Caribbean Journal of Science, Vol. 29, No. 3-4, 153-164, 1993 Copyright 1993 College of Arts and Sciences University of Puerto Rico, Mayaguez The Cuban Crocodile, Crocodylus rhombifer, from Late Quaternary Fossil Deposits on Grand Cayman 1 1 2 G ARY S. MORGAN, RICHARD FRANZ, AND RONALD I. CROMBIE 1Florida Museum of Natural History, University of Florida, Gainesville, Florida 32611 2Division of Reptiles and Amphibians, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560 ABSTRACT . – Fossil remains of crocodiles have been recovered from six sites on Grand Cayman, a small island in the northwestern Caribbean Sea about 250 km south of Cuba. The three most productive fossil faunas (Chisholm Cow Well, Crocodile Canal, and Furtherland Farms Cow Well) were deposited in aquatic depositional environments, based on the presence of dark organic sediments and the predominance of freshwater and estuarine vertebrates. Radiocarbon dates of 860 ± 50 yBP and 375 ± 60 yBP obtained on peat samples from the Crocodile Canal site confirm that crocodiles inhabited Grand Cayman until the late Holocene, although they are no longer found there. The Grand Cayman fossil sample contains four nearly complete skulls that closely resemble modern specimens of the Cuban crocodile, Crocodylus rhombifer Cuvier. The diagnostic cranial features they share include: comparatively short, broad, and deep rostrum; large orbits; strongly concave cranial roof and interorbital region; prominent ridges on dorsal margin of orbits and lateral edge of postorbital and squamosals, terminating in a rounded protuberance on pos- terolateral corner of squamosals; premaxillary/maxillary suture on palate transverse at level of first max- illary tooth; and 13 maxillary teeth. Most of the fossils in the extensive sample of C. rhombifer from Grand Cayman represent juvenile and subadult individuals. The name of the Cayman Islands is probably derived from the abundance of crocodiles prior to their local extinction during the last century. sources Study (MRCU) of the Cayman Is- INTRODUCTION lands government, Gary Morgan and Morgan and Patton (1979) first reported Richard Franz conducted a paleontological fossil crocodiles in the Cayman Islands survey of the Cayman Islands in January based on a single tooth collected in 1976 and February of 1986. Excavations at the from a small cave at the eastern end of Chisholm Cow Well and Furtherland Farms Grand Cayman. In the past ten years, croc- Cow Well sites produced abundant fossils odile fossils have been collected from five of crocodiles and other vertebrates. The additional sites on Grand Cayman. The crocodile bones recovered from these two three sites containing the richest samples deposits, as well as material from three ad- of crocodile bones, Chisholm Cow Well, ditional sites, Crocodile Canal, Connally Crocodile Canal, and Furtherland Farms Cow Well, and Prospect, form the basis for Cow Well, are composed of dark, peaty, the present report. organic sediments that probably formed in The Cayman Islands, including Grand a quiet estuarine or brackish depositional Cayman, Little Cayman, and Cayman Brat, environment such as a mangrove swamp are three small, low limestone islands lo- or coastal lagoon. The abundance of croc- cated in the northwestern Caribbean Sea odiles and the presence of several species between 19°15’N and 19°45’N latitude and of wading birds in the Grand Cayman sites 79°42’W and 81°26’W longitude. They are is unusual, as the vast majority of West situated slightly more than 200 km from Indian vertebrate fossil deposits occur in both Cuba to the north and Jamaica to the caves and are dominated by terrestrial taxa southeast, and about 500 km from the clos- (Morgan and Woods, 1986; Morgan, in est point in Central America. press). Grand Cayman is the largest of the is- At the invitation of the Mosquito Re- lands, with an area of 197 km2. Its maxi- search and Control Unit and Natural Re- mum elevation is about 20 m and more 153 154 G. S. MORGAN ET AL. than half the island’s area is under 5 m. Islands. In two deposits, the Chisholm Cow Low-lying areas are characterized by Well and the Furtherland Farms Cow Well, brackish lagoons, mangrove swamps, and the fossiliferous organic sediments were other swamp vegetative formations. deposited in small depressions or sink- Although they are low and flat, the Cay- holes in the limestone. These depressions man Islands represent the projecting peaks are called “cow wells” by residents because of high submarine mountains located along they are used as a source of drinking water the Cayman Ridge on the southern edge for cattle, especially after the organic sed- of the North American plate. The Cayman iments are removed to deepen the holes. Islands are surrounded by deep water in It is during the removal of these sediments all directions. Depths of 1000 m or more that fossils have been discovered. occur between the Cayman Islands and The third major locality for fossil croc- both the Greater Antilles and Middle odiles on Grand Cayman, Crocodile Canal, America. Depths of nearly 2000 m have is different from the other two sites, since been recorded between Grand Cayman and it did not form in a limestone depression. Little Cayman, and the narrow channel be- This site consists of a mangrove peat de- tween Cayman Brat and Little Cayman is posit uncovered during the excavation of almost 1000 m deep. These depths preclude a mosquito control canal. Although the cow land connections during Pleistocene low well deposits and the mangrove peat sea levels, either within the Cayman Is- formed under different depositional con- lands or between the Caymans and other ditions, they are similar in the high organ- land masses. ic content of the sediments and the abun- The following museums and institutions dance of C. rhombifer bones. These sites also possess specimens of fossil or extant croc- contain bones of many other vertebrates, odiles from the West Indies that we ex- including freshwater and terrestrial forms. amined during this study. The standard No strictly marine vertebrates have been abbreviation follows the name in paren- identified from either deposit. theses. Brief descriptions of the fossil deposits on Grand Cayman that have produced fos- American Museum of Natural History sil crocodiles are provided below. The lat- (AMNH) itude and longitude are given for each of Cayman Islands National Museum (CINM) the fossil sites, along with the six-digit co- Florida Museum of Natural History—for- ordinates based on the 1000 meter Uni- merly the Florida State Museum (UF) versal Transverse Mercator Grid (Grid Zone Mosquito Research and Control Unit and Designation 17Q) taken from the 1:25,000 Natural Resources Study (MRCU) topographic map series for the Cayman Is- Museum of Comparative Zoology, Har- lands, published in 1978 by the British Di- vard University (MCZ) rectorate of Overseas Surveys. The first United States National Museum of Natural three digits are east coordinates and the History (USNM) last three are north coordinates. The six- digit number is preceded by two letters D ESCRIPTION OF FOSSIL D EPOSITS corresponding to the 100,000 m2 identifi- Fossil deposits in the Cayman Islands that cation (MM is the designation for Grand have yielded remains of crocodiles are re- Cayman). More detailed locality data, field stricted to Grand Cayman. With the excep- notes, and photographs for these sites are tion of one tooth from Crab Cave at the available in the vertebrate paleontology lo- eastern end of the island (Morgan and Pat- cality files of the Florida Museum of Nat- ton, 1979), the fossils are preserved in dark ural History. A map showing the location brown to black, highly organic, peaty sed- of all vertebrate fossil sites on Grand Cay- iments. The most abundant species pre- man can be found in Morgan (in press). served in these organic sediments is the Chisholm Cow Well (19°21’N, 81°13’W; Cuban crocodile, Crocodylus rhombifer Cu- MM765391). — The Chisholm Cow Well is vier, which is now extinct in the Cayman located about 0.3 km south of Grape Tree FOSSIL CROCODILES FROM GRAND CAYMAN 155 Point and 1.8 km southwest of North Side. the cave entrance. A test pit 2 m long, 1 m This site is only a few meters above sea wide, and 0.5 m deep was dug under this level and consists of a small depression or ledge. Besides the single tooth of a croco- sinkhole in the limestone about 5 m long, dile (Morgan and Patton, 1979), Crab Cave 3 m wide, and 1-2 m deep. The bones are produced fossils of Cyclura nubila, smaller preserved in a dark, organic sediment at lizards, two species of snakes, birds, bats, the bottom of the sinkhole. The bones from and rich samples of capromyid rodents the Chisholm Cow Well are predominant- (Morgan, in press). Crab Cave was exca- ly those of the Cuban crocodile, Crocodylus vated by H. G. McDonald, G. S. Morgan, rhombifer, along with much smaller sam- and N. R. Thanz in March 1976 and by M. ples of the capromyid rodents Capromys and K. Langworthy and G. S. Morgan in April Geocapromys, the rock iguana Cyclura nubila 1980. Gray, and snakes. Bones were originally Crocodile Canal (19°19’N, 81°23’W; discovered at this site when sediments were MM603363). — The Crocodile Canal site was removed from the cow well during the dry discovered in a mosquito control canal season. The largest sample of bones from about 3 km north of George Town. The the Chisholm Cow Well was collected by canal was dug through a mangrove swamp Rolin Chisholm and the late Ira Thompson and is only slightly above sea level. The in the late 1970’s.