{{Short description|Extinct genus of trilobites}} {{Technical|date=January 2021}} {{Automatic taxobox |fossil_range = {{fossil range|Cambrian}}<ref name="SEPKOSKI 2002 560">{{cite journal |last=SEPKOSKI |first=J. |title=A compendium of fossil marine animal genera |journal=Bulletins of American Paleontology |volume=363 |pages=1–560 |year=2002 |url=http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=307&rank=class |access-date=2008-01-12 |url-status=dead |archive-url=https://web.archive.org/web/20060905162524/http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=307&rank=class |archive-date=2006-09-05 }}</ref> |image = Acontheus.jpg |taxon = Acontheus |authority = Angelin, 1851<ref>ANGELIN, N. P. 1851 (dated 1852). ''Palaeontologica Scandinavica, Pars 1, Iconographia crustaceorum formationis transitionis, Fasciculus I''. T. O. Weigel, Lund, 1 - 24.</ref> }}
'''''Acontheus''''' is a genus of trilobites belonging to the family Corynexochidae, order Corynexochida, and is geographically widespread having been recorded from middle Cambrian strata in Sweden, Newfoundland, Germany, Siberia, Antarctica, Queensland, China and Wales.
''Acontheus'' appears confined to the Drumian and Guzhangian Stages, uppermost two of three Stages subdividing the middle Cambrian Miaolingian Series and, if species assignments are correct, the genus ranges in terms of the Scandinavian sequence from at least the ''Hypagnostus parvifrons'' Biozone in Wales to the ''Lejopyge laevigata'' Biozone at various locations elsewhere.
==Taxonomy== ''Acontheus'' has been placed in the subfamily Acontheinae <ref>{{cite journal |last=GEYER |first=G. |title=Cambrian Corynexochid Trilobites from Morocco |journal=Journal of Paleontology |volume=68 |issue=6 |year=1994 |pages=1306–1320 |doi=10.1017/S0022336000034296 |jstor=1306232|bibcode=1994JPal...68.1306G |s2cid=130995363 }}</ref><ref>{{cite book |last=POLETAEVA |first=O. K. |year=1956 |chapter=Semejstvo Triniidae Poletayeva fam. nov. |pages=178–179 |editor-last1=KIPARISOVA |editor-first1=L. D. |editor-last2=MARKOVSKIJ |editor-first2=B. P. |editor-last3=RADCHENKO |editor-first3=G. P. |title=Materialy po paleontologii. Novye semejstva i rody. |trans-title=Materials on paleontology. New families and genera |publisher=Vsesoyuznyi Nauchno-issledovatel'skii Geologicheskii Institut (VSEGEI) |location=Trudy, Novaya Seriya |volume=12 |language=ru}}</ref><ref>{{cite journal |last=SUVOROVA |first=N. P. |year=1964 |title=Trilobity korineksokhoidy i ikh istoricheskoe razvitie|trans-title=Corynexochoid trilobites and their evolutionary history |journal=Trudy Paleontologichekogo Institita |volume=103 |pages=1–319 |language=ru}}</ref><ref>{{cite journal | title=Trilobites from the lower and lowermost middle Cambrian of the Kiya River reference section (Kuznetsk Alatau) | journal=Annales de Paléontologie | volume=85 | issue=1 | year=1999 | issn=0753-3969 | doi=10.1016/S0753-3969(99)80007-X | pages=3–56| last1=REPINA | first1=L. N. | last2=ROMANENKO | first2=E. V. | last3=FEDJANINA | first3=E.S. | last4=PEGEL | first4=T. V. | bibcode=1999AnPal..85....3R }}</ref> (See Cotton, 2001, p. 194).<ref>{{cite journal | last=COTTON | first=T. J. | title=The Phylogeny and Systematics of Blind Cambrian Ptychoparioid Trilobites | journal=Palaeontology | publisher=Wiley | volume=44 | issue=1 | year=2001 | issn=0031-0239 | doi=10.1111/1475-4983.00176 | pages=167–207| bibcode=2001Palgy..44..167C | s2cid=85967924 | doi-access= }}</ref>
Cotton (''op. cit.'') also erected Tribe HARTSHILLINI within the Acontheinae to accommodate the highly derived genera ''Hartshillia'' Illing, 1916 <ref>{{cite journal | last=ILLING | first=V. C. | title=The Paradoxidian Fauna of a Part of the Stockingford Shales | journal=Quarterly Journal of the Geological Society | volume=71 | issue=1–4 | date=1915-09-01 | issn=0370-291X | doi=10.1144/GSL.JGS.1915.071.01-04.17 | pages=386–450| s2cid=128634710 | url=https://zenodo.org/record/2397276 }}</ref> and ''Hartshillina'' Lake, 1940.<ref>{{cite journal | last=LAKE | first=P. | title=Monograph of the British Cambrian Trilobites. Part XII. Pages 273–306; Plates XL–XLIII | journal=Monographs of the Palaeontographical Society | publisher=Informa UK Limited | volume=94 | issue=421 | year=1940 | issn=0269-3445 | doi=10.1080/02693445.1940.12035665 | pages=273–306| bibcode=1940MPalS..94..273L }}</ref>
Öpik (1982, p. 77) <ref>{{cite journal |last=ÖPIK |first=A. A. |year=1982 |title=Dolichometopid trilobites of Queensland, Northern Territory, and New South Wales |journal=Bureau of Mineral Resources, Geology and Geophysics, Bulletin |volume=175 |pages=1–85}}</ref> placed Acontheinae in the Dolichometopidae “by virtue of the similarities in pygidial structure between ''Fuchouia'' (Dolichometopidae) and ''Acontheus tenebrarum''". However, ''Acontheus tenebrarum'', Öpik (1982, pl. 32, fig. 5) from a late part of the V-Creek Limestone of Queensland and Zone of ''Doryagnostus notalibrae'' (= the Zone of ''Pytchagnostus punctuosus'' associated with ''Goniagnostus nathorsti'') has since been excluded from the genus (Jago ''et al.''., 2011).<ref>{{cite journal |last1=JAGO |first1=J. B. |last2=BENTLEY |first2=C. J. |last3=COOPER |first3=R. A |year=2011 |title=A Cambrian Series 3 (Guzhangian) fauna with ''Centropleura'' from Northern Victoria Land, Antarctica |journal=Memoirs of the Association of Australasian Palaeontologists |volume=42 |pages=15–35 |issn=0810-8889}}</ref>
==Diagnosis== Small heteropygous Corynexochidae lacking eyes and dorsal sutures (Westergärd, 1950, p. 9; Hutchinson, 1962, p. 109, Pl. 16, figures 8a-b, 9; Jago ''et al.''., 2011, p. 29), although according to some authors including species with proparian sutures and possibly small palpepral lobes (see Remarks). Cephalon slightly parabolic in outline with rounded or acute genal angles; genae convex, subcircular to subtriangular in outline; lateral borders wide. Cephalic exoskeleton punctate or smooth. Thorax of 6 segments in species illustrated; pleurae bent strongly downwards abaxially, tips sharply rounded; pleural furrows straight, linked to posterior corners of axial rings by shallower oblique furrows. Axial furrows indistinctly defined. Pygidial axis composed of three (or four?) rings and a terminal piece. Pleural fields usually separated by axis; four pairs of pleural or interpleural furrows extend to margin; border broad with uniform convexities; margin entire.
Genus '''''Acontheus''''' Angelin, 1851 [= ''Aneucanthus'' Angelin 1854 (Obj.); ''Aneuacanthus'' Barrande, 1856 (Obj.)].<ref>{{cite journal |last=BARRANDE |first=J. |year=1856 |title=Bemerkungen über einige neue Fossilien aus der Umgebung von Rokitzan im Silurischen Beken von Mittel-Böhmen |journal=Jahrbuch der Kaiserlichen-königlichen geologischen Reich-sandstalt |volume=2 |page=355 |language=de}}</ref>
Characteristics of ''Acontheus'' are as for the subfamily.
'''Type species:''' By monotypy, ''A. acutangulus'' Angelin, 1851, from the Zone of ''Solenopleura'' s.l. ''brachymetopa'' (Andrarum Limestone), Andrarum, Scania (species redescribed and assigned to new subfamily Acontheinae by Westergård, 1950, p. 9, pl.18, figs.4-6).<ref>WESTERGÅRD, A. H. 1950. Non-agnostidean trilobites of the Middle Cambrian of Sweden, II. ''Sveriges Geologiska Underso Èkning, Series C'', '''511''', 1 - 56.</ref>
==Other species currently assigned== The genus needs revision and discovery of new material may lead to reassignment of certain species.
* ''Acontheus'' cf. ''acutangulus'', from the Guzhangian (probably lower ''Lejopyge laevigata'' Zone), Northern Victoria Land, Antarctica (Jago et al. 2011). * ''Acontheus inarmatus'', from the ''Paradoxides davidis'' Zone, southeastern Newfoundland (Hutchinson, 1962).<ref>{{cite journal |last=HUTCHINSON |first=R. D. |year=1962 |title=Cambrian stratigraphy and trilobite faunas of southeastern Newfoundland |journal=Geological Survey of Canada Bulletin |volume=88 |pages=1–156}}</ref> * ''Acontheus inarmatus minutus'', from the Drumian Stage, Franconian Forest (German: Frankenwald), Germany. (Sdzuy, 2000; Geyer, 2010).<ref>{{cite journal | last=SDZUY | first=K. | title=Das Kambrium des Frankenwaldes | journal=Senckenbergiana Lethaea | publisher=Springer Science and Business Media LLC | volume=79 | issue=2 | year=2000 | issn=0037-2110 | doi=10.1007/bf03043644 | pages=301–327 | s2cid=130018874 | language=de}}</ref><ref>{{cite book |last=GEYER |first=G. |year=2010 |chapter=Cambrian and lowermost Ordovician of the Franconian Forest |pages=78–92 |editor-last1=Fatka |editor-first1=O. |editor-last2=Budil |editor-first2=P. |title=The 15th Field Conference of the Cambrian Stage Subdivision Working Group, Abstracts and Excursion Guide |publisher=Czech Geological Survey |location=Prague}}</ref> * ''Acontheus patens'', from the late middle Cambrian of Siberia (Lazarenko, 1965).<ref>LAZARENKO, N. P., 1965. Some new Middle Cambrian trilobites from north central Siberia. ''Uchenye Zapiski Paleontologiya i Biostratigrafiya'' '''7''', 14-36.</ref> * ''Acontheus'' sp. nov. (figured above), from the ''Hypagnostus parvifrons'' Biozone (Menevia Formation) <ref>{{cite book|last1=REES|first1=A. J.|last2=THOMAS|first2=A. T.|last3=LEWIS|first3=M.|first4=H. E. |last4=HUGHES |first5=P. |last5=TURNER|title=Cambrian of SW Wales: Towards a United Avalonian Stratigraphy|url=https://books.google.com/books?id=xcykBQAAQBAJ&pg=PR1|volume=42|year=2014|publisher=Geological Society of London|isbn=978-1-86239-690-6|pages=1–31}}</ref> of Porth-y-rhaw, St. David's, Wales (mentioned by Cotton, 2001, p. 170, and listed Tables 1 & 2 and Text Figs. 2 & 3; also recorded from Locs. PR-4 & PR-13 of Rees et al., 2014, pp. 26, 27). According to Westergärd (1950, p. 9) the pygidium of ''A. acutangulus'' is apparently almost as large as its cephalon, whereas ''A.'' sp. nov. is distinctly heteropygous. * ''Acontheus burkeanus'', from the ''Lejopyge laevigata'' I Zone of Queensland, Australia (Öpik, 1961).<ref>{{cite journal |last=ÖPIK |first=A. A. |year=1961 |title=The geology and palaeontology of the headwaters of the Burke River, Queensland |journal=Bureau of Mineral Resources, Geology and Geophysics, Bulletin |volume=53 |pages=1–249}}</ref> * ''Acontheus rusticus'' Repina (''in'' Repina ''et al''., 1975, p. 131, pl. 15, figs. 12–15) and ''A. verus'' Repina (''in'' Repina ''et al''. 1975, p. 132, pl. 16, figs. 1–3) <ref>REPINA, L. N., YASKOVICH, B. V., AKSARINA, N. A., PETRUNINA, Z. E., PONIKLENKO, I. A., RÜBANOV, D. A., BOLGOVA, J. V., GOLEKOV, A. N., HAJRULLINA, T. I. & POSOKHOVA, M. M. 1975. Stratigraphy and fauna of Lower Palaeozoic in the northern Submontane Belt of Turkestan and Alai Ridges (southern Tyan-Shan). ''Akademia Nauk SSSR, Sibirskoe Otdelenie'' '''276''': 1-351. (in Russian)</ref> from the Middle Cambrian of Turkestan may be more appropriately assigned to ''Corynexochella'' Suvorova, 1964, based on features of their cranidia (according to Dean and Özgül, 1994, p. 15).<ref>{{cite journal | last1=DEAN | first1=W. T. | last2=ÖZGÜL | first2=N. | title=Cambrian rocks and faunas, Hüdai area, Taurus Mountains, southwestern Turkey | journal=Bulletin de l'Institut Royal des Sciences Naturelles de Belgique, Sciences de la Terre | volume=64 | issn=0374-6291 | pages=5–20 }}</ref> * ''Acontheus limbatus'' Egorova (''in'' Egorova ''et al''., 1982) <ref>{{cite journal |last1=EGOROVA |first1=L. I. |last2=SHABANOV |first2=Y. Y. |last3=PEGEL |first3=V. E. |last4=SAVITSKY |first4=V. E. |last5=SUCHOV |first5=S. S. |last6=TCHERNYSHEVA |first6=N. E. |year=1982 |title=Maiskii yarus stratotypicheskoy mestnosti (srednii Kembrii yogo-vostoka Sibirskoy platformy) |trans-title=The Mayan stage of the type locality (Middle Cambrian of the Siberian platform) |journal=Trudy Mezhvedomstvennyi Stratigraficheskii Komitet SSSR |volume=8 |page=146 |language=ru}}</ref> is '''excluded''' and has been reallocated to ''Clavigellus'' [<nowiki/>Type species, ''C. annulus'' Geyer (1994, p. 1314, fig. 6)].
==Ontogeny== thumb|Ontogenetic stages of ''Acontheus'' from the Menevia Formation (''H. parvifrons'' Biozone) of Porth-y-rhaw, St. David's, Wales: (a) '''Protaspis'''; (b) '''Meraspis''' degree 1; (c) '''Meraspis''' degree 2; (d) & (e) '''Holaspides''' with six thoracic segments.
Growth stages of ''Acontheus'' from the Menevia Formation (''H. parvifrons'' Biozone) of Porth-y-rhaw, St. David's, are housed in the National Museum of Wales, Cardiff - collection prefix 80.34G.
==Remarks== Hutchinson (1962, p. 109) noted that the ‘cranidium’ of ''A. inarmatus'' resembles that of the much larger ''Acontheus acutangulus'' Angelin, but differs in that the glabella is more strongly expanded frontally, glabellar furrows are weaker, genal spines are lacking (unless a marginal suture had removed the posterolateral border with genal spine) and the glabella and cheeks are punctate rather than smooth. He also credibly remarked that “according to modern usage, these differences are great enough to warrant generic distinction between the two forms” but nevertheless preferred to retain them in the same genus. Opik (1982, p. 77) remarked that “''Acontheus inarmatus'' Hutchinson, 1962 from Newfoundland and ''A. patens'' Lazarenko, 1965 from East Siberia are congeneric and distinguished by rounded (not spinose) cephalic corners” and, that “at all events the absence of eyes in ''Acontheus'' justifies an independent status for its subfamily”. This therefore places ''A. patens'' in the same clan as ''A. inarmatus'', ''A. inarmatus minutus'' and ''A''. sp. nov.
The cephalon of ''Acontheus'' sp. nov. particularly resembles that of ''A. inarmatus'', but differs in having larger genae and the glabella less expanded frontally; in ''A. inarmatus'' the maximum glabellar width is approximately twice that of each gena (tr.) whereas in the Welsh form the corresponding ratio numbers between 1.5 and 1.75. Punctation of the exoskeleton is also slightly less coarse than in ''A. inarmatus'', and the occipital ring is less elevated and without a median node. The thorax and pygidium of ''A. inarmatus'' are unknown.
''A. inarmatus minutus'' Sdzuy (2000, pl.3 fig. 5; Heuse et al., p. 113. Fig. 4, illustration 16),<ref>HEUSE, T., BLUMENSTENGEL, H., ELICKI, O., GEYER, G., HANSCH, W., MALETZ, J., SARMIENTO, G. N., WEYER, D. (2010). Biostratigraphy – The faunal province of the southern margin of the Rheic Ocean. ''In'' Linneman, U. & Romer, R. L. (eds.) ''Pre-Mesozoic Geology of Saxo-Thuringia – From the Cadomian Active Margin to the Variscan Orogen.'' Schweizerbart, Stuttgart, pp. 99-170.</ref> possibly based on an immature specimen, is clearly related to both ''A. inarmatus'' and ''A''. sp. nov.; the subspecies especially resembles ''A. inarmatus'' in the marked forward expansion of its glabella and small genae, but from the illustrations there is no clear evidence of lateral glabellar furrows as seen in Hutchinson's species. ''A''. sp. nov. has larger genae and the glabella less expanded frontally than in ''minutus''.
The pygidial axis in ''Acontheus'' cf. ''acutangulus'' (Jago ''et al.'', 2011, fig. 7, N-S) is narrower (tr.) than in the Swedish species and terminates slightly short of the posterior border furrow, whereas in both ''A. acutangulus'' Angelin, 1851 and ''A.'' sp. nov., the axis actually meets the border furrow and separates the pleural fields, as also observed in ''Clavigellus annulus'' Geyer (1994, figs. 6–8).
In their ‘Revised diagnosis’ of ''Acontheus'', Jago et al. (2011, p. 29) stated that pygidial pleural furrows are “wide, deep, extend to border with marked posterior deflection where they cross border furrow. Interpleural furrows effaced”. In ''Acontheus'' sp. nov., however, it is clearly the interpleural furrows that give the pygidial border its lobed character and, without rearward deflection as observed in the type species, ''A.'' cf, ''acutangulus'', Jago et al. (''op. cit.'') and ''Clavigellus annulatus'', Geyer (''op.cit.'').
Jago ''et al''. (''op. cit''., p. 31), also speculated “it is possible that ''A. burkeanus'' (Öpik, 1961, p.135, pl. 10, figs. 1-7; text-fig. 46) is a junior synonym of ''A. acutangulus'', but better specimens of both species are required before this can be determined”. However, the assignation of “''A''”. ''bukeanus'' (Öpik, 1961, p. 135, pl. 10, figs. 1–7; text-fig. 46) from the ''Ptychagnostus cassis'' Zone [= ''S. brachymetopa'' Zone] of Queensland, is somewhat tenuous. The pygidium is of similar design to that of the type species and if compared on its own could be regarded as congeneric, differing specifically from other forms in having one more rib and axial ring. The cephalon, however, displays characteristics somewhat intermediate between ''Corynexochus'' Angelin, 1854, [= ''Karlia'' Walcott, 1889] <ref>WALCOTT, C. D. 1889. "Description of new genera and species of fossils from the Middle Cambrian." ''Proceedings of the United States National Museum.'' '''11''' (738):441–446, 1 fig.</ref> and ''Acontheus'' . The fragmentary “cranidium” on which Öpik based his reconstruction (pl. 10, fig. 1) appears to possess a straight lateral margin which he tentatively suggested to be a proparian suture that separates a narrow librigena. The specimen appears also to have an ocular ridge extending possibly to a small palpebral lobe, though Öpik did not represent eyes in his reconstruction (text-fig. 46). Perhaps these features represent a difference at the generic level that will be more apparent when better preserved material and other parts of the exoskeleton become known.
==References== {{Reflist}}
{{Taxonbar|from=Q4674500}}
Category:Corynexochidae Category:Corynexochida genera Category:Cambrian trilobites of Europe Category:Fossils of Wales Category:Fossil taxa described in 1851 Category:Cambrian genus extinctions