the Upper Cambrian (Furongian) of Laurentia, Zootaxa 5422 (1), pp. 1-66 : 58

Machairagnostus cf. M. corrugatus ( Suárez-Soruco, 1975)

( Plate 7.20 )

Discussion. A single exfoliated pygidium from collection 5/22/08B conforms to the diagnosis of Naimark (2016) for Machairagnostus by exhibiting a clearly defined intranotular axis (=lanceolate field) bounded by well-impressed notular furrows, and a terminal node just inside the border furrow.Although a few exfoliated pygidia of Neoagnostus parki from the same collection display very faint notular furrows (e.g. Pl. 15 , fig. 10), none compare in depth of incision or are associated with scrobiculae on the pleural fields like those on the specimen here assigned to Machairagnostus . Moreover, this specimen is proportionally narrower (tr.) than N. parki pygidia of similar size. Given these differences, we consider it unlikely that this unique pygidium is a variant of N. parki .

The Windfall specimen differs from all pygidia illustrated for the seven species of Machairagnostus listed by Naimark (2016) in displaying a broader, flat border and much longer marginal spines that diverge posteriorly, rather than being subparallel. The pygidia of the type species, M. tmetus ( Harrington & Leanza, 1957) , and the species identified as M. sp. by Lazarenko et al. (2008, pl. 20, fig. 14, pl. 21, fig. 12) also differ in lacking scrobiculae on the pleural fields and having the sides of the posteroaxis more clearly defined by faintly impressed axial furrows. The pygidium of M. ornatus ( Lisogor, 1977, pl. 31, fig. 1) is less quadrate, more coarsely scobiculate, and lacks clear definition of the intranotular axis. The poor preservation of the illustrated pygidia of M. kentauensis ( Ergaliev, 1983, p. 43–44, pl. 3, figs 3–4) makes comparisons difficult. However, the Khazakhstanian specimens clearly display a much broader border furrow and narrower border of less variable width than the Windfall pygidium. Machairagnostus houchengensis ( Zhang, 1981, pl. 55, fig. 12) has a poorly defined intranotular axis, more strongly constricted acrolobe, and much smaller and more anteriorly positioned posterolateral spines than the Nevada pygidium. The axial features of the Windfall pygidium most closely resemble those displayed by the three pygidia of M. corrugatus ( Suárez-Soruco, 1975) illustrated by Tortello & Esteban (2003, figs 4.CC–EE). Indeed, the similarity is so strong that the Windfall pygidium might represent that species, with the wider border and longer, more posteriorly positioned spines attributable to intraspecific (ontogenetic) variation. Whether the Windfall pygidium is an ontogenetic variant of M. corrugatus or a new, closely related species cannot be determined at present owing to the poor preservation of the border and marginal spines in the smallest pygidium illustrated by Tortello & Esteban (2003, fig. 4.DD), which is similar in size to the Windfall specimen, and the lack of an associated cephalon in the Nevada collection.

Figures

PLATE 7. Miscellaneous Windfall taxa.Each white scale bar represents 2mm. 1-3. Lotagnostus cf. L. obscurus. USMN 775722, cephalon, from D3362-CO, dorsal, anterior, and left lateral views. 4-6. Rhaptagnostus cf. R. convergens. Cephalon, CM 41350, from 5/22/08B, dorsal, anterior, and right lateral views. 7-9. Lotagnostus cf. L. obscurus. Cephalon, CM 41351, from 5/22/08B, dorsal, anterior, and right lateral views. 10-12. Homagnostus?. Pygidium, CM 41356, from 5/22/08B, dorsal, posterior, and left lateral views. 13-15. Lotagnostus cf. L. obscurus. Pygidium, CM 41353, from 5/22/08B, dorsal, posterior, and left lateral views. 16. Homagnostus? sp. Pygidium, CM 41401, from 5/22/08B, dorsal view. 17-18. Micragnostus cf. M. intermedius. Cephalon, CM 41358, from 5/22/08B, left lateral and dorsal views. 19. Hedinaspis sp. Fragmentary thoracic and/or pygidial fragment, CM 41359, from 5/22/08C, dorsal view. 20. Machairagnostus cf. M. corrugatus. Pygidium, CM 41349, from 5/22/08B, dorsal view. 21-22. Homagnostus? sp. Cephalon, CM 41357, from 5/22/08B, dorsal and anterior views.

PLATE 15. Pygidia of Neoagnostus parki n. sp. Each white scale bar represents 2mm. 1-3. CM 41354 from 5/22/08B, dorsal, posterior, and right lateral views. 4. CM 41395, from 5/22/08B, dorsal view. 5. CM 41352, from 5/22/08B, dorsal view. 6. USMN 775753, from D3362-CO, dorsal view. 7. CM 41397, from 5/22/08B, dorsal view. 8. CM 41399, from 5/22/08B, dorsal view. 9. CM 41398, from 5/22/08B, dorsal view. 10-12. CM 41396, from 5/22/08B, dorsal, right lateral, and posterior views. 13. CM 41355, from 5/22/08B, dorsal view. 14. CM 41400, from 5/22/08B, dorsal view.