Contextual evolution on islands

On islands, large mammals get small and small mammals get large. However, there are many exceptions with a lot of scatter around the general, graded trend. So what with the island rule? What causes these deviations? Our idea is that insular body size of mammals results from various selective forces whose influence varies not only with characteristics of the focal islands and the focal species, but also with interactions among species (ecological displacement and release). Our results, based on regression tree analyses, support this hypothesis of contextual body size evolution of insular mammals. While there may exist a theoretical optimal body size for mammals, in general, the optimum for a particular insular population varies in a predictable manner with characteristics of the islands and the species, and with interactions among species. This study did, however, produce some unanticipated results that merit further study – patterns associated with Bergmanns rule are amplified on islands, and body size of small mammals appears to peak at intermediate and not maximum values of latitude and island isolation. Lomolino MV, Sax DF, Palombo MR, van der Geer AAE. Of mice and mammoths: evaluations of causal explanations for body size evolution in insular mammals. Journal of Biogeography 39 (5): 842-854. http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2699.2011.02656.x/abstract

Evolution of Island Mammals

Our book on the Evolution of Island Mammals, Adaptation and Extinction of Placental Mammals on Islands is out! Wiley-Blackwell published it this August. For a quick overview of contents and artwork, visit the online library of Wiley (move mouse over book cover shown to the right of this post). A second option is simply to order it.

If you are interested in the evolution of insular mammals, from the Eocene walking sirenian (Pezosiren portelli) of Jamaica to the recently extinct Falkland Wolf or Fox (Dusicyon australis) of the Falkland Islands and the still living island fox (Urocyon littoralis) of the Californian Channel Islands, this book is a must. It offers a complete overview of all fossil and most recently extinct mammals that once upon a time lived on islands somewhere on our planet. They were fully adapted to their environment, and often evolved bizarre features, like elongated, club-like antlers with hardly any tines (the deer Candiacervus of Crete), ever-growing front teeth (the bovid Myotragus of Majorca), enormous size (the cavia-like Amblyrhiza of the West Indies) or pygmy size (the hominid Homo floresiensis of Flores).

Unfortunately, the majority of them went extinct, often after spectacular long periods of gradual evolution in situ, when mainland colonisers discovered the islands and their fauna. Today, just a few islanders survived, in comparison with the number of islanders of the remote past. Their special features are unique, but in most cases less spectacular than seen in the fossil record, when elephants could shrink till a mere one or two percent of the body mass of their ancestral size (as in the case of Elephas falconeri).

To have an idea of how extreme evolution can be, you have to see the fossil islanders!

Dental eruption sequence of a fossil 'baboon' (Paradolichopithecus arvernensis)

The fossil remains of this large-sized 'baboon' found at Vatera, Lesvos, Greece (Plio-Pleistocene boundary, or Late Pleistocene now that the boundary has been moved up again) includes a mandible of a young male with unerupted wisdom molar. We X-rayed the specimen to reveal the unerupted elements and the mineralization of their crowns. The chronological, or individual age of our Paradolichopithecus at the time of its untimely death is estimated at between 5.0 and 5.3 years of age, based on eruption patterns and root formation times of similar-sized living papionins. Mandrills, yellow baboons and Japanese monkeys are in their puberty at this age and stage of dental development, from which we conclude that our male was in his puberty as well. During this period, young males disperse from their natal groups and live at the periphery of their troupes. Risks of predation, disease, and injury are higher than before; a quarter of male mandrills dies before reaching adulthood. Among known Paradolichopithecus specimens, nearly half have died before the third molars erupted. This may be explained either by some taphonomic factor or due to higher mortality levels during puberty.
We further found that the eruption sequence of the permanent mandibular dentition of this male is {m1 i1–2 m2} p4, p3, c, m3. The order of the already fully erupted elements (between curly brackets) is based on data from the living baboons, mandrills, macaques and geladas. The p4 p3 sequence as seen in our Paradolichopithecus occurs at high frequency is other papionins as well: Macaca nemestrina, M. mulatta, Mandrillus sphinx, Papio cynocephalus, but not in Papio anubis, Macaca fuscata and M. fascicularis. Theearlier root formation of p4 seems thus not to be related with body size or phylogeny. There is a considerable delay in the canine development relative to the premolars, as in other papioninmales; the m3 is delayed in formation relative to the premolars and the canine. In total, the dental eruption sequence of Paradolichopithecus is very similar to that of the living papionins.

Read more in Van der Geer, A.A.E., Dermitzakis, M. 2008. Dental eruption sequence in the Pliocene Papionini Paradolichopithecus arvernensis (Mammalia: Primates) from Greece. Journal of Vertebrate Paleontology 28 (4): 1238-1244. (Ask a pfd, geeraae@geol.uoa.gr)

'Hobbit' skull found in Indonesia is not human indeed

Since its first description in 2004, Homo floresiensis, or the Hobbit of Flores, has been attributed to a species of its own, a descendant of Homo erectus, Homo ergaster or another early hominid, such as Australopithecus. Non-believers however hold the new species for a pathological form of modern humans, Homo sapiens, or just a dwarf human like the Neolithic inhabitants of the very same island. Karen Baab and colleagues applied landmarks on the skull, and concluded Homo floresiensis is a species on its own, and related to hominins of 1.5 million years ago. We did the same, half a year earlier, and reached similar conclusion but dare to put one step further. We applied geometric morphometric analysis to the type skull of Homo floresiensis (LB1) and compared it with skulls of normal Homo sapiens, insular Homo sapiens (Minatogawa Man and Neolithic skulls from Flores), pathological Homo sapiens (microcephalics), Asian Homo erectus (Sangiran 17), African Homo habilis (KNM ER 1813), and Australopithecus africanus (Sts 5). Our analysis includes specimens that were highlighted by other authors to prove their conclusions. The geometric morphometric analysis separates the 'hobbit' from all modern humans, thus including both the pathological and the insular forms. It is further impossible to separate the 'hobbit' skull from Homo erectus. The very early hominin Australopithecus falls separately from all skulls.

Visual inspection of the skulls learned that the cranial shape of Homo floresiensis is most close to that of Homo erectus and not to that of any modern human. Apart from cranial shape, some features of Homo floresiensis are not unique but are shared with other insular taxa, such as the relatively large teeth (shared with Early Neolithic humans of Sardinia), and changed limb proportions (shared with Minatogawa Man).

We thus conclude that Homo floresiensis is a direct descendant of Asian Homo erectus and has no relation neither to primitive australopithecines nor to modern Neolithic pygmy people of Flores.

By G.A. LYRAS, M.D. DERMITZAKIS, A.A.E. Van der GEER, S.B. Van der GEER, J. De VOS. 2008. The origin of Homo floresiensis and its relation to evolutionary processes under isolation.© 2008 The Anthropological Society of Nippon

For free pdf, click here http://users.uoa.gr/~glyras/projects/Homo-floresiensis.pdf

Or go to the publisher http://www.jstage.jst.go.jp/browse/ase

The effect of insularity on the five-horned deer Hoplitomeryx (Late Miocene, Italy)

Island studies increase our understanding of the effects of habitat fragmentation. The study of the Tertiary paleo-island Gargano is an important contribution, because of the long-term isolation under less fluctuating climatic conditions, free from anthropogenic influences; such a situation does not exist in the Quaternary period nor in the Holocene period. This makes the Gargano a unique case to study the effects of insularity in isolation. Here, a highly endemic, unbalanced vertebrate fauna evolved including the five-horned deer Hoplitomeryx. Its post-cranial material contains four size groups, based on the metapodals. In this study, the humerus and radius are described. The question whether the morphotypes are chronomorphs or ecomorphs is addressed. Sexual dimorphism is ruled out as the underlying principle of size separation in this case, based upon body mass estimations and data from living deer. Chronomorphs is the best explanation for the Megaloceros cazioti lineage (Pleistocene, Sardinia) and the Myotragus balearicus lineage (Pliocene–Holocene, Mallorca). Ecomorphs are a better explanation for the size groups of Candiacervus (Late Pleistocene, Crete) and Cervus astylodon (Late Pleistocene, Ryukyu Islands, Japan). An adaptive radiation into several trophic types took place, promoted by the ecological meltdown of the ancestral niche. The drive behind this speciation is increased interspecific competition. For Hoplitomeryx, although the hypothesis of chronomorphs cannot be discarded, that of ecomorphs seems most likely, based upon the coexistence of two or more size groups per fissure, and upon the presence of a huge morphotype, larger than mainland species, in the younger fissures.

Read more in VAN DER GEER A.A.E. (2008). The effect of insularity on the Eastern Mediterranean early cervoid Hoplitomeryx: the study of the forelimb. Quaternary International 182, 1: 145-159. See http://dx.doi.org/10.1016/j.quaint.2007.09.021 or ask me a pdf (geeraae@geol.uoa.gr).

For more general information of this enigmatic Late Miocene 'deer', see my Wikipedia page at http://en.wikipedia.org/wiki/Hoplitomeryx

Locomotor behavior of Paradolichopithecus arvernensis as inferred from the functional morphology of its ankle and elbow

Taking all ankle and elbow elements of Paradolichopithecus into account, the picture emerges of a highly terrestrial monkey. This is not surprising as many fossil cercopithecines are found in open country habitats and show terrestrial adaptations, such as Dinopithecus (Late Pliocene, Africa), Procynocephalus (Late Pliocene, China and India), Paradolichopithecus (Pliocene, Spain and Asia), Theropithecus (Middle Pleistocene - Holocene, Africa) and, among the colobines, Paracolobus (Pliocene, East Africa) and Dolichopithecus (Pliocene, Europa) (Szalay & Delson, 1979). In addition, the larger species tend to be terrestrial, possibly as a response to predator pressure. This, too, makes a terrestrial adaptation of our large Paradolichopithecus very probable.
Body weight was carried more posterior, as the architecture of the olecranon and the trochlear notch are less apted for sustaining heavy load than is the case in the extant baboons. The morphology of the arm indicates an increased mobility in the elbow joint, with a departure from the sagittal plane during flexion. Paradolichopithecus could very well have used his strong arms for carrying food while walking or standing. Another option is the use of the arms in fights and defense.
The massive medial malleolus of the tibia also shows that a larger (part of the) body weight was carried on the hindlimbs. The suspensory facet for the fibular malleolus indicates an increased importance of the lateral malleolus in transferring body weight, and an increased fixation of the talus in the malleolar fork, formed by both the malleoli together.
As to the ankle joint, a remarkable parallel is seen with Australopithecus. Unique features that distinguish Paradolichopithecus, and probably also Procynocephalus, from the other papionins are seen also in Australopithecus, though the overall architecture of the Paradolichopithecus talus is typically cercopithecoid (pronounced lateral trochlear ridge, hardly developed groove for large toe flexor), whereas it is typically hominoid for Australopithecus (symmetrical trochlea, pronounced large toe flexor).The terrestrial traits in the postcranial elements show that this large monkey was clearly adapted to the habitat: an open savanna/bushland environment with seasonal availability of food, and large distances between the food sources.

Read more in SONDAAR P.Y., VAN DER GEER A.A.E., DERMITZAKIS M. (2006). The unique postcranial of the Old World monkey Paradolichopithecus: more similar to Australopithecus than to baboons. Hellenic Journal of Geosciences 41, 1: 19-28. Special volume in the memory of P.Y. Sondaar

and in VAN DER GEER A.A.E., SONDAAR P.Y. (2002). The postcranial elements of Paradolichopithecus arvernensis (Primates, Cercopithecidae, Papionini) from Lesvos, Greece. Annales Géologiques des Pays Helléniques 1e Série 39, A: 71-86. Free pdf at http://users.uoa.gr/~geeraae/publications/2002-agph-Paradolichopithecus.pdf .

and in SONDAAR P.Y., VAN DER GEER A.A.E. (2002). Arboreal and terrestrial traits as revealed by the primate ankle joint. Annales Géologiques des Pays Helléniques 1e Série 39, A: 87-98. Free pdf at http://users.uoa.gr/~geeraae/publications/2002-agph-terrestriality.pdf .

The postcranial of the deer Hoplitomeryx (Mio-Pliocene; Italy): another example of adaptive radiation on Eastern Mediterranean Islands.

During the Late Miocene a highly endemic vertebrate fauna evolved on Gargano Island (south-east coast of Italy), comprising others the giant soricid Deinogalerix, the giant barn owl Tyto gigantea, the giant hamster Hattomys, and the 'prongdeer' Hoplitomeryx with five horns and sabrelike ('moschid' type) upper canines. The Hoplitomeryx skeletal material forms a heterogenous group, containing four size groups; within the size groups different morphotypes may be present. All size groups share the same typical Hoplitomeryx features. These are: one central nasal horn and a pair of pronged orbital horns, protruding canines, complete fusion of the navicocuboid with the metatarsal, distally closed metatarsal gully, a non-parallel-sided astragalus, and an elongated patella. The different size groups are equally distributed over the excavated fissures, and are therefore not to be considered chronotypes. The hypothesis of an archipelago consisting of different islands each with its own morphotype cannot be confirmed.
The situation with several co-existing morphotypes on an island is paralleled by Candiacervus (Late Pleistocene, Crete, Greece). Opinions about its taxonomy differ, and at present two models prevail: one genus for eight morphotypes, or alternatively, two genera for five species. The second model is based upon limb proportions only, but these are invalid taxonomic features for island endemics, as they change under influence of environmental factors that differ from the mainland. Also in Hoplitomeryx the morphotypes differ in limb proportions, but here different ancestors are unlikely, because in that case they all ancestors must have shared the typical hoplitomerycid features. The morphosphere of Hoplitomeryx is too coherent to assume two or more different ancestors, and indicates a monophyletic origin of all morphotypes.
The large variation is instead explained as an example of adaptive radiation, starting when the Miocene ancestor colonized the island. The range of empty niches promoted its radiation into several trophic types, yielding a differentiation in Hoplitomeryx. The shared lack of large mammalian predators and the limited amount of food in all niches promoted the development of derived features in all size groups (apomorphies).

For full text, see VAN DER GEER, A.A.E. (2005). The postcranial of the deer Hoplitomeryx (Mio-Pliocene; Italy): another example of adaptive radiation on Eastern Mediterranean Islands.van der Geer. Monografies de la Societat d'Història Natural de les Balears 12: 325-336. Palma de Mallorca. For a free pdf [1,018 kb]: http://users.uoa.gr/~geeraae/publications/2005-IMEDEA-Hoplitomeryx.pdf. See my website http://users.uoa.gr/~geeraae for three more publications on this bizarre and enigmatic insular 'deer' of the Late Miocene.

For more general information of this enigmatic Late Miocene 'deer', see my Wikipedia page at http://en.wikipedia.org/wiki/Hoplitomeryx