Box The Messel Oil Shales Total Preservation Of Mammalian Fossils

The best-preserved fossils of mammals have been found in the middle Eocene (c. 49Myr ago) oil shales at Messel, near Frankfurt, Germany (Franzen, 1985; Schaal and Ziegler, 1992). All details of their hair, stomach contents and even internal organs are preserved in some cases.

The Messel deposits contain abundant plant remains - laurel, oak, beech, citrus fruits, vines and palms, with rare conifers, and ponds covered by water lilies, which indicate a humid tropical or subtropical climate. Invertebrate fossils include snails and insects, and fishes account for 90% of the vertebrate fossils. Rare frogs, toads and salamanders have been found, as well as six genera of crocodilians, several tortoises and terrapins, and some large lizards and snakes. The birds include dozens of species spanning most modern groups except passerines (e.g. Mayr and Daniels, 1998; Mayr, 2001).

The mammal fossils, although constituting only 2-3% of vertebrates found, have attracted most attention. Forty species belonging to 13 orders have been recorded so far. They include opossums, several primitive insect-eaters, a few true insectivores and rodents.

An unusual example of one of these mammals is Leptictidium, a small animal formerly classed with leptictids (see p. 329), but probably belonging to a related family (Rose, 1999). Leptictidium was a biped, standing only 200 mm tall, that dashed about like a long-tailed leprachaun (see illustration I). Three nearly complete skeletons (Storch and Lister, 1985) show that it has a long tail, a strong but short trunk region and relatively long hindlimbs and short forelimbs. The long tail suggests a balancing function, as in bipedal dinosaurs, and the short strong trunk also points to an ability to balance. Leptictidium was probably a facultative biped: it ran and walked on its hindlegs, but could have adopted a quadrupedal posture for slow locomotion and standing.

Trunk Region FrogPrehistoric Bipedal Mammals

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I The tiny bipedal insectivorous mammal Leptictidium from the Messel deposits, restoration of its running style. (After Storch and Lister, 1985.)

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I The tiny bipedal insectivorous mammal Leptictidium from the Messel deposits, restoration of its running style. (After Storch and Lister, 1985.)

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The extraordinary conditions of fossilization at Messel have allowed detailed studies of the diet of Leptictidium. In one specimen, several dozen pieces of bone were found, some of which could be identified as limb bones and vertebrae of a small reptile, possibly a lizard. A second skeleton contained bones of a small mammal and another contained fragments of chitin from the exoskeleton of large insects. The gut regions also show a variety of plant fragments, so that Leptictidium had a very varied diet.

Other small mammals from Messel include six species of bats, some of which have scales from butterfly wings and beetle exoskeletons preserved in their stomachs. There are two species of lemur-like primates and four of squirrel-like rodents. Carnivorous mammals include a creodont and two miacids (see p. 348), and ground-dwelling herbivores include a 'condy-larth' (see p. 332), three perissodactyls (early horses and tapirs) and three artiodactyls (relatives of modern cattle and deer).

Creep Darkness Illustrations

II Exceptional preservation of mammalian fossils In the Messel deposits, Germany: (a) the early horse-like animal Propalaeotherium parvalum, shoulder height 350 mm; (b) the dichobunid artiodactyl Messelobunodonschaefferi, shoulder height 220 mm; (c) the insectivore Pholidocercushassiacus, length of head and trunk 190 mm, showing a clear silhouette of the fur. (Courtesy of Jens Franzen, with permission of the Natur-Museum Senckenberg.)

II Exceptional preservation of mammalian fossils In the Messel deposits, Germany: (a) the early horse-like animal Propalaeotherium parvalum, shoulder height 350 mm; (b) the dichobunid artiodactyl Messelobunodonschaefferi, shoulder height 220 mm; (c) the insectivore Pholidocercushassiacus, length of head and trunk 190 mm, showing a clear silhouette of the fur. (Courtesy of Jens Franzen, with permission of the Natur-Museum Senckenberg.)

Two of the most remarkable finds from Messel are the ant-eater Eurotamandua (see p. 323) and the pangolin Eomanis (see p. 358). The former might belong to a South American group and the latter to a south-east Asian, so that, if these animals have been correctly identified (Rose and Emry, 1993), central Europe must have been a migratory cross-roads for mammals in the Eocene.

The Messel site seems to represent an Eocene lake that filled with organic matter periodically. Cadavers of land animals were washed in and birds and bats fell into the lake and sank to the bottom. The anoxic bottom waters prevented putrefaction and scavenging and the corpses were slowly covered by organic clays and preserved as near-perfect fossils (illustration II).

Read more and see images of the spectacular fossils from Messel in colour at http://www.senckenberg.uni-frankfurt.de/ messel_neu/_AusstellungME2002.htm, http://senckenberg.uni-frankfurt.de/sm/messel.htm and http://palaeo.gly.bris.ac. uk/Palaeofiles/Lagerstatten/Messel/index.html most notably the 'double-pulley' astragalus, previously seen as unique to artiodactyls (see below). Mesonychids are more primitive in that regard.

10.9.1 Artiodactyla: cattle, deer and pigs

The even-toed ungulates, the artiodactyls, are characterized by having an even number of toes, two or four, unlike the perissodactyls, which have an odd number (1, 3, or 5). There were some basal artiodactyls in the Eocene, and then later forms fall into two main groups, the Suiformes, the pigs and hippos (unless whales fall here too), and the Selenodontia, the cattle, deer, giraffes, camels and antelopes (Gentry and Hooker, 1988).

The oldest artiodactyls were small, rabbit-sized animals that fed on fruit, seeds and leaves, and had toes 3 and 4 enlarged to bear most of the weight of the body. Diacodexis from the lower Eocene of North America, Europe and Asia (Rose, 1982, 1996) is a slender long-limbed animal (Figure 10.31(a)) that has a key artio-dactyl feature, a 'double pulley' astragalus, which allows controlled bending between the lower leg and the ankle and restricts movement to a vertical plane.

The limbs are long and slender, and Diacodexis may have moved by leaping. The limbs are otherwise primitive: the fibula is still present, although reduced, the ulna is also retained, as is the clavicle in the shoulder girdle. Diacodexis has five fingers on the hand and four toes, but the main weight of the body is expressed through digits 3 and 4, which each bear small hooves.

Diacodexis shows unique artiodactyl characters in the skull: the facial portion of the lacrimal is enlarged, the orbitosphenoid is expanded and separates the frontal from the alisphenoid, and in the lower molar teeth the trigonid is narrow because the paraconid and metaconid are placed close together.

Basal artiodactyls continued into the Oligocene,but a major radiation of new forms occurred in the late Eocene, the first members of the Suiformes and Selen-odontia (Gentry and Hooker, 1988; Janis et al., 1998; Gatesy et al., 1999;Matthee et al., 2001).

10.9.2 Suiformes: pigs and hippos

The pig and hippo line of artiodactyls, the Suiformes or Bunodontia, are characterized by bulbous cusps on their molar teeth and powerful canine teeth that are triangular in cross-section. These dental features relate to an essentially omnivorous diet. Suiforms radiated from the late Eocene, and achieved modest diversity before declining to only seven or eight genera today. During the Oligocene, North America was populated by giant pig-like animals called entelodonts. These 2-3-m-long animals had long heavy skulls (Figure 10.31(b)) and they may have fed on a broad range of plants (? and animals). The deep lappets on the zygomatic arch and the knobs beneath the lower jaw may have been associated with sexual display activity.

The Suidae, pigs, arose in the upper Oligocene of Europe, and the Tayassuidae, peccaries, date from the upper Eocene of North America and Europe. Per-choerus, an early peccary (Figure 10.31(c)) from the Oligocene of North America, has long canines, used in feeding and in fighting.

Diacodexis
Fig. 10.31 Early artiodactyls and pigs: (a) the basal Eocene artiodactyl Diacodexis; (b) the Oligocene entelodont Dinohyus; (c) the Oligocene peccary Perchoerus. [Figure (a) modified from Rose, 1982; (b) after Zittel, 1925; (c) after Scott, 1940.]

Anthracotheriids and hippos form another major suiform evolutionary branch. Anthracotheriids, known from the Eocene to Pliocene, originated in Asia and later spread to Europe, North America and Africa. The first anthracotheriids were small, but later ones became as large as pigmy hippos. Hippos themselves have a limited fossil record, dating back to the mid-Miocene in Kenya. Two species survive today, Hippopotamus itself, a semi-aquatic grazer, and the pigmy hippo, Choeropsis, a forest browser, both restricted to Africa south of the Sahara.

10.9.3 Selenodontia: camels, cattle and deer

In contrast to the bunodonts, the selenodonts achieved high diversity, and there are more than 70 living genera of camels, cattle, sheep and deer (Vrba and Schaller, 2000). Selenodonts are characterized by specialized cheek teeth (Figure 10.32(a)) that show the selenodont pattern: the molars are square in outline and the cusps form pairs of crescent-shaped ridges (selenodont means 'crescent-moon tooth') that were durable grinders, effective for side to side chewing of leaves. Selenodonts share a number of other characters: the upper incisors are reduced or missing (or may be enlarged into sabre-like structures for display in the males, especially hornless species), the lower incisors and canines are small, spatulate and procumbent (they stick out forwards), the feet have two main toes, the metacarpals and metatarsals are fused into cannon bones in derived forms (made from metapodials 3 + 4) and the stomach is compound and adapted for fermenting the food.

Oreodont Astragalus

Fig. 10.32 Tylopod selenodont artiodactyls: (a) ventral view of the skull of the late Eocene oreodont Bathygenys; (b) the Oligocene oreodont Merycoidodon; (c) the late Eocene camel Poebrotherium,skeleton and hindfoot in anterior view, showing the divergent toes 3 and 4. [Figure (a) after Wilson, 1971; (b,c) after Scott, 1940.]

Fig. 10.32 Tylopod selenodont artiodactyls: (a) ventral view of the skull of the late Eocene oreodont Bathygenys; (b) the Oligocene oreodont Merycoidodon; (c) the late Eocene camel Poebrotherium,skeleton and hindfoot in anterior view, showing the divergent toes 3 and 4. [Figure (a) after Wilson, 1971; (b,c) after Scott, 1940.]

The Selenodontia falls into two subdivisions, the Ty-lopoda and Ruminantia. Tylopods (camels, protocer-atids and oreodonts) share some diagnostic characters of the teeth, jaws, vertebrae and ankle, but the group may be paraphyletic. The first tylopod radiation occurred from the late Eocene to the Miocene with the oreodonts (Merycoidodontidae and Agriochoeridae) ofNorth America. These low, pig-sized animals (Figure 10.32(b)) have four toes on each foot and were probably not very fast-moving. Large numbers of oreodonts have been collected in the Big Badlands of South Dakota and they evidently wandered the early North American wooded savannas in huge herds, browsing on low bushes.

The protoceratids were also exclusively North American, from the late Eocene to the Pliocene. They were rather deer-like forms, but with shorter, more primitive types of legs, and are distinguished by evolving horns convergently with the Ruminantia—not only above the eyes but also in the form of a single, sling-shot shaped horn on the nose.

Relatives of the oreodonts include the camels and llamas. An early camel, Poebrotherium from the upper Eocene ofNorth America (Figure 10.32(c)),is a slender, goat-sized animal. Like all camels, it has a long neck, long limbs and two toes (3 and 4). It still has hooves on these toes, but by Miocene times camels had broad pads as in modern forms. It is an unexpected fact that most of the evolution of camels took place in North America (Harrison, 1985), and it was only in the late Miocene and Pliocene that they reached their present areas of North Africa and the Middle East, and then passed into South America (llamas) during the Great American Interchange (see p. 320). They became extinct in North America at the end of the Pleistocene.

The main selenodont group is Ruminantia, cattle, sheep, antelope, deer and mouse deer (Hassanin and Douzery, 2003), so-called because they all ruminate or regurgitate their food. The cow has a four-chambered stomach. A mouthful of grass enters the rumen and part of the reticulum, where it is partially broken down by bacteria (foregut fermentation). The food is returned to the mouth for rumination or 'chewing the cud' and it then passes through the other two stomachs, which allows a cow to extract the maximum nutritive value from its food. Camels also have a primitive ruminating system, but other plant-eaters, such as pigs, rhinos and horses, lack the two-stage fermentation process. It has been suggested that the artiodactyls, and the ruminants in particular, proved so successful in comparison with the perissodactyls because of their amazing digestive system. The case is not proven, however (see Box 10.9).

Ruminants have also reduced or lost their upper incisors and have only a horny pad against which the lower incisors nip off food items. Hypertragulus, an early form from the upper Eocene and Oligocene of North America, is a small, rabbit-sized animal that shows the ruminant horny pad (Figure 10.33(a)). Its lower canine teeth look like incisors and the first pre-molars have taken on the canine role.

The early ruminants, the traguloids (a paraphyletic assemblage including the relatives of the modern mouse deer), were small, hornless animals that were fairly common until the early Miocene when the modern groups radiated (Scott and Janis, 1993). These, the pecoran ruminants, deer, giraffes, cattle and antelopes, nearly all have horns of one kind or another (Figure 10.33(b—g)): a bony horn core that is surrounded by a permanent horny sheath (cattle), a bony structure that is shed annually (deer antlers), permanent bony horns covered with skin (giraffes), or a bony nose prong whose outer sheath is shed (pronghorns). These types of horns probably evolved independently in the three main groups of ruminants as fighting structures. Males of the ruminant groups use their horns in head-butting (sheep) or 'antler-wrestling' (deer), which may follow displays establishing social dominance rank, winning females and patrolling feeding territories. Other plant-eaters such as horses or camels do not have horns or antlers because they live in open grasslands and eat less clumped food resources, so that territories are unnecessary (Janis, 1986).

10.9.4 Cetacea: evolution of the whales

The whales (Cetacea) are some of the most spectacular living mammals. Looking at a great blue whale, 30 m long, or a fast-swimming dolphin, it is hard to imagine how they evolved from terrestrial mammal ancestors, and yet that is what happened (Thewissen, 1998). One of the oldest known whales, Pakicetus from the lower Eocene of Pakistan (Gingerich and Russell, 1981), has a long-snouted skull with primitive carnivorous teeth lining its jaws (Figure 10.34(a)).The skeleton of Pakice-tus is incompletely known, and an early tentative reconstruction (Figure 10.34(b)) showed a semi-aquatic coast-dwelling carnivore. Subsequent evidence suggests that Pakicetus was still a land-dweller: for example, the ankle has the typical artiodactyl double-pulley astragalus.

Many taxa of early-mid-Miocene whales are now known from Pakistan (Thewissen et al., 1994, 2001; Gingerich et al., 2001), and one of these, Ambulocetus,is nearly complete. The limbs are adapted for swimming (Figure 10.34(c)), with short upper elements and paddle-like hands and feet. Ambulocetus could walk on land, even though its posture would have been rather crouched: probably it hauled itself around rather like a seal.

By the late Eocene, whales had become obligatorily aquatic and very large.Basilosaurus (Figure 10.34(d)) is over 20 m long and, unlike modern whales, it must have looked like a classic sea serpent because of its tiny head and long, thin body. Its hindlimbs are much reduced, but still present, with all elements in place (Gingerich et al., 1990). The pelvis has lost contact with the backbone and the lower limb and ankle are largely fused. This hindlimb would have been useless in swimming, but it may have been used as a copulatory guide. The head is relatively small and the teeth have a comb-like pattern of small pointed cusps.

After the Eocene, the whales radiated into two main groups, the toothed whales, such as dolphins and porpoises (Odontoceti), and the baleen whales such as the blue whale and humpback (Mysticeti). The sperm whale, the largest living whale to retain teeth, has generally been classified as an odontocete on morphological evidence,but early molecular analyses assigned it to the mysticetes. Current molecular studies (Gatesy et al.,

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