Sphyrna mokarran (Ruppell, 1837)

R D E Macphee, M A Iturralde-Vinent & Eugene S Gaffney, 2003, Domo de Zaza, an Early Miocene Vertebrate Locality in South-Central Cuba, with Notes on the Tectonic Evolution of Puerto Rico and the Mona Passage, American Museum Novitates 3394, pp. 1-43 : 32-37

Sphyrna mokarran (MNHNCu–P 3032).

Batomorphii: Myliobatidae

Myliobatis sp. (MNHNCu– P 3102 , fragmentary tooth with eroded occlusal surface, collected by M. Iturralde­Vinent, R. Rojas, and R. MacPhee, February 1994; MNHNCu– P 3120 , collected by R. MacPhee and M. Itrurralde­Vinent, June 1996).

Aetobatus arcuatus ( MNHNCu – P 3119 , collected by M. Iturralde­Vinent, R. Rojas, and R. MacPhee, February 1994; MNHNCu – P 3121 and 3122, collected at Zaza Dug­Out by M. Iturralde­Vinent, R. MacPhee, C. Flemming, and S. Díaz, June 1996) .

NOTES ON THE TECTONIC EVOLUTION OF PUERTO RICO AND MONA PASSAGE

The GAARlandia hypothesis (Iturralde­ Vinent and MacPhee, 1999; MacPhee and Iturralde­Vinent, 2000) proposes that, at approximately the time of the Eocene–Oligocene transition (~35–32 Ma), general uplift of the blocks comprising the northern Great­ er Antilles and the spine of the Aves Rise created a structure, the Greater Antilles–Aves Rise (or ‘‘GAARlandia’’) landspan that was briefly (±1 Ma?) connected to northwestern South America. Shortly thereafter, tectonic movements began to subdivide this structure in various complex ways. Biogeographically, the single most important implication of the GAARlandia hypothesis is that certain South American mammals, and possibly other terrestrial animals, may have been able to disperse up the landspan into the future Greater Antilles before subsidence and tectonic dismemberment occurred. The central importance of the idea of GAARlandia is that it provides a common­cause explanation for many aspects of faunal formation in the Greater Antilles. Naturally, the explanatory power of this hypothesis is dependent on whether these events have been correctly chronicled, and it is therefore important to test it against the results of other investigations, whether biological or geological, whenever opportunity offers.

Two recent studies (van Gestel et al., 1998, 1999) offer interpretations of the tectonic evolution of Puerto Rico and the Mona Passage that contradict critical aspects of the GAARlandia hypothesis. The relevant issues may be conveniently bundled into two relat­ ed series of topics: (1) The Mona Passage, which separates eastern Hispaniola and the Puerto Rico / Virgin Islands block, originated in the Early Oligocene. If this is correct, then subdivision of GAARlandia began almost immediately after the landspan’s emergence. (2) The Puerto Rico / Virgin Islands block (PRVIB) was completely submerged between the Late Oligocene and the Early Pliocene. If this is correct, no terrestrial biotic elements in Puerto Rico or the Virgin Islands could be older than 5.2 Ma (start of the Early Pliocene).

TIME OF ORIGIN OF THE MONA PASSAGE

On the basis of information available at the time, we ( Iturralde­Vinent and MacPhee, 1999) inferred that the Mona Passage first appeared as a marine basin in the Middle Miocene (~11–16 Ma). Our grounds for this inference were that (1) on both the Hispani­ olan and Puerto Rican sides of the Mona Passage, deformed Cretaceous/Paleogene arc sections outcrop below Quaternary limestones, suggesting that there had been a barrier to marine deposition (i.e., a land connection) until late in the Cenozoic; and (2) regional tectonic reconstructions implied that the Passage was of late or neotectonic origin (more specifically, that the Passage was formed as a consequence of Miocene–Pliocene extensional stress due to rotation of Puerto Rico [Larue and Ryan, 1998]). We also noted, however, that ‘‘additional data from the floor of the Mona Passage would help to clarify the history of this connection’’ ( Iturralde­Vinent and MacPhee, 1999: 35).

Van Gestel et al. (1998) have developed a different interpretation of the probable time of opening of the Mona Passage, based on their interpretation of a variety of newly collected as well as previously archived offshore seismic reflection profiles. Two profiles, based on lines shot along different tracks through the Passage (their figures 14 and 15), do indeed show a rather thick section of sedimentary rocks, the basal beds of which thin toward the latitude of Mona Island. On this basis the authors assume that the sediments flooring the Passage are probably the same age as those in the better­studied Puerto Rico North Coast Basin, where well­dated Early Oligocene–Early Pliocene carbonates drape arc­related basement rocks (middle column, fig. 16).

Although this is an interesting possibility, some cautionary notes have to be sounded. The absence of well corings or accessible outcrops and the lack of uniform thickness in the lower part of the seismostratigraphic section mean, in effect, that there is no direct evidence of the section’s actual age, nor any way currently to verify the proposed age for the sedimentary section filling the Passage. The oldest rocks exposed on Isla Mona and Isla Monito are relatively pure limestones generally regarded as Middle or Early Miocene in age (Kaye, 1959), although recently an Early Miocene or Late Oligocene age has been suggested on the basis of dolomitization studies (W. Ramírez, personal commun.). By contrast, the oldest Oligocene marine sediments exposed on land in Puerto Rico are of Early Oligocene age, and consist of thin beds intercalated within what is otherwise an essentially terrestrial section. Fully marine conditions are not encountered until the medial Oligocene ( fig. 16; see also MacPhee and Iturralde­Vinent, 1999). This evidence indicates that Oligocene transgression occurred in parts of the PRVIB and probably the Mona Passage area, but after rather than before ~30 Ma ( fig. 16). While we look forward to seeing further substantiation of the correlations suggested by van Gestel et al. (1998), we would prefer to see that come from something other than seismic data (e.g., seafloor drilling).

Notwithstanding the need for additional evidence, if indeed the Mona Passage opened in the Oligocene, we propose the earliest empirically defensible date should be approximately 30 Ma or slightly earlier (i.e., in the medial rather than the Early Oligocene), coincident with extensive development of marine rocks in the northern and southern late Tertiary basins of Puerto Rico (see Iturralde­ Vinent and MacPhee [1999: fig. 20] and the following section). It is also of interest that the seismic reflection profile data indicate that other marine basins in the area are much younger: thus St. Croix has been isolated from the Saba Bank only since the Middle Miocene, and the Virgin Passage (between the Puerto Rico bank and the Virgin Islands) has existed as a marine channel only since the Pliocene (van Gestel et al., 1998). As to western GAARlandia—essentially composed of the units making up Cuba and eastern His­ paniola—the tectonic and stratigraphic evidence continues to favor later (Miocene) subdivision ( Iturralde­Vinent and MacPhee, 1999).

An earlier opening of the Mona Passage (by 10 Ma or more compared to our original estimate) has some interesting implications. With regard to Antillean paleogeography, redating the opening strongly reinforces our suspicion that the period of maximum continuity and emergence of GAARlandia (see above) had to have been extremely short ( Iturralde­Vinent and MacPhee, 1999: 59). With regard to Antillean biogeography, redating also limits the period available for island–island vicariance. For example, species of the endemic choloepodine sloth genus Acratocnus are currently known from Cuba, Hispaniola, and Puerto Rico (Quaternary localities only). If they achieved this distribution via island–island vicariance rather than through several distinct episodes of overwater transport, as we think (see also White and MacPhee, 2001), then their last common ancestor must have lived in GAARlandia prior to 30 Ma. The partial sloth femur from the locality of Yauco ( Puerto Rico), dated to the Early Oligocene (MacPhee and Iturralde­Vinent, 1995b), is consistent with the early presence of sloths on that island, even if the specimen itself cannot be definitively allocated to Choloepodinae . The same logic applies to the history of nesophontid insectivores and heptaxodontid and heteropsomyine rodents, although for these groups there is as yet no fossil evidence from critical periods.

EMERGENCE OF PUERTO RICO

Another controversial interpretation proposed by van Gestel et al. (1999) in a second recent paper concerns the emergence of Puerto Rico (i.e., the appearance of terrestrial environments on the PRVIB). These authors present a tectonic history of the PRVIB during the Cenozoic that differs substantially from traditional interpretations (e.g., Meyerhoff, 1933; Monroe, 1980; Donnelly, 1992; Larue, 1994) and considerably conflicts with the GAARlandia hypothesis ( Iturralde­Vinent and MacPhee, 1999). In particular, van Gestel et al. (1999) do not accept that portions of this block have been continuously emergent since the latest Eocene, preferring instead a three­phase tectonic evolution that allows for considerable periods of nonemergence. The three phases, largely based on their interpretations of offshore seismic reflection data from the North Coast Basin, are as follows: phase 1, Cretaceous to Middle Eocene, active vulcanism, uplift, emergent land along axis of island arc; phase 2, medial Oligocene to Early Pliocene, ‘‘period of tectonic quiescence’’, complete subsidence of the PRVIB, carbonates deposited across latter’s entire width; and phase 3, Early Pliocene to Holocene, uplift, re­emergence, and acquisition of present coastlines. Importantly, the very short interval stipulated for Neogene emergence (Pliocene and later) is plainly inconsistent with a lengthy history of vertebrate colonization of the PRVIB, whether this was accomplished by overwater or landspan dispersal.

In our view this scenario of complete submergence from the medial Oligocene to Early Pliocene is contradicted by available geological, vertebrate paleontological, and paleobotanical information—most of which is not discussed or referenced by van Gestel et al. (1999). The reader is referred to our summary paper ( Iturralde­Vinent and MacPhee, 1999) for background information and to figure 16 for the position and relationship of various formations named in the following paragraphs.

Geological Evidence: Phase 1 of the mod­ el developed by van Gestel et al. (1999) conflates two quite different tectonic regimes experienced by the PRVIB—the Paleocene– Middle Eocene volcanic arc phase, and the Late Eocene to medial Oligocene uplift phase. This is recognized, for example, by Larue et al. (1998: fig. 16) in their recent reconstruction, which divides the Tertiary tectonic history of Puerto Rico into four rath­ er than three phases. Separate recognition of the Late Eocene/Early Oligocene phase of uplift is critical, because it covers the ‘‘post arc’’ orogenic deformation which had wide effects along the whole Greater Antilles/ Aves Rise (and, not incidentally, led to the formation of GAARlandia). Supporting evidence for this uplift event as it affected the PRVIB is diverse (Monroe, 1980; MacPhee and Iturralde­Vinent, 1995b; Larue et al., 1998; Iturralde­Vinent and MacPhee, 1999; Iturralde­Vinent, 2001): (1) the angular unconformity between the late Tertiary section and the oldest arc section, marking a long period of emergence and subsequent subaerial erosion; (2) the absence of Late Eocene marine rocks (or even reworked Late Eocene marine fossils) in younger rocks within the Puerto Rico /Virgin Island block; (3) the occurrence of?latest Eocene/Early Oligocene conglomerates of terrestrial origin at the base of both the San Sebastian (north basin) and Juana Díaz Fms (south basin). These facts cannot be explained unless the block was continuously emergent through the Eocene– Oligocene transition.

Equally problematic is the characterization of phase 2 by van Gestel et al. (1999) as a ‘‘period of tectonic quiescence’’ and complete subsidence from the mid­Oligocene to Early Pliocene. Long­established facts lead to just the opposite conclusion. Although the northern and southern flanks of Puerto Rico differ stratigraphically in many ways, in both areas sections dating from the beginning of the Oligocene to the end of the early Middle Miocene (33–14 Ma) typically contain terrigenous clastic material eroded from the igneous­sedimentary Cretaceous–Eocene core (Monroe, 1980; Iturralde Vinent and Hartstein, 1998), which means that emergent land had to have been present through this entire interval. The following points lend further support to this contention (see fig. 16; Frost et al., 1983; MacPhee and Wyss, 1990; Iturralde­Vinent and MacPhee, 1999; Iturralde­ Vinent, 2001): (1) the main San Sebastian Fm in the northern half of Puerto Rico and the upper Juana Diaz Fm and unnamed formation of Frost et al. (1983) in the south contain nonmarine sediments and fossils; (2) the marine limestones making up Lares Fm (Late Oligocene–Early Miocene) occur locally only, and interfinger with terrestrially derived sediments of the San Sebastian Fm and Mucarabones Sands (eastern lateral equivalent of Lares and lower Cibao Fms); (3) in several localities the Cibao Fm (Early Miocene) directly overlies structural highs in the Cretaceous and Paleogene volcanic arc sections, indicating that the latter were uplifted and emergent until the Early–Middle Miocene; (4) the Cibao Fm contains abundant nonmarine intercalations and other sedimentological indications of the occurrence of land; (5) the Aguada Fm (Middle Miocene) represents a carbonate shelf and fringing reef, but contains some terrigenous clastic quartz and very rare beds of lignite as well as desiccation cracks; and (6) later Miocene and Pliocene formations (Aymamon, Camuy, Angola, and Ponce Fms; ‘‘unnamed formation’’ of Frost et al., [1983]) are typical marine­shelf facies, similar to those which today surround places in the Cuban archipelago characterized by low relief or low sediment output (or both) (Pushcharovsky et al., 1988). Mere presence of such sediments is not dispositive of the absence of land, and, as always, should be evaluated in relation to all other relevant evidence.

Monroe (1980: 57) stated that, notwithstanding the absence of clastics in Aymamon rocks, land of ‘‘very low relief that probably stood not far above sea level’’ was still present in the Middle Miocene. Perhaps most importantly, he also noted that there is no positive evidence that the interior of Puerto Rico was transgressed in the Neogene. In other words, no vestige of Neogene sedimentary units have ever been identified in Puerto Rico outside the carbonate units framing the coasts, despite very active programs of outcrop exploration in the last century (Monroe, 1980). Absent an unprecedented rate of erosion in central Puerto Rico, or the complete failure of carbonate rocks to form except along the coasts, this evidence by itself strongly indicates that no mid­Tertiary or lat­ er transgression of this island could have been complete.

In summary, the preponderance of geological facts establish, contra van Gestel et al. (1999), and in line with Meyerhoff (1933) and Iturralde­Vinent and MacPhee (1999: figs. 6–8), that significant portions of the Puerto Rico /Virgin Island block have been emergent since the latest Eocene, although doubtless there was variation in its areal extent and relief.

VERTEBRATE PALEONTOLOGICAL AND PALEO­ BOTANICAL EVIDENCE: San Sebastian and Juana Diaz Fms present abundant evidence of terrestrial plant remains, including petrified wood ( Graham and Jarzen, 1969; Graham, 1996), as well as good examples of paleosol development (MacPhee and Wyss, 1990; MacPhee and Iturralde­Vinent, 1995b). These are unambiguous indicators that land was present during the latter part of the Oligocene, perhaps land of high relief ( Graham and Jarzen, 1969). Furthermore, in Early Miocene sediments in both areas terrestrial plant and animal fossils have been recovered ( Graham, 1996; MacPhee and Wyss, 1990; MacPhee and Iturralde­Vinent, 1995b; Iturralde­Vinent and Hartstein, 1998; Iturralde­Vinent, 2001), including specimens that indisputably indicate terrestrial conditions (boid and?iguanid fossils from locality AMNH PR 88­1, Cibao Fm; MacPhee and Wyss, 1990). In fact, van Gestel et al. (1999: 279) agree that in Late Oligocene/Early Miocene Lares Fm, ‘‘siliclastic sedimentation... may have represented the final stages of the erosional event that produced the underlying San Sebastian Formation’’, but they failed to include this point in their characterization of phase 2 paleogeography.

In closing, we emphasize that van Gestel et al. (1999) did not misinterpret their basic paleogeographical data, which concern many other topics of no direct relevance here. Rather, the problem is that, in making their specific inductions about emergence vs. nonemergence of Puerto Rico, they failed to consider sources of contrary evidence (e.g., vertebrate paleontology). This underscores how important it is to consider carefully all relevant information when making paleogeo­ graphical investigations, and to pay particular attention to apparent contradictions in datasets before reaching settled conclusions ( Crother and Guyer, 1996).