CHAPTER 8
CONCLUSIONS
In this PhD dissertation the evolution of the Neogene Euro-North African sirenian species has been investigated through time and space. A better chronostratigraphic setting, a review of the main specimens, a cladistic analysis, a palaeoecological study and a palaeobiogeographic reconstruction have been provided.
The systematic study reveals the existence of nine valid species along the Euro – North African coasts during the Neogene plus the Late Oligocene “Halitherium” bellunense. For each species, chronostratigraphic and palaeogeographic distribution; systematic position; palaeoecology (established on the basis of cranial features, C and O stable isotopes analysis and depositional environment) have been discussed.
“Halitherium” bellunense is known from only one specimen, referred to the Chattian
(Upper Oligocene), and not to the Lower Miocene as previously considered. It is regarded as a dugongine according to Domning (1989b; 1996) and the cladistic analysis establishes its systematic position as sister taxon of the clade Corystosiren varguezi + Rytiodus spp. In fact, “Halitherium” bellunense shows a modified nasal process as in Rytiodus and Corystosiren, together with a certain specialization in tusks. Its δ13C and δ18O values are respectively 2.03‰ and 27.17‰, indicating a specialized seagrass diet
and a mixture of freshwater to marine habitats, according respectively with its anatomical features and its depositional environment. In fact, the animal lived in a sheltered inland sea in front of a delta system (in the Belluno syncline, northern Italy); even if it moved between marine and fresh waters, it fed nearly exclusively on seagrasses like the other dugongine species.
This attribution demonstrates that the dugongines also lived along the Euro-North African coasts during the Palaeogene, and the use of tusks in excavating seagrass rhizomes could have evolved from the Late Oligocene onward in the derived lineage of dugongines including “Halitherium” bellunense, Rytiodus spp. and Corystosiren varguezi.
Miosiren kocki is known with confidence from only one specimen referred to the
Aquitanian, and not to the lower Burdigalian as previously considered. Miosiren kocki is confirmed to be a trichechid close to the Oligocene Anomotherium langewieschei,
forming the subfamily Miosireninae, as concluded by Domning (1994). Its cranial features are consistent with a diet including hard and/or abrasive food. This type of food could consist of molluscs with shells or abrasive plants like true grasses (Gramineae). The second hypothesis is consistent with the systematic position of Miosiren kocki. Miosiren kocki, in fact, belongs to the Trichechidae, a family now represented by the euryhaline mixed-feeding manatees. On the whole, the Trichechidae (or at least the subfamily Trichechinae) are considered a family specialized for eating abrasive plants such as true grasses. The miosirenines Miosiren kocki and Anomotherium langewieschei can be seen as adapted to abrasive foods by developing thick enamel, a very thick palate, and also a thick lamina orbitalis of the frontal; while the modern trichechines seem to have adapted to a similar type of diet by developing supernumerary molars with horizontal replacement. This last adaptation appears to be successful and the Trichechinae are still extant; on the contrary the Miosireninae appear to have died out during the Miocene. Stable isotope results indicate a mixture of freshwater to marine habitats (δ18O value of 27.94‰) and a mixed diet (δ13C value of -6.29‰), in accord with the systematic position of Miosiren kocki; but the δ13C value could be altered by inclusion of a milk signal, and it does not rule out a molluscan diet.
In conclusion, Miosiren kocki appears to have been a euryhaline mixed-feeder, living in isolation on the southernmost border of the North Sea Basin during the Early Miocene, moving between marine and fresh waters looking for, most probably, several types of plants including true grasses.
Prohalicore dubaleni is referred to the Middle Miocene, according to Domning (1996:
393), and specifically to the Serravallian and not to the Lower Miocene, as considered by Flot (1887). It is known only from a partial mandible, which does not show unambiguous differences compared with those of Anomotherium langewieschei; and therefore Prohalicore dubaleni is referred to the Trichechidae and in particular to the Miosireninae. According to this interpretation, the miosirenine record extends into the Serravallian.
The validity of the species Prohalicore dubaleni and its exact systematic position can not be established because of its fragmentary nature, the absence of differences compared with Anomotherium langewieschei, and the impossibility of comparison with Miosiren kocki because the mandible of this last species is unknown.
Rytiodus capgrandi from the Aquitanian – lower Burdigalian deposits of the Aquitaine
Basin (SW France) and Rytiodus sp. from the Aquitanian - Burdigalian deposits of the Sirte Basin (Libya) are confirmed to be two distinct dugongine species. In particular, the cladistic analysis assigns the genus Rytiodus to the dugongine clade including also “Halitherium” bellunense and Corystosiren varguezi. The Libyan Rytiodus sp. appears to be more derived in having a larger infraorbital foramen and in having lost the dorsal contact between premaxilla and frontal.
The genus Rytiodus is characterized by specialized broad, mediolaterally compressed, bladelike tusks and a modified nasal process of the premaxilla. It is regarded as a specialized seagrass feeder, using its tusks in excavation of seagrass rhizomes (Domning & Beatty, 2007). This interpretation is supported by δ13C and δ18O values for Rytiodus sp. from Libya, which are respectively 2.27‰ and 30.00‰ and indicate a specialized seagrass diet and a predominantly marine environment.
Metaxytherium krahuletzi, M. petersi, M. medium, M. serresii and M. subapenninum
are considered the only valid Euro-North African Metaxytherium species, according to Bianucci et al. (in press). On the basis of cranial features, these Metaxytherium species are regarded as generalized consumers of seagrass blades and rhizomes of smaller seagrass species, without using tusks in feeding; but the isotopic analysis indicates a wider feeding niche for this genus, with δ13C and δ18O values ranging respectively from
-5.1‰ to +3.8‰ and from 26.0‰ to 32.0‰. On the whole, these isotopic data corroborate and accentuate the generalist feeder role of the whole genus Metaxytherium. This dietary flexibility could have represented the key to its success, allowing Metaxytherium populations to adapt their diets on the basis of food resources present in their habitats. A generalist diet appears to be a derived condition in the genus Metaxytherium, whereas the primitive sirenians appear to be specialized on seagrasses as indicated by the high δ13C values of the Eocene and Oligocene sirenian specimens analyzed by MacFadden et al. (2004) and by Clementz et al. (2006). The only exception appears to be represented by the holotype of the oldest and most primitive sirenian species Prorastomus sirenoides (BMNH 44897) from the Lower Eocene deposits of Jamaica, which shows a relatively low δ13C value of –4.49‰ (Clementz, pers. comm.) indicating a mixed diet according to Savage et al. (1994), who, on the basis of the morphological features, regarded this species as a selective browser on floating and emergent aquatic plants and, to a minor degree, on seagrasses.
Metaxytherium krahuletzi has been recently revised by Domning and Pervesler (2001).
They recorded the species with confidence only from the Burdigalian Central Paratethys deposits of Austria and Switzerland and maybe also from Horné-Strháre (Slovakia) and Léognan (France). Moreover, they cited the skeleton, now lost, from the Burdigalian of Beaucaire (France) named Metaxytherium beaumontii by Christol in Blainville (1844) and supposed to represent M. krahuletzi by Depéret and Roman (1920:31); some specimens from Klattau (Bohemia, Czech Republic) referred by Pia and Sickenberg (1934) to M. krahuletzi; and two skull fragments from the Burdigalian of Gebel Zelten (Libya) described as Metaxytherium sp. indet. by Heal (1973), which might be referable to M. krahuletzi, but need further study.
In this study I also assign to Metaxytherium cf. krahuletzi a new French specimen (MPNRL-MAN2000) represented by a skull with associated thoracic vertebrae and ribs, from the late Burdigalian deposits of Manosque (Provence, southern France), the humerus from the upper Burdigalian of Quinta da Farinheira (Lisbon, Portugal) reported by Zbyszewski (1944), Metaxytherium aquitaniae from the Aquitanian deposits of northwestern France, and Metaxytherium catalaunicum from the upper Burdigalian – Langhian deposits of southeastern Spain.
On the whole, the range of M. krahuletzi is extended from the Aquitanian to the upper Burdigalian – lower Langhian; and along the European Atlantic coasts too.
Metaxytherium petersi is presently under study by Domning and Pervesler, so I do not
include it in my systematic and cladistic studies. It apparently evolved in isolation in the Carpathian Basin during the Middle Miocene as a short-lived and localized offshoot of the M. krahuletzi-M. medium stem (Domning, pers. comm.),
Metaxytherium medium has been reviewed and its wide chronostratigraphic and
palaeobiogeographic distribution confirmed. The comparison reveals that the species appears to be morphologically indistinguishable from M. crataegense and very similar to M. floridanum; therefore a specific distinction could be questionable and the Euro-North African M. medium together with the West Atlantic – Caribbean M. floridanum and M. crataegense could represent three populations of only one species with a wide geographical distribution comparable to that of the living dugongine Dugong dugon.
Metaxytherium serresii has been recently reviewed by Carone and Domning (2007),
who described the new records from Calabria together with the known records from Libya and France. I can just report a rib cage from southern Spain, studied by Sendra et al. (1998). They did not offer a description of the specimen, but it falls in the chronostratigraphic range of M. serresii, having been found in the basal sediments of
the Pliocene Cuevas Formation dated between 5.25 and 4.98 Ma (Fortuin et al., 1995). According to this interpretation, this record could represent the oldest post-MSC M. serresii specimen, reducing the gap between pre- and post-MSC M. serresii records.
Metaxytherium subapenninum has been reviewed, with the inclusion of two new
Spanish records. It is considered the latest Metaxytherium species and also the latest halitheriine species, and close to M. serresii. Its resemblances to the Pacific species Metaxytherium arctodites and Dusisiren jordani are considered convergences due to a similar evolutionary trend.
On the whole, the Neogene Euro-North African sirenians appear to be fully aquatic herbivorous mammals strictly linked to coastal environments. They inhabited the Euro-North African coasts during a crucial period characterized by a long-term trend of climatic cooling (e.g. Zachos et al., 2001) and of decreasing marine influence with correlative reduction in size of the marine depositional domains (e.g. Rögl, 1998; Meulenkamp et al., 2000; Meulenkamp & Sissingh, 2003; Popov et al., 2004), whose repercussions on sirenian evolution can be observed in the fossil record:
During the Early Miocene, which was a period of global warming culminating in a climatic optimum from 17 to 15 Ma (Zachos et al., 2001), representatives of at least three different sirenian lines lived along the Euro-North African coasts: the species Miosiren kocki (Trichechidae, Miosireninae), living in isolation on the southernmost margin of the North Sea; the generalist small-tusked species Metaxytherium krahuletzi (Dugongidae, Halitheriinae), which appears to have been widespread along the Euro-North African coasts; and the specialized large-tusked genus Rytiodus (Dugongidae, Dugonginae), recorded with confidence just in the Aquitaine Basin (SW France) and in the Sirte Basin (Libya), where it most probably lived sympatrically with Metaxytherium krahuletzi.
From the Middle Miocene onward, however, the only sirenians surviving in the Euro-North African region belonged to the generalist cosmopolitan halitheriine genus Metaxytherium. The only exceptions are the miosirenine trichechid Prohalicore dubaleni and, maybe, a partial skull described as Miosiren canhami (Flower, 1874) and considered by Domning (1996) as a possible synonym of M. kocki, possibly reworked from Miocene deposits into the Red Crag Formation of England. Therefore, Trichechidae could have moved southward along the northeastern Atlantic coasts and
survived until the Serravallian, probably living together with the generalist Metaxytherium.
During the Late Miocene the generalist and cosmopolitan genus Metaxytherium became extinct throughout the world except along the Euro-North African coasts, and by the end of the Miocene it appears to have survived just in the Mediterranean Basin with the relictual dwarfed species M. serresii.
M. serresii specimens are recorded at about 7.24 Ma from southern Italian deposits; at about 6.8 Ma in Libyan deposits; at 5.25 - 4.98 Ma in Spanish deposits; and at 4 – 5.3 Ma in French deposits. The M. serresii record is here considered continuous from the uppermost Tortonian to the early Pliocene in the Mediterranean Basin. There is no evidence to support the disappearance of M. serresii from the Mediterranean Basin during the MSC and its recolonization after the crisis from the northeastern Atlantic coasts. A lot of data suggest that normal marine conditions were present in the Mediterranean during the evaporitic episode and in the later portion of the post-evaporitic ‘Lago-mare’ phase (see Chapter 7); therefore suitable habitats for sirenians could also have existed during the MSC. Moreover, stable isotope values for Metaxytherium species, and in particular for M. serresii, indicate a high adaptability in diet and habitats; therefore M. serresii appears to have been well adapted for living in small basins, lagoons and deltas, feeding on seagrasses and/or other aquatic plants. The dwarfism in M. serresii is here interpreted as a method to increase the fitness of a relictual species, permitted by geographic restriction and reduction in the genetic pool. Moreover, the increase in tusk size observed in M. serresii, in comparison with its supposed ancestors M. krahuletzi and M. medium, could denote a shift in behaviour, with tendencies to territoriality and competition for females and space.
In the Early Pliocene, immediately after the end of the Messinian, sirenians were still about the same small size, as documented by the Lower Pliocene record of Metaxytherium serresii from the sands of Montpellier (France). However, the subsequent, and latest, Metaxytherium species M. subapenninum was larger and also with larger and, probably, sexually dimorphic tusks.
The supposed sexually dimorphic tusks of M. subapenninum could be interpreted as a successive step in change in behaviour, an indication of competition for females and space that favoured the development of a sexual dimorphism similar to that observed in the living Dugong dugon. On the contrary, the rapid increase in body size could be
interpreted as a response to the long-term climatic cooling. In fact the Mediterranean region (like the Earth in general) underwent a net cooling during the Late Miocene and Pliocene (Zachos et al., 2001). Sirenians, with their low metabolic rates, would be expected to adapt to cooler climate by an increase in body size, as the hydrodamaline sirenians did in the North Pacific (Domning, 1978). Although Hydrodamalis survived into historic times, eating algae in cold waters, Metaxytherium did not. The anatomical feeding adaptations and the stable isotope analysis of the last Metaxytherium species M. subapenninum do not indicate a clear change in diet and habitat in comparison with the Miocene species. M. subapenninum did not respond to climatic cooling by changing its diet and habitat as the Hydrodamalinae did, and therefore it became extinct during the Middle Pliocene, most probably during the acceleration of climatic cooling recorded by Capozzi and Picotti (2003) after 3 Ma.
But the history of Euro-North African sirenians does not appear to be concluded; in fact, the remains cited by Ennouchi (1954), coming from the Pliocene deposits of the Atlantic coast of Morocco, are referred to the Piacenzian, and not to the Zanclean as previously considered. They represent the last northeastern Atlantic sirenian record, but they are too fragmentary for a specific assignment, being just referred here to a dugongid with teeth. They could represent a dugongine close to the Pliocene Caribbean-Western Atlantic dugongines; in fact the Moroccan skullcap, as illustrated by Ennouchi, bears more resemblance to that of Corystosiren varguezi (Domning, 1990) than to those of Metaxytherium (Domning, pers. comm.).
