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INTRODUCTION
On Gran Canaria (Canary Islands, Spain; Figure 1) marine deposits at La Esfinge site (Figure 2) host a considerable accumulation of Janthina typica (Figure 3). These deposits have been recently dated (Meco et al., 2015) using 40Ar/39Ar to 4.2 Ma from samples of pillow lava that entered into contact with it. The aim of the present paper is to show the geographic distribution of J. typica in the Atlantic and Pacific oceans during the early Pliocene. The paper presents hypotheses that could explain its notable dispersal as a floating animal in warm water.
Figure 1 Location of La Esfinge site in northeastern Gran Canaria, where Janthina typica has been found.
Figure 2 Geological context at La Esfinge (Gran Canaria), (a) Section showing the fossiliferous marine deposit (2) between pillow lavas with a 4.2 Ma 40Ar/39Ar age (1) (from Meco et al, 2015) and Pleistocene volcanics (3); (b) detail oflayer 2: 2a marine sands with fossils inside and 2b aeolian sands; (c) accumulation of Janthina typica at the top oflayer (2).
Figure 3 Janthina typica (Bronn, 1861) showing a) apertural view b) apical view c) lateral view (LE2051) from the early Pliocene (4.2 Ma) at La Esfinge (Gran Canaria, Spain). Note that it has faint but visible folds; d) drawing of apertural view from early Pliocene (Opoitian) at southwest Aukland, New Zealand (Beu and Maxwell, 1990); e) apertural view (MGF3472) from the Pliocene of Japan (Tomida et al. 2013); f) apertural view (MFM112203) from Upper Pliocene of Japan (Tomida and Kitao, 2002) as Hartungia japónica (Tomida and Itoigawa, 1984).
Janthina typica: ecology and palaeodistribution
The geographic distribution of fossil specimens of the tropical gastropod Janthina typica (Bronn, 1860) (= Hartungia typica Bronn =Janthina Hartungi Mayer = Hartungia chuberti Chavan) is striking for the great distances between the sites where this taxon has been found: from the Canary Islands (Meco et al., 2015), the Azores (Bronn, 1860; Mayer, 1864), Madeira (Mayer, 1864), Selvagem Grande (Joksimowitsch, 1911) and Atlantic Morocco (Chavan, 1951) to the antipodean locations of New Zealand, Australia and Japan (Tomida and Kitao, 2002; Beu and Raine, 2009).
The current Janthinidae comprise a family of pelagic gastropods that live in colonies in warm and temperate seas, floating on a bubble-like raft of their own making, bound by mucus secreted from the organisms' feet, and they feed on hydrozoans such as Velella and Physalia (Nicklès, 1950; Laursen, 1953; Powell, 1979; Janssen, 2007; Churchill et al., 2011). This unusual adaptation enables them to travel enormous distances by drifting along with the currents, and sometimes they are cast onto beaches in significant numbers by on-shore winds (Beu and Raine, 2009).
J. typica is a fossil species from the early Pliocene (5.3 Ma to 3.6 Ma; Gradstein et al., 2004). The other localities where J. typica is found span much of the Pliocene. On Santa Maria Island in the Azores, specimens have been found in Feteirinhas and Pinheiro on the southeastern tip of the island (Reiss and Bronn, 1863). In a nearby locality, Pedra que Pica, the marine deposits have been attributed to the Messinian based on an 87Sr/86Sr age of 5.51 Ma (Kirby et al., 2007). The oldest shield of Madeira Island has yielded a 40Ar/39Ar age >4.6 Ma (Geldmacher et al., 2000) to 5.3 Ma (Klügel, 2009); therefore, the marine deposits at the Sao Vicente locality are also probably early Pliocene. In Selvagem Grande the marine deposits accumulated before the Pliocene volcanic cycle occurring at 3.4 Ma (Geldmacher et al., 2001).
The marine deposits at Aïn s eb aa near Casablanca in Morocco are referred to the Piacenzian stage (Chavan, 1951). In New Zealand, J. typica has been found in deposits assigned to the Kapitean to Waipipian stages (Beu and Raine, 2009), which are roughly contemporaneous with the uppermost Messinian-Lower Zanclian to Lower Piacenzian stages (ca. 6 Ma to ca. 3 Ma). In southeastern Australia (Victoria) an d western Australia (Perth) the deposits that yielded J. typica were included in the Kalimnan stage (Tate, 1893; Beu and Raine, 2009), which corresponds roughly to the Upper Zanclian and Lower Piacenzian (ca. 4.4 to ca. 3 Ma). The >. typica specimens found in Kyushu, South Japan, were included in the earliest Pliocene (Tsuma Formation ca. 5 Ma) and in the latest Pliocene (Takanabe Formation, 2.6-2.5 Ma; Tomida et al., 2013).
In the early Pliocene (5 Ma to 4 Ma), Earth's climate was warm in many regions and temperate in others. The gradual cooling that followed the early Pliocene led to the establishment of modern temperature patterns (Fedorov et al. 2013). Thus, the age of the Gran Canaria marine deposits with J. typica (4.2 Ma) falls within the early Pliocene warm period, but is close to the transition into the cooling period that followed.
The geological characteristics and sedimentary interpretation of the Gran Canaria marine deposits that contain a significant accumulation of J. typica shells, along with their isotopie age (4.2 Ma; Figure 2), were reported in a previous paper (Meco et al., 2015). Other marine deposits of Las Palmas de Gran Canaria are also early Pliocene but these sediments provided a 40Ar/39Ar age of 4.8 Ma (Meco et al., 2015) and no fossil specimens of J. typica have been found in them. The fossils contained indicate a coastal and littoral habitat (e.g., Patella ambroggii Lecointre) with a warm intertropical climate (e.g., Persististrombus coronatus Defrance), Nerita emiliana Mayer).
In the present paper the ecology of Janthina is discussed and the oceanic currents of ca. 4 Ma are summarized, mainly from the findings of the Ocean Drilling Program (ODP), which allow sites where fossil specimens of J. typica have been found to be connected. The dispersal of this species from the eastern Atlantic to the western Pacific (or vice-versa), must have been determined by the sea surface currents at that time, which were in turn controlled by tectonic plate movements, and pressure gradients and the effect of the Earths rotation (the Coriolis effect).
INFLUENCE OF TECTONIC PLATE MOVEMENTS
Around 4.2 Ma ago, the geographic distribution of land masses (and so of the seas) differed significantly from today. Two important changes took place around this time that were caused by the movement of tectonic plates, and resulted in the closure of inter-ocean connections. One change caused the closure of the Central American Seaway, as the South American, Caribbean and North American plates moved closer together, and the Atlantic Ocean became isolated from the Pacific Ocean (Schmidt, 2007). In the other major change, the Indonesian Seaway closed as the Australian and Eurasian plates moved closer together, bringing about a partial isolation of the Pacific Ocean from the Indian Ocean (Gallagher et al., 2009).
Closure of Central American seaway
The closure of the Central American Seaway (also called the Panamanian Seaway) can be deduced from the progressive differentiation in salinity between the Caribbean Sea and the Pacific Ocean which was taking place about 4 Ma ago (Keigwin, 1982). This change is documented in isotopie analyses of foraminifera extracted from ODP oceanic drilling samples, including ODP Site 1241 (Groeneveld et al., 2006) in the eastern equatorial Pacific Ocean, and ODP Site 999 (Steph et al., 2006) in the Caribbean Sea. Salinity was virtually the same on both sides of present day Panama 4.2 Ma ago (Sarnthein et al., 2009), but shortly thereafter began the first phase of an increase in Caribbean salinity that concluded some 3.7 Ma ago. This time interval is considered to reflect the initial stage in the closure of the Central American Seaway (Chaisson and Ravelo, 2000). Thus, 4.2 Ma ago, the seaway still existed, and the Circumtropical Current that flowed east-to-west from the Caribbean to the Pacific, was still operative (Iturralde-Vinent and MacPhee, 1999; Mestas-Nuñez, 2014).
The Pacific Ocean warm pool
A comparison of foraminifera from ODP Site 806, near the Solomon Islands, and ODP Site 847, near the Galapagos Islands, reveals important changes 4.2 Ma ago that establish the timing of salinity differentiation at the latitude of Ecuador between the eastern and western sides of the Pacific Ocean. From this point onwards an enlargement of the West Pacific Warm Pool began and an eastwards lengthening of the cold tongue was initiated. The evolution of plank-tonic foraminifera and their ecology suggest that surface water cooling was taking place between 4.5 Ma and 4.0 Ma (Chaisson and Ravelo, 2000, Li et al., 2006), which is interpreted to be a consequence of the Panamanian Seaway closure.
Indonesian Seaway closure
Foraminiferal analyses of Miocene to Holocene strata of the northwestern continental shelf off Australia have been used to chart the influence of the West Pacific Warm Pool (Gallagher et al., 2009). Between 10 Ma and 4.4 Ma ago, the collision of Australia and Asia narrowed the Indonesian Throughflow. This "S" shaped current connects the West Pacific Warm Pool and the Indian Ocean, passing from south of the Philippines through the Makassar Strait (between Borneo and Sulawesi), turning southwards through the Flores Sea and the Timor Sea, ultimately heading southwest to the Indian Ocean (Figure 4). Warmer waters became trapped in the Pacific, creating a central West Pacific Warm Pool marine biogeographic province spanning a zone from the equator to 26°N (Gallagher et al., 2009).
Figure 4 Biogeography or Janthina typica and reconstruction of the océanographie dispersal route around 4.2 Ma in accordance with the most likely hypothesis. Important localities (pink squares) are shown with numbered circles: (1) Santa Maria I. (Azores Islands), (2) Madeira I., (3) Selvagem Grande I., (4) Casablanca (Morocco), (5) Gran Canaria I. (Canary Islands), (6) Auckland (New Zealand), (7) Victoria (Australia), (8) Perth (Australia) and (9) Kyushu (Japan). ODP Sites (black circles). Dispersal route of Janthina typica (black arrows) from the East Atlantic Warm Pool (EAWP), through the Warm Canary Current (WCC), the Circumtropical Current (cc), the Central America Seaway (CAS), and the South Equatorial Current (SEC), and to the Warm East Australian Current (WEAC); or trough the Western Pacific Warm Pool (WPWP) to the Proto Kuroshio Current (PKC). Indonesian Through Flow (ITF). T: Trade winds (red arrows).
Between 4.4 Ma and 4 Ma, In do-Pacific marine taxa migrated to waters off of northwestern Australia possibly because of a limited Indonesian Throughflow, and the absence of these taxa after 4 Ma indicates the possible restriction of this current. The Leeuwin Current, which flows from south of Java to the south of Australia along its west coast, did not begin until much later, in the Early Pleistocene (Gallagher et al., 2009), and so offered no viable route for faun al migration 4.2 Ma ago. At the same time, beginning with the northward displacement of New Guinea and the emergence of the islands of Indonesia 5 Ma ago, exchange between the Pacific and Indian Oceans through the Indonesian Seaway was interrupted between 4 Ma and 3 Ma (Cane and Molnar, 2001). This had the effect of redirecting oceanic circulation from the southern Pacific warm waters northward to Japan surrounded by relatively cold waters of the northern Pacific. The presence of J. typica at ca. 5 Ma in southern Japan shows that the warm Kuroshio Currents flowed strongly in the earliest Pliocene (Tomida et al, 2013).
Southern Australia
The Pliocene climatic and environmental evolution of southeastern Australia (Bass Strait) indicates that relatively stable and warmer marine conditions than the present day prevailed throughout most of the early Pliocene (Gallagher et al., 2003). This corresponds to a period of low marine δ18O values (corresponding to warmer waters and/or lower ice volume) from 4.2 Ma to 4.0 Ma (Shackleton et al., 1995). This was also a time of relatively low ice volume in the Antarctic (Bart 2001; Gallagher et al., 2003).
POSSIBLE HYPHOTHESES
There are no significant age differences among the sites where J. typica has been collected in the Pacific and Atlantic. This suggests that two opposing hypotheses may explain the path of its biogeographic dispersal: (a) migration from the eastern Atlantic Islands to the Australian coasts; or (b) migration from the western Australian coast to the eastern Atlantic islands.
Hypothesis 1: dispersal of J. typica from the eastern Atlantic Islands to the western Australian coast
At present, the Azores, Madeira, Salvagen and Canary Islands are situated within the North Atlantic subtropical gyre. Around 4.2 Ma, the waters were warmer and this gyre would have been weaker (Meco et al., 2015). Nevertheless, the trade winds and associated currents likely existed, and would have allowed J. typica to disperse into the Caribbean Sea, and cross the Central American Seaway to the equatorial Pacific. Once it reached the equatorial Pacific Ocean, the trade winds of the intertropical zone could have carried J. typica westward into the southern hemisphere toward Indonesia. Because the Indonesian Troughflow was closed at the time, once the Equatorial Current reached the western Pacific, separate currents may have formed, one moving northward in the direction of Japan, and another southward to Australia and New Zealand (Figure 4).
Hypothesis 2: dispersal of J. typica from the western Australian coast to the eastern Atlantic islands
The reverse route, from the western Pacific to the eastern Atlantic, would have posed significant difficulties. Two possible paths could be invoked: (1) following the oceanic gyres through the Indian Ocean into the South Atlantic, and from there into the North Atlantic. This would have endangered the survival of J. typica, a warm water species, as it would have required passage through long sections of cold waters in both the northern and southern hemispheres; or (2) following the Equatorial Counter Currents eastward across the Pacific, though this would have meant overcoming the obstacle of the "doldrums", the area of low pressure around the equator where prevailing winds are calm. It should be noted that the Pacific Equatorial Countercurrent originated in the west and advanced eastward as modern climate conditions began to set in (Li et al., 2006). Some 4.2 Ma ago, however, the countercurrent was shorter and the oceanic gyres were only in the formative stage of their modern configurations (Li et al., 2006). The Pacific currents were no doubt different from those of today and in some areas may not have even existed. It would have been difficult for J. typica to migrate from the western Australian coast to the eastern Atlantic Ocean ca. 4.2 Ma ago.
Other older species of Janthina are very similar morphologically to the Canary I. J. typica (4.2 Ma), but designated Hartungia instead in the literature (for a taxonomie study, see Beu and Raine, 2009). Hartungia elegans occurred along the Pacific coast of southern and central Japan during the Late Miocene (planktonic foraminiferal Zone N. 17, at ca. 68 Ma; see Tomida and Nakamura, 2001) and Hartungia pehuensis occurs at Taranaki in northwestern New Zealand also during the Late Miocene (Marwick, 1926). The Late Miocene oceanic circulation pattern (Iturralde-Vinent and MacPhee, 1999, fig. 10; Li et al., 2006, flg. 8) may have set the precedent for the early Pliocene dispersal route of J. typica from the Atlantic Ocean to the Pacific Ocean.
CONCLUSIONS
The principal explanation for the extraordinary geographic dispersal of J. typica are a combination of factors of different scale: ecological, geological and astronomical. Its ecology as a floating animal in warm waters and capable of long-distance transport in a short period of time (for a modern analog, see Bryan et al., 2004) and the tectonic plate configuration that permitted an open Central American Seaway and also brought about a restricted Indonesian Seaway. The ancestor of J. typica is unknown. The great similarity between the Pacific and Atlantic forms (Figure 3) suggests a rapid dispersal rather than a geographic speciation. Accordingly, this species must have originated in the western Atlantic during the Late Miocene or earliest Pliocene and experienced a remarkable dispersal, crossing into the Pacific during the early Pliocene before reaching the vicinity of present-day Perth in Western Australia, as well as southern Japan. The presence of J. typica in these diverse localities indicates that warm waters existed throughout the dispersal route, and that the Canary Current was at that time warmer. The Circumtropical Current must have extended to the western Pacific, while the cold Humboldt Current in the area of the Galapagos Islands must have been weak or absent. On reaching Panama, the Equatorial Countercurrent must have been deflected southward. This would allow connection with the Pacific South Equatorial Current, and end in a bifurcation in Indonesia with one branch heading north (Kuroshio Current) and the other south (East Australian Current). This latter current would extend through the Bass Strait and along the southern coast of Australia as far as modern-day Perth, which would also require some distancing of the West Wind Drift.
