Heterosphaeroma priscum (Milne-Edwards, 1868)

Robin, Ninon & Vonk, Ronald, 2020, Marine-like isopod Heterosphaeroma priscum from a Late Paleocene freshwater system in Sézanne, France revisited, Acta Palaeontologica Polonica 65 (4), pp. 793-798 : 794-796

publication ID	https://doi.org/ 10.4202/app.00767.2020
persistent identifier	https://treatment.plazi.org/id/115C879F-FFF6-1F5F-7A24-FA8BFA8BFBC6
treatment provided by	Felipe
scientific name	Heterosphaeroma priscum (Milne-Edwards, 1868)
status

Treatment

Heterosphaeroma priscum (Milne-Edwards, 1868) View in CoL

Figs. 1–3 View Fig View Fig View Fig .

1868 Sphaeroma priscum ; Milne-Edwards 1868: 426, 2 figures.

1872 Heterosphaeroma priscum View in CoL ; Munier-Chalmas 1872: 166.

1928 Heterosphaeroma priscum View in CoL ; Van Straelen 1928b: 40.

1969 Heterosphaeroma priscum View in CoL ; Moore and Brooks 1969: 377.

1977 Heterosphaeroma priscum View in CoL ; Secrétan 1975: 319.

2014 Heterosphaeroma priscum View in CoL ; Giusberti et al. 2014: 81.

Type material: Holotype: SU.PAL.2017.0.2.61 (Munier-Chalmas old collection), outer mold ( Fig. 1A View Fig ). Despite the existence of several known specimens as early as 1868, it is clear that Alphonse Milne-Edwards had only seen a single specimen when he provided the species description. He writes: “The false abdominal legs of the last pair are lacking in the exemplar I could examine” (Milne-Edwards 1868: 427) leaving no doubt in that matter. Consequently, SU.PAL.2017.0.2.61 is the holotype, which originally consisted of both the outer and inner molds, the latter being now lost. The fossils in Sézanne are preserved as voids in the travertine. Munier-Chalmas recovered these fossils by pouring gypsum/stearine solution and subsequently dissolved the calcareous rock in acid leaving moulds for investigations. Consequently, the specimens SU.PAL.2017.0.0.4 and SU.PAL.2017.0.2.4, exist only in the form of casts and the original specimen has been dissolved.

Type locality: Sézanne , Marne department, Grand-Est region, France .

Type horizon: Upper Thanetian , Palaeocene, Cenozoic .

Material.— MNHN.F.B71849, plaster cast of lost SU.PAL.2017. 0.2.61 (inner mold, Munier-Chalmas old collection) made by André Tournouër in 1882; SU.PAL.2017.0.2.4–6, 0.2.11, 0.2.12, 0.3.9, 0.2.3.1, 0.2.3.2-1, 0.2.3.2-2 (Munier-Chalmas old collection).

Emended diagnosis.—Body oblong to ovoid in dorsal view, length 17 mm, width 10 mm, greatest width at pereonites 4 and 5; cephalon separate from pereonite 1, semi-enclosed by it laterally; pleonites narrow, four in number with first one mostly covered by seventh pereonite, fourth pleonite broadened mid-dorsally, covering or fusing with others at mid-dorsal part; pleotelson regularly rounded with an inflated mid-dorsal section, upper margin with S-form hinge locks on both sides making strong connection to fourth pleonite; uropods with broad and flat, not clearly surpassing in length lower rim of pleotelson, exopod on robust peduncle, much smaller than endopod with apparently smooth outer margin.

Description.—Cephalon semi-ovate and surrounded laterally by the first pereonite. Ophthalmic ridge visible. Eyes with lower half extended posteriorly to the anterior margin of first pereonite ( Figs. 1C View Fig , 2B View Fig ). Antennule with robust peduncle segments and much thinner flagellum of which separate segments cannot be seen, end covered by matrix. Antenna with peduncle partly visible, no separations between segments visible. Buccal mass ( Fig. 1C View Fig ) protruding out strongly from well-defined perimeter at the underside of cephalon. Frontal lamina not fused with clypeus but coming forward from mid-frontal margin of cephalon ( Fig. 4A View Fig ). Clypeus triangular with clear lateral lobes pointing downward and semi-enclosing labrum. Labrum semi-triangular and prominent, frontal margin slightly concave and downward part tapering toward blunt triangular point with rounded posterior margin. Mandible visible, showing palp on left side which is directed forward with geniculate last segment. Maxilla 1 (maxillule) ( Fig. 1C View Fig ), 2 (maxilla), and maxilliped of normal sphaeromatid form insofar their outlines can be seen. Basis of maxilliped L-shaped (with retinacula that lock opposing plate and visible endopodite). Pereopod 1 (second thoracopod) ( Fig. 1C View Fig ) with sharp triangular coxal plate and clear suture where plate connects to thoracic segment. Basis slender, broken off at ischium, rest of the leg not preserved. Pereopod 2 Figs. 1C View Fig , 2A View Fig ) with half of basis preserved. Coxa suture not as prominent as in pereopod 1. Pereopods 3 to 6 ( Figs. 2A View Fig , 3A View Fig ) slender, preserved partly on both sides, no protruding lobes of separate segments present, pereopod 7 without basal segments and with only long dactylus and claw present. Pleonites ( Figs. 1A, B View Fig , 2C View Fig ), four in number interrupted in middle on dorsal side by enlargement of fourth pleonite. Fourth pleonite has on both sides a hinging and locking process connected with pleotelson. Pleotelson ( Fig. 1A, B View Fig ) robust and bulbous in middle with one smooth surfaced bulge protruding mid-dorsally. Apex rounded without indentations, tunnel-shaped in fossil imprint. Uropods prominent ( Fig. 1A View Fig ), endopod broad and flat, exopod much smaller, not well preserved, condition of outer margin not discernible.

Remarks.—The fourth pleonite extension overlapping the other three visible pleonites dorsally ( Figs. 1A, B View Fig , 2C View Fig ) seems a unique character not reported from other fossil Sphaeromatoidea . There are extant species with four pleonites visible dorsally that also have the sutures not completely spanning the entire mid-dorsal region such as Exosphaeroma waitemata Bruce, 2005 and species in the genus Gnorimosphaeroma Menzies, 1954 . However, in extant species five pleopods are always present on the ventral side and no overlapping structure extending from the fourth pleonite is completely covering the others. Species of the enigmatic genus Parvireia Chilton, 1925 , provisionally attributed to the Sphaeromatidae ( Brökeland et al. 2001) , also show four pleonites dorsally but have five functional pleopods. Comparisons to other, extant isopods living in travertine depositing conditions do not feature similar pleonite configurations. The presence of a medial tube-like folding (= tunnel) ( Fig. 1A, B View Fig ) on the posterior margin of the pleotelson is typical for sphaeromatoideans ( Jones et al. 2014).