Journal of Paleontology, 91(3), 2017, p. 407–416 Copyright © 2017, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2016.83
New azooxanthellate genus of Scleractinia (Flabellidae) from the Australian Cenozoic
Stephen D. Cairns Department of Invertebrate Zoology, NationalMuseum of Natural History, Smithsonian Institution,Washington, DC 20560, USA ⟨
cairnss@si.edu⟩
Abstract.—A new genus of flabellid scleractinian coral, Periplacotrochus, is described from the late Eocene to middle Miocene of Victoria, South Australia, and Tasmania. It differs from Placotrochus in having a pedicellate base, and from Flabellum in having a lamellar columella. Six previously described species are redescribed, illustrated, and reassigned to this genus (P. deltoideus Duncan, 1864, P. corniculatus Dennant, 1899, P. elongatus Duncan, 1864, P. pueblensis Dennant, 1903,
P.inflectus Dennant, 1903, and P. magnus Dennant, 1904), and one new species is described: P. cudmorei. One species, Placotrochus elegans Tennison-Woods, 1878a, is considered as a nomen dubium. A key is provided to the seven species placed in this genus.
Introduction
Duncan (1864) noted that there were ‘paired’ flabellid genera, or maybe only groups of species, differing from one another by only one character, such as the presence or absence of a lamellar columella. He called these ‘mimetic’ genera, or those that mimic one another, except for one character. He also noted that groups of flabellid species might also differ from another by having a pedicellate (attached) base or having transverse division, being otherwise similar. He categorized 10 flabellid species into four permutations or categories (Fig. 1): (1) pedicellate and lacking a columella (typical Flabellum Lesson, 1831, Fig. 1, Category 1), (2) pedicellate but with a lamellar columella (unnamed higher taxon, herein Periplacotrochus, Fig. 1, Category 2), (3) trans- verse division and lacking a columella (future Truncatoflabellum Cairns, 1989b, Fig. 1,Category 3), and (4) transverse division but with a lamellar columella (typical Placotrochus Milne Edwards and Haime, 1848, Fig. 1, Category 4). Species in the third cate- gory mentioned in the preceding are quite common, represented by at least 38 species, occurring in water depths of 2–3,010m around the world, as well as in the fossil record as early as the middle Eocene (Cairns, 2016). This group was frequently discussed (see Cairns, 1989b) but not named until 1989, when Cairns (1989a, b) suggested that transverse division conferred an evolutionary advantage to this genus. He called the genus Truncatoflabellum Cairns, 1989b. A similar situation occurs between Category 4 (Placotrochus) and Category two; however, because species belonging to Category two occur exclusively as fossils in southern Australia and are collected, this species group has never been named. Cairns (1989b, p. 75) had implied the need for such a name, calling this group of species the ‘Placotrochus’ deltoideus species complex (Cairns, 1989b, table 4). The purpose of this paper is to name this genus and redescribe the species contained in it.
According to Duncan (1864), the genus Flabellum could be
considered mimetic to Truncatoflabellum, differing only in lacking transverse division. In the same way, Periplacotrochus could be considered mimetic to Flabellum, differing only in having a lamellar columella. Likewise, Truncatoflabellum could be considered mimetic to Placotrochus, differing only in lacking a lamellar columella, and Placotrochus could be considered mimetic to Periplacotrochus, differing only in having transverse division (Fig. 1). Whatever way interpreted, Flabellum is the oldest genus (Late Cretaceous) and could have been the ancestor of Periplacotrochus (earliest known occurrence lower Eocene) and Truncatoflabellum (earliest known occurrence middle Eocene). Placotrochus, known only from the Recent, could have evolved from either Periplacotrochus or Truncatoflabellum by one character state change. Because of the solitary nature of the corallum and the
placement of the genus in the family Flabellidae, species of Periplacotrochus are assumed to have been azooxanthellate. Because of its similarity to Placotrochus and Truncatoflabellum and other associated south Australian Cenozoic corals, especially those found at Muddy Creek and Balcombe’s Bay, Victoria, the species of Periplacotrochus are assumed to have lived in the middle to lower shelf depths, i.e., 50–200 m.
Materials and methods
The terminology used in the descriptions is defined and illu- strated in Cairns and Kitahara (2012). Abbreviations used in the text include the following: Cx, Sx = costae or septa, respec- tively, of cycle x; GCD = greater calicular diameter; GCD: LCD = ratio of greater calicular diameter to lesser calicular diameter of a corallum; H:GCD = ratio of corallum height to its greater calicular diameter; Sx>Sy = in the context of a septal formula, septa of cycle x are wider than those of cycle y.
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