Favocassidulina tuberculata, a new benthic foraminiferal species from Integrated Ocean Drilling Program Site U1391 drilled on the southwest Portuguese continental slope, is described. This new species has a slit-shaped apertural opening oblique to the test periphery instead of paralleling the periphery as the type species Favocassidulina favus (Brady) does. And more notably, this species possesses a test surface ornamented with tubercles, quite different from the coarse honeycomb wall structure of F. favus and the finely irregular cancellate surface ornamentation of other Favocassidulina species. This paper for the first time reports the presence of the genus Favocassidulina in the Atlantic Ocean.
The genus Favocassidulina was originally proposed by Loeblich & Tappan (1957), based on Pulvinulina favus Brady described from a detached rock of unknown age in recent deep water sediments of the Pacific Ocean by Brady (1877). Favocassidulina is characterized by radial wall structure (Belford, 1966; Eade, 1967), biserially coiled and elongate chambers, an internal toothplate (Nomura, 1984), and an ornamented test surface (Eade, 1967). This genus is considered to belong to the family Cassidulinidae because it is closely related to the genus Cassidulina in having biserially enrolled chambers (Loeblich & Tappan, 1988). The presence or absence of an internal toothplate and surface ornamentation to the test is the major difference between these two genera.
So far, four species in this genus, Favocassidulina favus (Brady), F. australis Eade, F. subfavus Resig, and F. indica Gupta & Srinivasan, have been reported. These species appear to be limited to lower bathyal and abyssal environments of the Indo-Pacific. Favocassidulina favus characterizes the southern-sourced deep Pacific waters (>2,500 m) in the western Equatorial Pacific and the South China Sea (Burke, 1981; Jian & Wang, 1997). Favocassidulina australis occurs at water depths between 1,500 and 2,400 m off Great Barrier Island, New Zealand (Eade, 1967). Favocassidulina subfavus inhabits water depths ranging from 3,954 to 4,088 m on the Nazca Plate (Resig, 1982) and from 2,200 to 3,000 m in the area of the Ontong Java Plateau (e.g., Favocassidulina sp. A of Burke, 1981 in Resig, 1982). Favocassidulina indica was found at water depths of 1,665 m and 2,247 m in the late Oligocene to early Pliocene sediments from the Indian Ocean (Gupta & Srinivasan, 1990). Hitherto, there is no report of a Favocassidulina species in the Atlantic Ocean.
This paper describes a new Favocassidulina species from the Northeast Atlantic intermediate water off Portugal that possesses an entirely different surface ornamentation from other Favocassidulina species.
Location, Material, and Methods
Selected specimens were scanned using a SU3500 scanning electron microscope (SEM) to capture the characteristics of test surface and aperture, and using a Zeiss Xradia 520 Versa (high-resolution X-ray tomography) to observe the three-dimensional structure of specimens, especially the arrangement and the shape of chambers which were obscured by rough test surface. The scans were performed at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences.
Superfamily CASSIDULINACEA d'Orbigny, 1839
Family CASSIDULINIDAE d'Orbigny, 1839
Subfamily CASSIDULININAE d'Orbigny, 1839
Genus FavocassidulinaLoeblich & Tappan, 1957
Type species: Pulvinulina favusBrady, 1877
Favocassidulina tuberculata Guo, n. sp.
Description. Test small, lenticular, nearly symmetrically biconvex, slightly lobulated in outline, composed of six to six and a half pairs of biserially enrolled chambers in the last whole; chambers elongate, narrower near the margin, more clearly visible when the test is immersed in water; sutures slightly curved, flush with the test surface or slightly depressed; test surface, except for the area around the aperture which is smooth, covered with protuberances that tend to be bigger on the early chambers and smaller on the later chambers, thus obscuring the sutures; wall calcareous, optically radial, finely perforated; pores situated in the center of protuberances; periphery broadly rounded; aperture a short slit near and oblique to the test margin, extending upward from the base of the last chamber with a narrow lip that is turned inward at the anterior side but does not join an internal structure and with a toothplate on the posterior side.
Variation. There is little variation in the test surface ornamentation and in the location and the shape of aperture from juvenile stage to adult stage (Fig. 2).
Holotype. The specimen illustrated in Figure 3.1, deposited in Micropaleontology Section, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Register no. NIGP 180537.
Paratypes. Specimens illustrated in Figs. 2.1–2.18, 3.2–3.3, and additional specimens are deposited in the collection of the author.
Materials. 425 specimens.
Type locality. IODP Site U1391 on the southwest Portuguese continental slope; latitude 37°21.532’ N, longitude 9°24.656’ W, water depth 1,085 m, Northeast Atlantic Ocean.
Type level. Marine Isotope Stages (MIS) 15 to 12 (∼608 to ∼428 ka), Middle Pleistocene (Guo et al., 2020; Guo & Li, 2022, and this study).
Dimensions. For holotype, the length, width and thickness are 0.37 mm, 0.35 mm, and 0.22 mm, respectively. For paratypes, the length varies from 0.11 to 0.39 mm, the width from 0.09 to 0.35 mm, and the thickness from 0.06 to 0.22 mm.
Remarks. This new species is placed in the genus Favocassidulina because it matches the characteristics of this genus (radial wall structure, biserially coiled and elongate chambers, an internal toothplate, and an ornamented test surface). Test surface ornamentation is an important feature in distinguishing Favocassidulina species. In contrast to the coarse honeycomb wall structure of F. favus (Brady, 1877; Loeblich & Tappan, 1957; Nomura, 1984), and the finely irregular cancellate surface ornamentation of F. subfavus, F. australis, and F. indica (Eade, 1967; Resig, 1982; Gupta & Srinivasan, 1990), the test surface of F. tuberculata is covered with prominent protuberances that form a secondary growth of the wall separate from the smooth apertural face. Compared with other Favocassidulina species, F. tuberculata shows a smaller size with maximum diameter of no more than 0.40 mm. The holotypes of F. favus and F. australis are more than twice and that of F. subfavus is about one and a half times as large as F. tuberculata. All of these species have a similar slit-shaped aperture. Favocassidulina tuberculata resembles F. subfavus and F. indica but differs from F. favus and F. australis in the position of the aperture. The former three species possess an aperture oblique to the test periphery and the latter two species have an aperture parallel to the periphery.
The name tuberculata (Latin tuberculata tubercle) is given to this species because the surface of the whole test except around the aperture densely decorated with raised tubercles.
Ecology. This species first appeared at ∼608 ka within interstadial MIS 15 during which bottom water bathed by warm and saline MOW prevailed at Site U1391 (Guo et al., 2020). The high-resolution study of Site U1391 by Guo & Li (2022) suggested that the frequent occurrences (about 3%) of F. tuberculata (Favocassidulina sp. of Guo & Li, 2022) coincided with the presence of MOW indicator Planulina ariminensis, the dominance of Uvigerina peregrina parva and Melonis affinis in the benthic foraminiferal population, and high epibenthic δ13C values during the late MIS 13. Favocassidulina tuberculata probably preferred MOW-related high oxic and mesotrophic to slightly eutrophic bottom-water environments. Given the limited paleoecological information, I speculate that this species probably lived off Portugal at a maximum depth of 1,500 m, corresponding to the boundary between MOW and the underlying Northeast Atlantic Deep Water.
Geographic and stratigraphic distribution. No additional F. tuberculata specimens were found in the >63-μm size fraction outside the depth interval of 124.61–168.15 m at Site U1391. Thus F. tuberculata is known only in Middle Pleistocene (∼608 to ∼428 ka) sediments retrieved from IODP Site U1391 drilled in the Northeast Atlantic intermediate-depth water off the Portuguese margin. Given the number of studies of Pleistocene benthic foraminifera in the Atlantic Ocean, F. tuberculata is probably a taxon with a limited distribution. Tiny propagules passively entrained into the water column and transported by currents are the most likely mechanism of dispersal of species along the margins and across an ocean as well as between oceans (Alve & Goldstein, 2010; Gooday & Jorissen, 2012; Murray, 2013), in view of the fact that the active movement of free-living species across the seafloor is too slow to account for wide-scale dispersal (Gooday & Jorissen, 2012). Favocassidulina tuberculata might have evolved from another species’ propagules that had been transported to the Portuguese continental margin and underwent rapid evolution at Site U1391. According to Hemleben (1975), pustules on the test surface of planktonic foraminifera may be anchoring points for pseudopodia to the test and be largely controlled by environmental factors. The tubercular surface ornamentation of F. tuberculata off Portugal is quite different from the honeycomb-like and cancellate patterns of other Favocassidulina species in the Indian and Pacific Oceans, which may result from the huge differences between MOW and deep waters of the Indo-Pacific in environmental parameters, such as temperature and salinity at which these species live. Since the first appearance at ∼608 ka, F. tuberculata was discontinuously distributed with decreasing relative abundance in the Middle Pleistocene sediments at Site U1391 (Guo et al., 2020). After ∼428 ka, the end of MIS 12a, there have been no fossil records of this species (Guo & Li, 2022; this study). Its disappearance was probably the result of increased coldness and decreased nutrition of Glacial North Atlantic Intermediate Water (GNAIW) that impacted on the studied site during the glacial MIS 12 (Guo & Li, 2022), combined with its inability to produce propagules (Murray, 2013). Considering limited drilling data used in this study, future research is needed to determine its geographic and stratigraphic distribution.
Samples used in this research were provided by the Integrated Ocean Drilling Program (IODP). The author would like to thank the IODP technical staff from Expedition 339. The author would also like to thank the Associate Editor and two anonymous reviewers, who all provided constructive suggestions to improve the quality of the manuscript. This work was supported by National Natural Science Foundation of China 41806069 and the State Key Laboratory of Palaeobiology and Stratigraphy (20182104).