Heliodiscidae Haeckel, 1881

Dumitrica, Paulian, 2019, Cenozoic Spumellarian Radiolaria With Eccentric Microsphere, Acta Palaeontologica Romaniae 15 (1), pp. 39-60 : 51-52

Family Heliodiscidae Haeckel, 1881 , emend. Dumitrică, 1984

Type genus. Heliodiscus Haeckel, 1862 .

Diagnosis. Test consisting of a double medullary shell with eccentric microsphere and a cortical shell that can be lenticular, spherical, discoid and simple or multiple, and connected to medullary shell by numerous beams directed to the two faces.

Remarks. Hollande & Enjumet (1960) seem to be the first researchers who remarked the eccentric position of the microsphere in the outer medullary shell. In the Addendum of their monograph they described two new taxa: Excentrodiscus echinatus , a new genus and species with a spherical cortical shell and a double medullary shell with an eccentric microsphere, and Astrophacus viminalis , a new species with external characters of Astrophacus Haeckel , but with eccentric instead of concentric microsphere. Due to this similarity they questioned the accuracy of Haeckel’s observation concerning the position of the microsphere in this genus.

Regarding Heliodiscus phacodiscus Haeckel , the type species of the type genus of the family, we have no information concerning the position of its microsphere within the outer medullary shell because the central part of the type specimen illustrated by Haeckel (1862, pl. 17, figs. 5-7) is obscured, probably due to the thickness of the cortical shell. In the specimen of the same species illustrated by Tan & Chen (1999, fig. 5-118), as well as in other Quaternary species of this genus illustrated with drawings by them, the microsphere is also invisible. However, given its Quaternary age and the eccentric position of the microsphere in other Quaternary species of the genus Heliodiscus , we can rather confidently ascertain that it is also eccentric in the type species because the Phacodiscidae and Coccodiscidae were already extinct before the Quaternary and even the Pliocene. My assertion could be considered groundless because Haeckel (1862, pl. 28, figs. 11, 12), in his monograph on the living Radiolaria from Messina, illustrated the first coccodiscid ( Coccodiscus darwinii Haeckel ) and even described in detail the structure of its soft body. He wrote ( Haeckel, 1862, p. 485-486) that he found the only specimen in the water from Messina and he preserved it in liqueur to study its soft body later. Knowing generally from my practice the occurrence of the Coccodiscidae and Phacodiscidae I was always intrigued by the presence of this genus and species in the living plankton and I was feeling that something is not in order with it. In my opinion, it had no place among the living plankton from Messina. I thought that Haeckel either cheated or made a mistake when he included this species in his monograph. But how could he describe in detail the soft body of this species? Imagination? I raised this problem last year during the last meeting of our group of co-authors of the Project of Catalogue of Neozoic Radiolaria, which included Jean-Pierre Caulet, Luis O’Dogherty, Noritoshi Suzuki and myself. I expressed my strong doubt about the presence of the species Coccodiscus darwinii in the living plankton illustrated in Haeckel monograph. I expressed my feeling that Haeckel knew the Eocene fauna from Barbados at the time when he wrote this monograph and he included this species to have one more genus in it. Noritoshi Suzuki who had worked in the “Joint Haeckel and Ehrenberg Project (2009) didn’t agree with me arguing that at that time Haeckel did not had samples from Barbados. However, looking in the collection of photos he had made from Haeckel’s house and collection in Jena, he fortunately found and showed us a photo of an individual slide with Haeckel’s hand writing on which it was written: “ Coccodiscus Darwinii Haeckel, 1862 , Barbados ”. It was the holotype of this species. This was exactly the proof I had looked for long time, the proof that Coccodiscus darwinii was not a living species but an Eocene one as most of the coccodiscids.

A somehow similar problem could raise Heliodiscus asteriscus Haeckel. Tan & Chen (1999, p. 208 , fig. 5-121) illustrated with a drawing a specimen of this species in whose centre of the outer medullary shell they drew a darker spot suggesting probably that the microsphere is centrally disposed. If so, this drawing is certainly wrong. A photo of the same species published in Dumitrică (1978, pl. 2, fig. 4) shows clearly an eccentric microsphere represented by an eccentric dark spot. To be completely sure of the position of the microsphere I sectioned a specimen of this species according to the method I described in De Wever et al. (2001, p. 436-439). The specimen had been found in the late middle Miocene sample PROA 96P in which the microsphere was very difficult to see due to the thick cortical shell, but it became rather well visible in eccentric position after scratching out a part of this shell. Nigrini (1967, pl. 3, figs. 1a, 1b) and Kawakami (2001, pl. 1, fig. 13) illustrated also specimens of this species with well visible eccentric microsphere. This suggests that the two species ( H. phacodiscus and H. asteriscus ) are congeneric and that both belong to the genus Heliodiscus .

According to the emended definition ( Dumitrică, 1984; De Wever et al., 2001) the main characteristic of this family is the presence of a double medullary shell with eccentric microsphere and of a latticed cortical shell that has various geometrical shapes: (lens, disc or sphere) and consists of a variable number of layers.

The fossil record proves that the spherical heliodiscids are the oldest members of the family. They are known from the late Santonian-early Campanian from western Kamchatka ( Vishnevskaya & Dumitrică, 2003; Vishnevskaya, 2006). The lenticular and disc-shaped heliodiscids ( Heliodiscus and Excentrococcus ) are known since the Miocene and they have probably their origin in horizontal gene transfers between Excentrodiscus and a phacodiscid species probably during the late Paleogene or early Miocene when the lens-shaped phacodiscids were still frequent.

Range and occurrence. The oldest representatives of the family are known in late Santonian - early Campanian from western Kamchatka Peninsula ( Vishnevskaya & Dumitrica, 2003; Vishnevskaya & Bernard, 2004; Vishnevskaya, 2006). At this stratigraphic level the family is represented by spherical species with 3 or 4 shells belonging to the genera Excentrodiscus and probably Excentrosphaerella . Excentrosphaerella seems also to occur in the early Paleocene, represented probably by Haliomma teuria Hollis (1997) . Until present no other species of this genus is known in the Cenozoic except the type species occurring in the late middle Miocene of Romania. In exchange, the genus Excentrodiscus is rather well represented by several species in the Eocene and Oligocene.