The first Oligocene sea turtle (Pan-Cheloniidae) record of South America

The evolution and occurrence of fossil sea turtles at the Pacific margin of South America is poorly known and restricted to Neogene (Miocene/Pliocene) findings from the Pisco Formation, Peru. Here we report and describe the first record of Oligocene (late Oligocene, ∼24 Ma) Pan-Cheloniidae sea turtle remains of South America. The fossil material corresponds to a single, isolated and well-preserved costal bone found at the Montañita/Olón locality, Santa Elena Province, Ecuador. Comparisons with other Oligocene and extant representatives allow us to confirm that belongs to a sea turtle characterized by: lack of lateral ossification, allowing the dorsal exposure of the distal end of ribs; dorsal surface of bone sculptured, changing from dense vermiculation at the vertebral scute region to anastomosing pattern of grooves at the most lateral portion of the costal. This fossil finding shows the high potential that the Ecuadorian Oligocene outcrops have in order to explore the evolution and paleobiogeography distribution of sea turtles by the time that the Pacific and the Atlantic oceans were connected via the Panama basin.

Recently, a late Oligocene fossil site at the Pacific coast of Ecuador, Motañita-Olón locality ( Fig. 1) has shown being rich in marine vertebrates, including a new genus and species of dolphin Urkudelphis chawpipacha (Tanaka et al., 2017), abundant sharks and actinopterygian fish teeth (J Carrillo-Briceño et al., 2018, unpublished data) and isolated turtle remains. Here we describe an isolated costal bone belonging to a sea turtle from this site, which constitute the first record of Oligocene Pan-Cheloniidae sea turtles in South America. In addition, we discuss the importance of this fossil site for understanding the evolution and paleobiogeography of sea turtles in the American continent.

Locality and age
Montañita/Olón locality, between the towns of Montañita and Olón, Santa Elena Province, Ecuador (1 • 48 50.64 S, −80 • 45 24.18 W). Here, we provisionally identify the source horizon for UPSE-T0036 as the Zapotal Member of the Dos Bocas Formation following Whittaker (1988). However, the age of this horizon is well constrained based on the occurrence of fossil shark Carcharocles angustidens, indicating that is it late Oligocene in age (Bristow, 1975;Tanaka et al., 2017).
Description UPSE-T0036 corresponds to a right costal 4 (14.5 cm length, 3.8 cm width as preserved) (Figs. 2A-2B). We use a specimen of the extant Eretmochelys imbricata MTKD D 8295 to indicate the anatomical position of UPSE-T0036 in a complete turtle carapace (Fig. 2C). UPSE-T0036 is a rectangular costal bone with almost the same medial and lateral regions width, lacking of fully ossified lateral region, which allows the exposure of the distal end of the costal rib. On its dorsal surface the bone exhibits a sculpturing pattern that varies along its width, being of dense vermiculation at the vertebral scute region (medial portion of the costal) ( Fig. 2D), changing to anastomosing to almost parallel pattern of grooves at its lateral portion (Fig. 2E). The sulci between pleural and vertebral scutes are well defined, indicating that the vertebral scute covered 1/3 of the total surface of the bone, ending laterally in an acute tip. The sulcus between pleurals separates the bone in two almost equal portions. On its ventral surface (Figs. 2F-2G) the outline of the costal rib is defined along the length of the bone, showing a well-developed rib-head of the costal for the attachment with the thoracic vertebra.

Taxonomical attribution and comparisons
UPSE-T0036 costal bone is attributed as belonging to Pan-Cheloniidae by sharing with some of the fossil and extant representatives of this clade the following characteristics: lack of lateral ossification but keeping a considerable thickness (5-7 mm), allowing the dorsal exposure of the distal end of ribs; dorsal surface of bone sculptured, changing from dense vermiculation at the vertebral scute region to anastomosing pattern of grooves at the most lateral portion of the costal. Lateral reduction in ossification of costals allowing  the exposure of costal ribs occur also in some other turtles as for example Chelydridae (snapping and alligator turtles), however in these turtles the bone thickness is extremely reduced and the dorsal surface is smooth and developing ridges or knobs. Other groups of turtles that also exhibit reduction in lateral ossification of costals is the Tryionichidae (soft-shelled turtles), but in contrast to chelydrids and pan-cheloniids they develop a very distinct pitted dorsal bone sculpturing and absence of sulci from keratinous scutes. Among pan-cheloniids UPSE-T0036 resembles the sculpturing pattern of other Cenozoic fossil forms from North and South America, as for example Ashleychelys palmeri Weems & Sanders (2014) from Charleston, South Carolina, USA, and the Miocene Pacifichelys urbinai Parham & Pyenson (2010) from Peru. However, it differs from the first one in having a narrower covering of the costal by the vertebral scute (as indicated by the sulcus). Unfortunately, the posterior region of the carapace is unknown for P. urbinai, avoiding to establishing if sculpturing pattern and scutes arrangement was similar as in UPSE-T0036. Other Oligocene sea turtles from South Carolina: Procolpochelys charlestonensis Weems & Sanders (2014) and Carolinochelys wilsoni Hay (1923) differ from UPSE-T0036 by having faintly sculptured to almost smooth dorsal carapacial bones. Table 1 shows the comparisons between UPSE-T0036 and Cenozoic taxa from American continent.
UPSE-T0036 resembles in geometry, sulci and medial to lateral sculpturing pattern variation of the posterior costals of some extant sea turtles, as for example Lepidochelys olivacea YT-Ver-0002 (Figs. 2H-2M), differing from this particular specimen by a wider covering of the vertebral scute on the costal surface. The width of vertebral scutes exhibit intraspecific variation as we observed in specimens of Caretta caretta NMW 31531 and 1858; and Eretmochelys imbricata NMW 1853 and MTKD D 8295 (Fig. 2C), for example in this last specimen the posterior vertebral scutes almost reach the most lateral portions of costal bones.

Importance of Montañita-Olón locality for South American sea turtle evolution understanding
The marine fossil vertebrates (cetaceans, sharks and turtles) recently discovered and described from the Oligocene, Montañita/Olón locality of Ecuador (Tanaka et al., 2017, J Carrillo-Briceño et al., 2018 represent the first occurrences of each of these groups in Paleogene (Oligocene) sequences of tropical South America; and for the particular case of turtles, the first Oligocene record of Pan-Cheloniidae marine turtles for the whole South America. Even though the material described herein corresponds to a single and isolated bone-reason why we avoid to formulate any further systematic or phylogenetic affinity hypotheses; it sets up a very promising scenario for future exploration and finding of new and more complete specimens that could elucidate if for instance the already known Oligocene sea turtle taxa from North America (Weems & Sanders, 2014;Weems & Sanders, 2017) inhabited also the tropical Pacific coast of South America; a hypothesis that it seems to be possible considering that during the Oligocene, the Pacific and the Atlantic oceans were connected via the Panama basin (Pindell, 1994;Boschman et al., 2014;Jaramillo, 2018) (Fig. 3A).
Thus, more complete sea turtle specimens from Montañita/Olón could shed light in establishing relationships with younger marine taxa from South America, as for example with the Miocene Pacifichelys urbinai Parham & Pyenson (2010) from Peru (Fig. 3B), or potentially being direct ancestors of any of the five extant representatives that inhabit the Pacific coast of tropical South America (Turtle Taxonomy Working Group et al., 2017) (Fig. 3C).
The fossil sea turtle material from Montañita/Olón also increases the knowledge on the fossil turtle paleobiodiversity of Ecuador, being the first record of a marine fossil turtle in the country and an addition to the already known occurrences of Pleistocene freshwater and terrestrial fossil turtles from Santa Elena Province (Cadena, Abella & Gregori, 2017).

CONCLUSIONS
The costal bone descrided herein is the first undisputable record of Oligocene (late Oligocene, ∼24 Ma) marine turtles Pan-Cheloniidae of South America. This fossil finding shows the high potential that the Ecuadorian Oligocene outcrops have in order to explore the evolution and paleobiogeography distribution of sea turtles by the time that the Pacific and the Atlantic oceans were connected via the Panama basin. More complete specimens will have to be found in the Montañita/Olón in order to establish in detail the taxonomy and phylogenetic relationships of the Oligocene sea turtles that inhabited this part of South America. We hope this finding will encourage more paleontological expeditions and support for this type of studies in Ecuador and northern South America.

MTKD
Senckenberg Museum of Natural History, Dresden collections, Germany.

NMW
Natural History Museum of Vienna, Austria. UPSE paleontological collection, Universidad Estatal de la Peninsula de Santa Elena La Libertad, Santa Elena Province, Ecuador.

ACKNOWLEDGEMENTS
Thanks to K Guerrero for assistance in the logistics during the visit of the first author (E Cadena) to Santa Elena Province. JA thanks the Marine Biology students and staff from UPSE, who actively participated in the fieldwork, and the ''Comuna de Montañita'' and ''Comuna de Olón'' for their support. We thank the Instituto Nacional de Patrimonio Cultural (INPC) for the prospection and excavation permits. We thank the three reviewers (J Parham, I Danilov, and R Hirayama) for their comments and suggestions to improve this manuscript.

ADDITIONAL INFORMATION AND DECLARATIONS Funding
Funding for this work was provided by the Alexander von Humboldt Foundation of Germany under a return fellowship given to the first author and funds from the School of Geological Science and Engineering of Yachay Tech. ''Proyecto Prometeo'' of the ''Secretaria de Educación Superior, Ciencia Tecnología e Innovación'', Republic of Ecuador.