ABSTRACT
We report and describe the first sirenian endocranial casts from the West Indies based on three specimens collected from two quarries of the upper Oligocene–lower Miocene Colón Formation, in the province of Matanzas, western Cuba. We assign them to Dugongidae incertae sedis, based on a phylogenetic analysis of fossil and extant sirenians. Thus, these new specimens provide a unique opportunity to describe the endocranial neuroanatomy of this family. The endocasts are suggestive of dugongids with limited vision and olfaction, based on the diminished olfactory and optic nerves. Additionally, we provide a geological reinterpretation of the Colón Formation and its paleoecological setting. Altogether, these data provide further insight into the diversity and evolution of sirenians, especially Caribbean dugongs.
RESUMEN
Se describen los primeros moldes endocraniales de sirénido hasta ahora reportados de las Antillas. Los tres especímenes que reportamos provienen de dos canteras con afloramientos de la Formación Colón, en la provincia de Matanzas, Cuba. Esta formación se considera de edad Oligoceno tardío a Mioceno temprano, pero nosotros asignamos los moldes al Mioceno temprano. Conjuntamente, proveemos una lista de caracteres y un análisis filogenético que puede ayudar aclarar su posición sistemática. Por falta de material comparativo, los moldes endocranianos no se pudieron atribuir a una especie, y por ende los asignamos a la familia Dugongidae incertae sedis, por su posición en el árbol filogenético. Estos especímenes permiten un acercamiento a la historia, diversidad y evolución de los sirénidos, y en especial de los dugongídos en el neógeno caribeño.
INTRODUCTION
The West Atlantic and the Caribbean are considered areas of high interest for the study of sirenians because of the region’s rich fossil record for this mammalian group (Domning, Citation2001). The oldest fossil sirenians derive from the region and are assigned to the primitive Prorastomus sirenoides Owen, 1855, and Pezosiren portelli Domning, Citation2001, described from the early and early middle Eocene of Jamaica, respectively (Domning, Citation2001; Savage et al., Citation1994).
During the Cenozoic, the West Atlantic-Caribbean region was inhabited by at least 10 genera and 19 species of the family Dugongidae, the last of which became extinct during the Pliocene (Domning, Citation1988, Citation1989a; Vélez-Juarbe et al., Citation2012). Dugongids were later replaced in the region by the Trichechidae, the modern manatee family (Domning, Citation1989b, Citation1997; Domning and Aguilera, Citation2008; Vélez-Juarbe, Citation2014; Vélez-Juarbe and Domning, Citation2014a, Citation2014b, Citation2015).
The fossil evidence so far corroborates the West Atlantic-Caribbean region as a center of Sirenia evolution and radiation (Domning, Citation1988; MacPhee and Wyss, Citation1990; Vélez-Juarbe and Domning, Citation2014b, Citation2015). However, there are areas in the Greater Antilles, such as the islands of Cuba and Hispaniola, where sirenian fossils remain largely limited, poorly studied, or unpublished. Two taxa, Metaxytherium (M. cf. crataegense) and Nanosiren, have been reported from Cuba (Varona, Citation1972; MacPhee et al., Citation2003; Domning and Aguilera, Citation2008), but these finds have consisted of isolated cranial, rib, and vertebral fragments (Trelles-Duelo, Citation1936; Varona, Citation1972; MacPhee et al., Citation2003; Domning and Aguilera, Citation2008; Vélez-Juarbe et al., Citation2012). As far as we know, endocranial casts, natural or otherwise, which provide a unique opportunity to approximate sensory characteristics and distinct neuroanatomical characters (Edinger, Citation1939; Pilleri, Citation1990; Furusawa, Citation2004; Balaguer and Alba, Citation2016), have not been reported from this region. Anatomical descriptions of additional fossils, particularly endocranial casts, from this region could greatly enhance our knowledge of the history of sirenians in the Greater Antilles, as fossils from Puerto Rico already have done (Vélez-Juarbe and Domning, Citation2015), and thus deepen our understanding of sirenian evolution, diversity, and radiation in the West Atlantic-Caribbean region.
Here, we report three natural cranial endocasts (steinkerns) from late Oligocene–early Miocene (Colón Formation) quarry deposits from central Matanzas Province, Cuba. By using computed tomography (CT) technology, we illustrate, describe, and reconstruct the morphology of the endocranium and compare it with other known sirenian endocasts. We also provide a list of characters from the endocranium that provide insights into the systematics of the group. Our geological field observations provide additional data on the depositional setting and age of the formation and thus help to reconstruct the environment of this marine herbivore.
Institutional Abbreviations—AMNH, American Museum of Natural History, New York, New York, U.S.A.; J4, code given to schools and quarries in this region of Matanzas, Cuba; MGB, Museo de Geología de Barcelona, Barcelona, Spain; MNHNCu, Museo Nacional de Historia Natural, Havana, Cuba; NHMUK, Natural History Museum, London, U.K.; SMF, Forschungsinstitut und Natur-Museum Senckenberg, Frankfurt, Germany; TMNH, Takikawa Museum of Art and Natural History, Hokkaido, Japan; UCMP, University of California Museum of Paleontology, Berkeley, California, U.S.A.
GEOLOGICAL SETTING
The most complete sirenian endocast, MNHNCu. P71.005310, was collected in 2014 at a limestone quarry named Beruvides’s Quarry, near Agramonte, province of Matanzas, Cuba. Specimens MNHNCu. P P71.005311 and MNHNCu. PP71.005312 were collected at J4, another limestone quarry ca. 5 km south of Berovides’s Quarry in the central region of the province of Matanzas ().
FIGURE 1. Geological map indicating the location of Quarry Beruvides and J4 near the town of Jagüey Grande, province of Matanzas, Cuba. Colón Formation constitutes older outcrops of late Oligocene–early Miocene limestones (N1), which are extensively covered by the Güines Formation, of middle–late Miocene-aged (N2–3) limestones. Colón Formation in dark brown (Oligocene in the key) to differentiate it from the younger outcrops.
The limestone that crops out at both quarries has been assigned to the Colón Formation (Brödermann, Citation1945), a late Oligocene and early Miocene (∼28–16 Ma) massive unit exposed in the area (Varela and Rojas-Consuegra, Citation2011a). Core samples collected near this location suggest a maximum thickness no greater than 65 m, of which the upper 5–20 m is exposed at these quarries (Franco et al., 1992). The fossils reported here were extracted from the uppermost levels, ca. 20 m from the current quarry floor. Geological samples were taken at the J4 quarry, from the same level where the specimens were found. Five macroscopic samples and five thin sections were made from these samples and analyzed to provide a reinterpretation of the age and setting of the outcrop at these levels.
There are four known lithological facies for the Colón Formation. The first is a biocalcarenite-biocalcirudite, which is characterized by a fine, marly, and micritic matrix, with the large benthic foraminifers Lepidocyclina and Heterostegina, in addition to coral fragments. The second is a biomicritic limestone, also with Lepidocyclina. The third is a polymictic sandstone with an argillaceous-calcareous matrix. The fourth is a marl calcirudite, intercalated with calcarenites (Huelbes, Citation2014). Their sequence has not been correlated, and only one member (Coliseo) is recognized (Huelbes, 2014).
Our specimens were set in cream-colored, bioclastic, marl limestone referable to the first biocalcarenite facies, which outcrops at both quarries. Other sirenian fossils have been recovered from that calcarenite facies (E. Abreu, pers. comm., 2014). Sirenian fossilized ribs, vertebrae, and teeth are frequently found in both quarries, but these remain unstudied and formally unreported.
The fauna previously reported from the Colón Formation at these localities is characterized by its abundance of benthic and planktonic foraminifera, ostracods, mollusks, decapod crustaceans (crabs), echinoderms, bony fishes, and cartilaginous fishes such as the sharks Otodus megalodon (sensu Cappetta, Citation2012), Hemipristis sp., and Galeocerdo sp., in addition to chelonians and crocodilians (Varela and Rojas-Consuegra, Citation2011a, Citation2011b, Citation2011c; Varela and Schweitzer, Citation2011; Rojas-Consuegra and Viñola, Citation2013; Jiménez Vázquez et al., Citation2014; Viñola and Rojas-Consuegra, Citation2016; Viñola et al., Citation2017).
The Colón Formation is conformably overlain by the Güines Formation, which crops out extensively in this area () and is currently correlated with the Lagunitas Formation in central Cuba, from which other sirenian remains have been previously reported (Varona, Citation1972; MacPhee et al., Citation2003).
MATERIALS AND METHODS
Specimens
Our study is based on three natural endocranial casts: MNHNCu. P71.005310, a nearly complete endocast (, ); MNHNCu. P71.005311, an endocast partially embedded in matrix (); and MNHNCu. P71.005312, an endocast associated with a frontosupraorbital (frontal) skull fragment (not illustrated). The most complete specimen, MNHNCu. P71.005310, was imaged with computed tomography (CT) to allow us to create a volume rendering of the specimen, acquire detailed measurements, and explore its composition. Specimen MNHNCu. P71.005312 was too fragmentary to be diagnostic, but its association with the corresponding frontosupraorbital (‘frontal’ hereafter) skull fragment is briefly discussed.
FIGURE 2. Digital volume rendering of MNHNCu. P71.005310, a dugongid endocast, in A, ventral, B, dorsal, C, posterior, and D, anterior views.
FIGURE 3. Additional digital volume rendering of MNHNCu. P71.005310, a dugongid endocast, in A, right lateral, B, left lateral, and C, oblique dorsolateral views. Several molds and casts of mollusks are visible on the specimen.
The CT data were acquired with a General Electric Lightspeed VCT scanner, resulting in 572 images of dimensions of 512 × 512 pixels. The scanner utilized 120 peak kilovolts (kVp), with a 200 mA current, and produced images with a slice thickness of 0.625 mm in the axial (horizontal or transverse) plane without overlap or interpolation of data. The volume rendering, sagittal (median), coronal (frontal), and axial views were reconstructed from the raw data on a General Electric ADW Workstation (4.8).
Linear measurements were taken directly from the specimens with digital calipers and are reported to the nearest 0.1 mm. The volumetric measurements were produced from the CT data on a General Electric ADW Workstation (4.8) using a standard algorithm, without segmentation. All measurements are listed in .
TABLE 1. Linear and volumetric measurements (in mm) of endocasts MNHNCu. P71.005310 and MNHNCu. P71.005311.
Sirenian brain and endocast terminology follow Owen (Citation1875), Breathnach (Citation1955), Furusawa (Citation1988, Citation2004), Pilleri (Citation1989), and Gingerich et al. (Citation1994). Character states are modified from those given by Edinger (Citation1939), Pilleri et al. (Citation1988), Pilleri (Citation1989, Citation1990), Gingerich et al. (Citation1994), Furusawa (Citation2004), and Macrini et al. (Citation2007). We followed Voss and Hampe (Citation2017) in including the species Halitherium schinzii Kaup, 1838 in the genus Kaupitherium Voss and Hampe, Citation2017. However, because the endocast specimen described by Furusawa (Citation2004) for H. schinzii (SMF-M3921) was not included in Voss and Hampe’s revision, we refer to it here as Kaupitherium sp. Comparisons and characters described for other taxa in the text were interpreted or taken from the cited literature see Appendix 1 and 2. A digital endocast of Trichechus senegalensis (AMNH 53939; Macrini, Citation2006) was used for comparative purposes.
Phylogenetic Analysis
For the phylogenetic analysis, 17 endocranial and 69 additional craniodental characters from 10 taxa, from Vélez-Juarbe and Domning (Citation2014a), were used to determine the relationships of MNHNCu. P71.005310 and MNHNCu. P71.005311 within Sirenia. However, only endocranial characters were available for MNHNCu. P71.005310 and MNHNCu. P71.005311.
Character states were obtained from the supplemental data provided by Vélez-Juarbe and Domning (2014) and endocranial characters were scored directly from specimens or from the literature in the group (from supplemental data 1 of Vélez-Juarbe and Domning, Citation2014a). In some cases, it was necessary to combine skeletal characters from one taxon with endocranial characters from another taxon in the same family (Protosirenidae: Protosiren heali + Ashokia antiqua; Trichechidae: Trichechus manatus + Miosiren kocki). Similarly, skeletal characters from Metaxytherium medium were combined with endocranial characters from two specimens of Metaxytherium sp. from the Miocene of Spain (Pilleri, Citation1990; Bianucci et al., Citation2008). The characters were treated as unordered. The matrix was analyzed with MrBayes 3.2.6 by using the Markov chain Monte Carlo method with the following settings: 10 runs of 1,000,000 generations with a sampling frequency of 10,000 and a burn-in factor of 0.25. Prorastomus sirenoides was the outgroup (Appendix 2).
SYSTEMATIC PALEONTOLOGY
Order SIRENIA Illiger, Citation1811
Family DUGONGIDAE Gray, Citation1821
DUGONGIDAE indet.
Referred Specimens—MNHNCu. P71.005310, complete endocast; MNHNCu. P71.005311, endocast fragment embedded in matrix; and MNHNCu. P71.005312, fragment of a frontal endocast associated with a frontosupraorbital skull fragment. These are all assignable to Dugongidae. MNHNCu. P71.005310 and MNHNCu. P71.005311 are here tentatively assigned to the Dugonginae subfamily, but they may represent more than one species.
Horizon and Locality—MNHNCu. P71.005310 is from Beruvides’s Quarry, near the town of Agramonte, province of Matanzas, northwestern Cuba. MNHNCu. P71.005311 and MNHNCu. P71.005312 are from quarry J4, ca. 5 km south of Berovides’s Quarry, province of Matanzas (). Colón Formation, upper Oligocene and lower Miocene.
DESCRIPTION
Forebrain
The forebrain region of the endocast is dominated by the massive cerebral hemispheres, the superior view of which shows the extent of the neocortex. The cerebrum is best preserved in one of the specimens (, , ), but incomplete in the others (). In lateral view, the endocast is domed, with prominent temporal and parietal lobes of the cerebrum rising posteriorly (). The endocast is generally lissencephalic or smooth, without any visible gyrification.
FIGURE 4. MNHNCu. P71.005311, dugongid endocast specimen in limestone matrix, in A, oblique superior and B, anteroposterior views. Abbreviations: IS, inferior sulcus of mesethmoid; MS, median sulcus; Ol-Ocx, olfactory lobe-olfactory-piriform cortex (?); Pl, parietal lobe.
FIGURE 5. Neuroanatomy of the dugongid natural endocast MNHNCu. P71.005310 in A, dorsal, B, left lateral, C, ventral, and D, anterior views. Abbreviations: asSF, swelling anterior to Sylvian fissure; Cbl, cerebellum; Cbr, cerebrum; CE, cavum epiptericum; FL, frontal lobe; Fmc, foramen magnum cast; Hypf, hypophyseal fossa; iam, internal auditory meatus; IS, inferior sulcus; MS, median sulcus; Ncx, neocortex; NT, trigeminal nerve (cranial nerve V); Ob, olfactory bulb; Obt, olfactory bulb tracts; P, pons; Pc, parietal cortex; PL, parietal lobe; RhF, rhinal fissure; SF, Sylvian fissure; TL, temporal lobe; Tr, temporal region; TS, transverse sinus.
Diminutive olfactory bulb casts are located anteriorly and slightly inferiorly on the endocast, extending like a low crest at the base of the frontal lobes. Viewed in anterior aspect, the olfactory bulbs are elliptical in outline, not round. The olfactory bulbs are widely separated by a deep fissure (), likely resulting from a robust crista galli, as seen in other sirenians (Gingerich et al., Citation1994). Thin and somewhat laterally flattened olfactory tracts extend posteriorly from the olfactory bulbs to the piriform lobe of the cerebellum (). Striae are not visible on the olfactory tracts (, ). The accessory olfactory bulbs are not present on the endocast ().
The cerebral hemispheres are elongated, extending about half the length of the endocast in dorsal view (). The hemispheres are somewhat peanut-shaped in dorsal view, being wide at the ends with a slight waisting at the level of the Sylvian fissure (), the pseudo-lateral sulcus of Furusawa (Citation2004). The right and left hemispheres are separated by a deep and wide median sulcus, the fissure longitudinalis cerebri of Furusawa (Citation2004), reaching a deep and wide end before the post–Sylvian-temporal fissure that separates the cerebrum from the cerebellum. The frontal, temporal, and parietal lobes of the cerebrum are discernible on the endocast. The frontal lobe casts are elongated but do not extend laterally beyond the maximum width of the temporal region. The Sylvian fissure marks the posterior extent of the frontal lobes and the medial border of the temporal lobe with the rest of the cerebral hemispheres. The parietal lobes are well inflated. The rhinal fissure is represented by a thin line that separates the cast of the frontal lobe of the cerebrum from the cast of the trigeminal nerve (cranial nerve V) and cavum epiptericum (). Other than the rhinal and Sylvian fissures, the cerebral hemisphere casts are smooth.
The ventral side of the forebrain region of the endocast is preserved with a fair degree of detail. The frontal lobes of the cerebrum are widely separated in their ventral aspect, presumably by a robust crista galli of the ethmoid (). Just posterior to this groove lies the hypophyseal fossa, which is generally not deep and is somewhat elliptical in outline (). Besides the olfactory tracts of cranial nerve I, which are described above, the trigeminal nerve (cranial nerve V) leaves the most distinctive impression on the ventral side of this endocast. The cavum epiptericum runs lateral and along the entire length of the hypophyseal fossa (). Anteriorly, this space terminates at the sphenorbital fissure of the skull, where several cranial nerves exit the skull in many extant mammals (e.g., cranial nerves II, III, IV, V1, and VI). There is no clear indication from the skull and endocast of separate optic foramina (for cranial nerve II) in Trichechus senegalensis () or the extinct taxon examined here (), contra the condition in Hydrodamalis (Furusawa, Citation2004). Instead, cranial nerve II likely passed through the sphenorbital fissure in both taxa examined here.
FIGURE 6. Coronal CT sections of the dugongid endocast MNHNCu. P71.005310. The slice location is indicated by the reference image. Note the difference between the inferior and median sulci. The median sulcus is shallower and wider. The inferior sulcus is deeper and narrower.
The ear region is defined by a circular depression, impressed by a deep ridge on the temporal side of the endocast. This region lies posteriorly to the Sylvian fissure (). However, this region, and the rest of the petrosal region, although it seems intricate and detailed on this specimen, seems to be only partially intact; therefore, the flocculus, the paraflocculus, and the internal auditory meatus are not preserved on this endocast.
Midbrain and Hindbrain
The midbrain is not visible on the dorsal surface of the endocasts. Like modern sirenian brains (Edinger, Citation1939; Pilleri, Citation1988), the overgrown cerebral hemispheres likely pressed against the cerebellum during life to cover up the inferior and superior colliculi (corpora quadrigemina) of the midbrain tectum in this extinct taxon. Furthermore, a robust tentorium cerebelli and the transverse sinus embedded within the dura matter cover up this region and thus obscure discernment of brain structures on the endocast as in other sirenian endocasts (Gingerich et al., Citation1994; ). Likewise, the dorsal surface of the cerebellum was likely mostly covered by the falx cerebelli of the dura mater during life and thus is not represented on the endocast. Additionally, damage to this region of the skull further obscures the corresponding region of the endocast. The morphology on the ventral surface of the endocast in this region suggests that the meninges and cisterns significantly covered the brain stem, preventing brain structures on the endocast (e.g., pons and medulla oblongata) from being visible as in the modern West African manatee ().
Species Identity
The most complete endocast (MNHNCu. P71.005310) has a volume of 551.76 cm3 (ml), which is comparable to those of Metaxytherium (305–500 mL) and Dugong dugon (390–455 ml), but smaller than that of Hydrodamalis gigas (1650 ml) and larger than those of Trichechus inunguis (165–390 ml), T. manatus (396 ml), and T. senegalensis (340–478 ml) as reported by Pilleri (Citation1990). The specimen of Trichechus senegalensis examined in this study (AMNH 53939; ) has a volume of 374.5 ml (Macrini, Citation2006). The volumes in Protosiren fraasi and Eotheroides aegyptiacum are reported as 185 and 150 ml, respectively (Gingerich et al., Citation1994). Based on total endocast length, the primitive Eotheroides, Masrisiren (= Eotheroides; Domning, Citation1996), Eosiren, and Protosiren are smaller (<102 mm) than the specimen described here (174 mm total, 160 mm from anterior-most to cerebellum). Specimen MNHNCu. P71.005310 is also larger than the two specimens of Metaxytherium sp. (113–125 mm) and Dugong dugon (117 mm) mentioned in Pilleri (Citation1990).
Overall, two of the natural endocasts (MNHNCu. P71.005310 and MNHNCu. P71.005311) represent an extinct dugong similar in size to Metaxytherium. The endocast MNHNCu. P71.005311, although slightly smaller than MNHNCu. P71.005310, seems to be assignable to the same taxon as the latter. The endocast associated with the frontal skull fragment (MNHNCu. P71.005312) is too incomplete to be diagnostic or compared with MNHNCu. P71.005310 and MNHNCu. P71.005311. Moreover, the discrete characters of this specimen suggest that it may represent an undescribed dugongine, which will be described elsewhere (Viñola and Domning, in prep.).
The presence of multiple sirenian species in the same ecosystem has been well documented and may represent a case of niche partitioning (Domning, Citation2001; Vélez-Juarbe and Domning, Citation2015). Apparently, such an association was not uncommon, as inferred from the sirenian fossil record, during the Neogene of the Western Atlantic and Caribbean region (Vélez-Juarbe et al., Citation2012; Vélez-Juarbe and Domning, Citation2015), and from Cuban and Puerto Rican fossils (MacPhee et al., Citation2003). Unfortunately, due to the lack of comparative material, a genus and species cannot now be assigned confidently to any of the specimens, and we refer them to Dugongidae incertae sedis.
Geological-Petrographic Observations on Age and Environment of Sedimentation
The five thin sections analyzed revealed that the general microscopic composition of the limestones is classifiable as a bioclastic, marl limestone, with a microcrystalline texture and micritized bioclasts (bioallochems). These were enveloped in sparite composed of a stubby calcite coating and sparse sparite cement. The composition included <15% bioallochems, >50% well-rounded but poorly sorted mineralized clasts, peloids, and intraclasts, plus <1% aggregate minerals (largely angular, polished quartz). The microfossils are poorly preserved, likely an effect of meteoric or phreatic diagenesis, as suggested by several generations of sparite formation and sparite cement with secondary (authigenic) sparite matrix.
The microfossils consisted mostly of Foraminifera, including the planktonic Globigerina sp. cf. G. ciperoensis, Globigerina praebulloides, Globigerinoides sp. cf. quadrilobatus, Globorotalia sp. cf. archeomenardi, Whiteinella sp. (?), and Cassigerinella sp. Also present were the small and large benthic foraminiferans Heterostegina cf. H. antillea, Lepidocyclina sp., Amphistegina sp., Nummulites sp. cf. dia, Sorites sp. cf. S. marginalis, Rosalina sp., Elphidium sp., Cibicides sp., Pyrgo sp., Bolivina cf. mexicana, Uvigerina sp., and Textularia sp. (see Huelber, 2014, for additional fauna). Bryozoans, algae, pteropods, and ostracods were poorly preserved and are mostly present as traces (see Scholle and Ulmer-Scholle, Citation2003).
Several imprints of mollusk shells are present on the surface of the endocasts. The carbonate, fine-grained mud (carbonate mud?) seems to have infilled the sirenian skull cavity soon after burial. Over time, it may have harbored a benthic bivalve and gastropod mollusk fauna whose traces and casts can be seen on the cross-axial sections of the endocasts ().
FIGURE 7. Sagittal CT section of specimen MNHNCu. P71.005310. Note the presence of mollusk (gastropods and bivalves indicated by arrows) ghosts within the matrix. This matrix and its fauna suggest that a calcareous mud filled the skull cavity of this specimen, forming the natural endocast. Arrows point to casts of a Siphocypraea or Oliva gastropod species. Abbreviations: A, anterior; Cb?, cerebellum; CE, root of the cavum epiptericum; FL, frontal lobe; P, posterior; PL, parietal lobe; NT, trigeminal nerve.
DISCUSSION
Phylogenetic Analysis
A Bayesian consensus tree () from 345 trees with 99% support was obtained from the analysis and then visualized in Mesquite 3.04. The consensus tree has a length of 115 steps with a consistency index (CI) of 0.748 and a retention index (RI) of 0.701. All nodes with posterior probability below 0.5 were collapsed. Our Bayesian consensus tree supports the nesting of our specimens (MNHNCu. P71.005310 and MNHNCu. P71.005311) within the Dugongidae, most precisely within the subfamily Dugonginae, where they seem to be more closely related to Dugong dugon and Rhytiodus heali () than to the Hydrodamalinae or Metaxytherium spp.
FIGURE 8. A Bayesian final consensus tree obtained from 345 trees with 99% support, a length of 115 steps, a consistency index (CI) of 0.748, and a retention index (RI) of 0.701. This tree supports the nesting of the endocranial casts reported here within the subfamily Dugonginae. See Appendix 1 and 2.
The general topology of the tree agrees with previous phylogenies of sirenians (Vélez and Domning, 2014, 2015; Balaguer and Alba, Citation2016; Díaz-Berenguer et al., Citation2018) but differs from most of them in considering the family Trichechidae to be nested within the family Dugongidae. Similar results were obtained by Balaguer and Alba (Citation2016), with the difference that the relationship between Trichechidae and Eotheroides aegyptiacum is unresolved here. Specimens MNHNCu. P71.005310 and MNHNCu. P71.005311 are nested within the subfamily Dugonginae, a group of sirenians that was very diverse during the Neogene in the Caribbean region, along with the Hydrodamalinae + Metaxytherium spp. clade (Vélez-Juarbe et al., Citation2012; Vélez-Juarbe and Domning, 2014, Citation2015). Specimen MNHNCu. P71.005312 seems to nest within the same clade but may represent a different species.
Sensory Structures
In comparison with extant taxa, the features of our endocasts suggest that these species had reduced olfaction, given the reduction of the olfactory bulbs and tracts, but enhanced hearing and motor sensory capabilities due to the development of the temporal-parietal lobes. The endocast of MNHNCu. P71.005310 was, overall, less massive and more elongated, with more pronounced temporal-parietal regions and wider cerebellar regions, than that of the Catalonian Metaxytherium illustrated by Pilleri et al. (Citation1989), but similar, in this aspect, to Protosiren fraasi (Edinger, Citation1939:pl. 2). This is consistent with limited underwater vision, but with greater hearing and locomotion than other sensory stimuli as already suggested by Piggins et al. (Citation1983; Supin et al., Citation2001). Their senses and memory, however, seem to have been already as advanced as in the extant taxa, Dugong and Trichechus, and their last common ancestors (Benoit et al., Citation2013a, Citation2013b).
Reinterpretation of Age and Depositional Environment
The environment of sedimentation for the Colón Formation has been interpreted as a deep sublittoral setting with slight reef development, and possible pockets within the infralittoral zone (Huelbes, 2014). However, based on the presence of decapod crustaceans, Varela and Rojas-Consuegra (Citation2011a) interpreted the setting as a restricted, shallow intertidal, near-estuarine deposit. This last interpretation does not agree with our observations.
The presence of the planktonic Globigerina sp. cf. G. ciperoensis, Globigerina praebulloides, Globigerinoides sp. cf. quadrilobatus, Globorotalia sp. cf. archeomenardi, and Cassigerinella sp. suggests open access to more neritic waters. These taxa also serve as index taxa, suggesting a late Oligocene–early Miocene age that is consistent with previous assessments (Franco et al., 1992; Huelbes, 2014). Heterostegina cf. antillea and Lepidocyclina sp. further corroborate this age (Boudagher-Fadel, Citation2008). Interestingly, in the previous lexicon, G. fohsiperipheroronda is reported (Franco et al., 1992). Similarly, Huelbes (2014) reports Globorotalia (Fohsella) fohsi for this formation. This is problematic for the age correlation because G. fohsi and G. fohsiperipheroronda are index species of the middle Miocene (stage M-9; Wade et al., 2011). We found specimens that may resemble these species in our thin sections, but they were not confidently identified due to their orientation and poor preservation. An effort is underway to clarify this issue (Orihuela and Viñola, in prep.). If these species are confirmed, the time range for this formation and the fossils described here may be extended to the middle Miocene.
The small benthic Bolivina and Uvigerina, along with the planktonic forms, suggest an open, deeper neritic-shelf environment. The presence of the angular quartz, rotaliids, and Textularia suggests proximity to an unrestricted offshore platform (Boudagher-Fadel, Citation2008; Holbourn et al., Citation2013; Poag, Citation2015). The larger benthic foraminifers (Boudagher-Fadel, Citation2008, Citation2008), the planktonic forams Globigerina praebulloides (Iturralde, Citation1967, Citation1969a, Citation1969b), plus the bryozoans, corals, and decapods, suggest a warm tropical, relatively shallow shelf, forereef setting (Boudagher-Fadel, Citation2008; Holbourn et al., Citation2013). Thus, we interpret that the environment of deposition was not very deep, and although it may have been near tidal or infralittoral zones, these fossils likely deposited in a more offshore, back reef-ramp region.
Altogether, our preliminary study of the microfauna from the thin sections considered with the known vertebrate and invertebrate fauna (Jiménez Vázquez et al., Citation2014; Rojas-Consuegra and Viñola, Citation2013; Viñola and Rojas-Consuegra, Citation2016; Viñola et al., Citation2017) suggests a shallow, tropical, likely unrestricted marine depositional environment, such as a shallow shelf, with isolated keys and reefs (especially forereef and nearby tidal settings). Fibrous and blocky cement observed in the thin sections is suggestive of normal salinity (Scholle and Ulmer-Scholle, Citation2003).
CONCLUSIONS
Three sirenian endocranial casts were collected from the early Miocene shallow-marine limestones of the Colón Formation. These endocasts are similar in size and share several discrete characters with the dugongid family but are difficult to allocate taxonomically due to the lack of comparative material. The phylogenetic analysis suggests that all three could belong to two or more undescribed species or genera within the Dugongidae, specifically nested within the subfamily Dugonginae. Because of this limitation, we consider them here to be Dugongidae incertae sedis.
The structures observed on the complete natural endocast suggest that this sirenian taxon had similar sensory characteristics to the extinct and extant sirenians compared here. Likely as an underwater adaptation, it had poor or limited eyesight and smell but was better at hearing and sensory-tactile stimuli. In addition to our interpretation of the geological setting, this taxon likely inhabited shallow, warm marine environments such as inner neritic, shallow platforms or an embayment with proximal reef ecosystems and muddy-sandy substrate where abundant benthic organisms existed. These specimens are the first sirenian endocranial casts reported from the West Indies and provide new insight into the history of sirenian evolution, particularly dugongid diversity in the Caribbean during the early Miocene.
ACKNOWLEDGMENTS
We thank A. Tejedor for the revision of an earlier manuscript. E. Robinson, F. Maurrasse, L. S. Collins, and J. Alvarez Licourt for their guidance, generally stratigraphy and microfossils, particularly foraminifera, and use of their paleolaboratories. We also thank O. Jiménez and D. Domning for suggestions and guidance, E. Abreu, O. Gil, and R. Figueroa for discussing their observations on fossil occurrence at the quarries, the quarry workers who discovered the fossils discussed here, M. Voss for answering questions regarding specimen SMF-M3921, and E. Przybyszewski for his help with the phylogenetic analysis. We also thank J. Vélez-Juarbe, an anonymous referee, and the handling editor O. Lambert.
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APPENDIX 1. Phylogenetic characters. Unless noted otherwise, character numbers follow Macrini et al. (Citation2007). Characters 1, 9, 10, 13, and 14 seem to be the most diagnostic and useful phylogenetically among the taxa compared (Appendix 2).
Olfactory bulbs, when viewed from an anterior perspective (anteroposteriorly): elongated, compressed laterally, resulting in an elliptical outline (0); circular or spherical in outline (1). This is character 21 in Furusawa (Citation2004). Our specimen scored as a ‘0’ because the olfactory bulbs when viewed in this aspect have an elliptical outline (, ). Kaupitherium sp., Dusisiren jordani, Hydrodamalis cuestae, Eotheroides aegyptiacum (Gingerich et al., Citation1994:fig. 2; synonym E. libycum from Owen, Citation1875; NHMUK 46722), Metaxytherium sp. (MGB 30.531, described by Pilleri Citation1989, Citation1990, from the Miocene of Cerro Gordo, Spain), and Trichechus manatus (Edinger, Citation1939:pl. II) were scored as ‘0.’ Hydrodamalis spissa and H. gigas were scored as ‘1’ for having circular outlines. Based on our work on AMNH 53939 (), Trichechus senegalensis was scored as ‘1.’ This character seems to be polymorphic within the Hydrodamalinae (Furusawa, Citation2004:fig. 2). See Appendix 2 for comparison of character scores.
FIGURE 9. Digital rendering of AMNH 53939, a cranial endocast of Trichechus senegalensis, in A, dorsal, B, anterior, C, ventral, D, posterior, E, left lateral, and F, right lateral views. Sylvian fissure = pseudo-lateral sulcus of Furusawa (Citation2004); median sulcus = fissure longitudinalis cerebri of Furusawa (Citation2004). All images shown to the same scale. Abbreviations: Cb, cerebellum; Cbr, cerebrum; CE, cavum epiptericum; FL, frontal lobe; Fmc, foramen magnum cast; Hypf, hypophyseal fossa; iam, internal auditory meatus; MS, median sulcus; NT, trigeminal nerve (cranial nerve V); Ob, olfactory bulb; Obt, olfactory tracts; PL, parietal lobe; RhF, rhinal fissure; SF, Sylvian fissure; SphF, sphenorbital region; TL, temporal lobe; Tr, temporal region; TS, transverse sinus.
Accessory olfactory bulbs: absent (0); visibly present (1). This is character 2 of Macrini et al. (Citation2007). All taxa were scored as ‘0.’ See .
Olfactory bulb tracts: not present or visible on endocasts (0); visible (1). These are defined as the projections that lead to the telencephalon, following Butler and Hodos (Citation1996). Our specimen was scored as ‘1’ because the tracts are visible and traceable on the ventral surface of the endocast, leading up to the hypophyseal fossa (). The other taxa mentioned above also scored as ‘1.’ Metaxytherium sp., the dugong Trichechus, and Eotheroides all scored as ‘1.’ The Hydrodamalinae seem to also be ‘1,’ but this state is questionable or undefined (?) because Furusawa’s (2004:fig. 2) illustrations are not clear in this aspect.
Circular fissure: absent (0); present (1). The circular fissure results from a ventral projection of the frontal bone called the annular ridge. All taxa were scored as ‘0.’ In other mammals, this feature separates the olfactory bulb tracts from the rest of the brain. This feature is absent on all the sirenians mentioned above. The olfactory bulbs (and olfactory system in general) of sirenians are greatly reduced compared with terrestrial mammals (Berta, Citation2006; Pihlstrom, Citation2008); consequently, the bulbs do not contact the roof of the cranial cavity. Therefore, there is no impression from the annular ridge of the frontal bone.
Surface of the cerebral hemisphere casts: smooth or lissencephalic (0); gyrencephalic (1). All taxa were scored as ‘0.’ Few shallow sulci have been described for Dugong dugon (Husar, Citation1978), suggesting that low gyrification, and thus likely lissencephaly, is the normal state in dugongids. A similar condition is observable in manatees (Trichechus). Lissencephaly seems to be the plesiomorphic condition in Mammalia (Kielan-Jaworwska et al., 2004). This condition is especially consistent in the endocasts of some large mammals, such as proboscideans, cetaceans, and hominids. This is because their brain is normally encapsulated with thick meninges, cisterns, or dural sinuses that separate the brain from the braincase bones, limiting the imprint of the gyri of the brain on their endocasts (Bauchot and Stephan, Citation1967; Macrini et al., Citation2007).
Rhinal fissure cast on endocast: absent (0); present (1). This is character 6 of Macrini et al. (Citation2007). This structure is defined as the lower border of the neocortex where it meets the piriform cortex, following Rowe (Citation1996) and Macrini et al. (Citation2007). This fissure is not always visible on endocasts, but its absence (state 0) is considered the ancestral condition (Macrini et al., Citation2007). Our endocast scored as a ‘1’ for having a marked rhinal fissure. In the lateral view, it lies above the trigeminal nerve cast and cavum epiptericum. It is present in Eotheroides aegyptiacum, Hydrodamalis spissa, the other Hydrodamalinae examined by Furusawa (Citation2004), and Metaxytherium sp. from Cerro Gordo (Pilleri, Citation1990). However, the rhinal fissure on this specimen of Metaxytherium sp. is not well marked, maybe because of the preservation of the specimen. This feature is also present in endocasts of Trichechus (three species; ).
Inferior sulcus on the endocranial cast: absent (0); present (1). This structure results from a robust crista galli of the ethmoid (Edinger, Citation1939; Gingerich et al., Citation1994). All of the ingroup taxa were scored as ‘1.’ This sulcus is only on the underside of the endocast (see ) and therefore cannot be caused by the falx cerebri. Instead, it seems to be a result of an enlarged crista galli of the ethmoid (see Gingerich et al., Citation1994). The falx cerebri causes the median sulcus on the superior side of the endocast, but this sulcus is not continuous with the inferior sulcus.
Extent of the inferior sulcus: anterior to the trigeminal nerve cast, thus not reaching it (0); reaching its vertices (1). Our specimen and Metaxytherium sp. (Pilleri et al., Citation1988; Pilleri, Citation1989, Citation1990) were scored as a ‘0’ for not reaching the origin of the trigeminal nerve (CN V), being instead at the level of the optic nerve (CN II). Eotheroides aegyptiacum, Hydrodamalinae (Furusawa, Citation2004), Dugong dugong, Trichechus manatus, and T. senegalensis were scored as a ‘1’ (). The inferior sulcus likely results from a robust crista galli of the ethmoid as described for Protosiren fraasi and Eotheroides aegyptiacum (Edinger, Citation1939; Gingerich et al., Citation1994).
Lateral extent of the cerebral hemisphere casts: less or up to the cast of the temporal-lateral/parafloccular region (0); extends well beyond it (1). This is a modified version of character 7 of Macrini et al. (Citation2007). MNHNCu. P71.005310, Trichechus, and Dugong were scored as ‘0.’
Superior sagittal sinus or medial sulcus: not visible on the dorsal surface of endocast (0); visible but very shallow (1); visible and deep (2). This is a modified version of character 8 of Macrini et al. (Citation2007). The hydrodamalines Hydrodamalis cuestae (1?), H. spissa, and H. gigas (1) were scored as ‘1.’ MNHNCu. P71.005310, Trichechus, Dugong, Metaxytherium sp. (Pilleri, Citation1988, Citation1990), Eotheroides aegyptiacum, and the hydrodamalines Kaupitherium sp. and Dusisiren jordani (Furusawa, Citation2004) were scored as ‘2.’ However, as with other characters, the preservation state and degree of detail of the endocast can greatly influence the score of a feature. A shallow median sulcus is described for Protosiren fraasi, but not a bony falx cerebri (Edinger, Citation1939; Gingerich et al., Citation1994).
Parietal lobes: less than or equal to the temporal lobe height (0); higher (1). All taxa were scored as ‘1,’ which is especially apparent in Hydrodamalis gigas (UCMP 23050) (Furusawa, Citation2004).
Sylvian sulcus/fissure: absent or not visible on the endocast (0); present or visible (1). We scored MNHNCu. P71.005310 as ‘1,’ although it is faintly represented on the endocast. State 1 is well represented in the genera Trichechus and Dugong (1), but faint in Protosiren fraasi, Eotheroides, Metaxytherium, and the Hydrodamalinae, apart from H. spissa (TMNH 0001; Furusawa, 1994), in which the Sylvian sulcus of which seems to be pronounced.
Swelling anterior to the Sylvian fissure: absent (0); present (1). We scored MNHNCu. P71.005310 as ‘1’ based on the swelling anterior to the cast of the frontal lobe, best seen in dorsal and anteroposterior views (). Eotheroides and Kaupitherium (Gingerich et al., Citation1994; Furusawa, Citation2004) were also scored as ‘1.’ The area is not visible on the casts of Metaxytherium sp. (Pilleri, Citation1990) and the Hydrodamalinae described in Furusawa (Citation2004).
Depth of the transverse sulcus (called differently in Owen, Citation1875, in Gingerich et al., Citation1994:fig. 2:45), which separates the cerebellum from the parietal-temporal lobes: shallow transverse sulcus (0); deep sulcus (1). We scored MNHNCu. P71.005310 as ‘1’ because the sulcus separating this region is wide and deep. The same condition pertains to Metaxytherium sp. (Pilleri, Citation1990) and Kaupitherium. This sulcus is tightly ‘V’-shaped in Trichechus when seen in anteroposterior view, but wider (i.e., the ‘V’ shape is more opened) in Dugong. Following Gingerich et al. (Citation1994), Protosiren frassi was scored as ‘0.’
Exposure of the midbrain (superior and inferior colliculi) on the dorsal surface of endocast: absent (0); or present (1). This is character 11 of Macrini et al. (Citation2007). All ingroup taxa were scored as ‘0.’
Petrosal impression extent: well anteriorly after the transverse sulcus and posteriorly up to just before the cerebellum (0); from the transverse sulcus and below, reaching the posterior maximum extent of the cerebellum (1). MNHNCu. P71.005310, Trichechus, and Eotheroides, were scored as ‘1.’ Metaxytherium sp. from Cerro Gordo was tentatively scored as ‘1.’ Dugong was scored as ‘0.’ Hydrodamalinae could not be assessed for this feature. Because the petrosal often separates from the skull of sirenians after death, the petrosal region is not often well preserved on endocasts. This structure is quite intricate and detailed in MNHNCu. P71.005310, but we have no specimens to compare it further.
Depth of the hypophyseal fossa relative to length: deeper than long, aspect ratio >1 (0); longer/wider than deep, aspect ratio <0.9 (1); or approximately equal (2). We scored MNHNCu. P71.005310 as ‘1.’ Deeper than wide seems to be the plesiomorphic condition (Macrini et al., Citation2007).
FIGURE 10. Ventral reconstruction of dugongid specimen MNHNCu. P71.005310. Abbreviations: CE, cavum epiptericum in blue; Hypf, the hypophyseal fossa in pink; L, extent of the inferior sulcus; NO, optic nerve (cranial nerve II) in yellow; NT, trigeminal nerve in blue; Ob, the olfactory bulb in green; Obt, olfactory tract in green. See Appendix 1 and 2.
