Mesozoic mammals of Madagascar

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Several mammals are known from the Mesozoic of Madagascar. The Bathonian (middle Jurassic) Ambondro , known from a piece of jaw with three teeth, is the earliest known mammal with molars showing the modern, tribosphenic pattern that is characteristic of marsupial and placental mammals. Interpretations of its affinities have differed; one proposal places it in a group known as Australosphenida with other Mesozoic tribosphenic mammals from the southern continents (Gondwana) as well as the monotremes, while others favor closer affinities with northern (Laurasian) tribosphenic mammals or specifically with placentals. At least five species are known from the Maastrichtian (late Cretaceous), including a yet undescribed species known from a nearly complete skeleton that may represent a completely new group of mammals. The gondwanathere Lavanify , known from two teeth, is most closely related to other gondwanatheres found in India and Argentina. Two other teeth may represent another gondwanathere or a different kind of mammal. One molar fragment is one of the few known remains of a multituberculate mammal from Gondwana and another (UA 8699) has been interpreted as either a marsupial or a placental.

Contents

Jurassic

Jaw fragment of Ambondro mahabo seen from the inner (lingual) side. Scale bar is 1 mm. Ambondro lingual.jpg
Jaw fragment of Ambondro mahabo seen from the inner (lingual) side. Scale bar is 1 mm.

Ambondro mahabo was described from the middle Jurassic (Bathonian, about 167 million years ago) of northwestern Madagascar in 1999. It is known from a single lower jaw fragment with three teeth, probably the last premolar and first two molars. The molars have been interpreted as showing the tribosphenic pattern that is characteristic of modern mammals; Ambondro is the oldest known mammal with such a pattern. This led its discoverers to propose that the ancestors of tribosphenic mammals arose in the south (Gondwana), not, as generally assumed, in the north (Laurasia). In 2001, however, paleontologist Zhe-Xi Luo and colleagues alternatively proposed that Ambondro was part of a clade with Ausktribosphenos from the Cretaceous of Australia and the monotremes that developed tribosphenicity independently from other mammals (Boreosphenida). This clade, Australosphenida, has since been expanded with more recently discovered species from Argentina ( Asfaltomylos and Henosferus ) and Australia ( Bishops ). Other paleontologists have disagreed with this interpretation and proposed different models; for example, in 2001 Denise Sigogneau-Russell and colleagues proposed that although Ausktribosphenos and monotremes were related, Ambondro was not and was in fact more similar to boreosphenidans, and in 2003 Michael Woodburne and colleagues excluded monotremes from Australosphenida and placed the remaining australosphenidans close to placentals. The deposits that produced Ambondro have yielded some reptiles, but no other mammals. [1]

Cretaceous

The Mahajanga Basin of northwestern Madagascar has produced a rich late Cretaceous fauna, including various dinosaurs and crocodyliforms as well as mammals, found by the team of David W. Krause since 1993. Many of these taxa show affinities with similarly aged South American and Indian animals, also parts of Gondwana. [2] The mammalian fauna consists of several taxa known only by isolated teeth and a single reasonably complete skeleton, none of which can be plausibly related to the Recent Madagascar fauna (see list of mammals of Madagascar). [3] The fossils come from the Maastrichtian (latest Cretaceous) of the Anembalembo Member of the Maevarano Formation. [4]

Two teeth, one complete and one damaged, form the known material of the gondwanathere Lavanify , first described in 1997. The teeth are high-crowned and curved; one contains a deep cementum-filled furrow and the other at least one deep pit (infundibulum). Lavanify appears to be most closely related to the Indian gondwanathere Bharattherium and more distantly to the other gondwanatheres, which are known from Argentina. [5] Two other teeth, not yet fully described, may represent different tooth positions of another gondwanathere. One, a fragmentary molariform (molar or molar-like premolar—the identities of gondwanathere tooth are poorly understood) is larger and lower-crowned than the Lavanify teeth and the other, which is complete and unworn, is yet lower-crowned and has the surface obliquely oriented. Its crown consists of a W-shaped ridge with the parts separated by deep infundibula. This second tooth may also represent a completely different, yet unknown mammalian group. [6]

A fragmentary molar, preserving two cusps, is identified as from a multituberculate. Although multituberculates are common in nearly contemporaneous deposits in Laurasia, this tooth is one of the few records from Gondwana; [7] a few fragmentary remains, the multituberculate affinities of some of which are disputed, are also known from South America ( Argentodites ), Africa ( Hahnodon ), and Australia ( Corriebaatar ). [8] Another fragmentary tooth, UA 8699, is recognizable as a tribosphenic lower molar. Krause identified it in 2001 as a marsupial, but in 2003 a group led by Alexander Averianov instead argued that the tooth was placental and related to zhelestids (a primitive group possibly related to ungulates). [9] Both placentals and marsupials are mostly known from Laurasia during the Cretaceous. [10]

In addition to these fragmentary teeth, the Maevarano Formation has also yielded a nearly complete, articulated skeleton of an immature, cat-sized mammal that has not yet been fully described. [11] It is the most complete mammal known from the Mesozoic of Gondwana. [12] Its skull is damaged, but its unusual dentition is preserved. The incisors (two on each side of the upper and one on each side of the lower jaw) project forwards and are separated from the three or four cheektooth in each side of the lower and upper jaws by a large diastema (gap). It shows primitive features, such as the presence of epipubic bones (in the pelvis), a septomaxilla (a small bone placed between the premaxilla and the maxilla in the upper jaw), [Note 1] and a deep zygomatic arch (cheekbone). On the other hand, it has derived traits like the presence of a well-developed trochlea on the distal (far) end of the humerus (upper arm bone), the absence of a rim at the dorsal (upper) margin of the acetabulum (the opening in the pelvis which receives the head of the femur), a small lesser trochanter of the femur (upper leg bone), reduced contact between the fibula (the smaller of the two lower leg bones) and the calcaneum (heel bone), and the dentition. [11] In a 2000 abstract, Krause identified it as a therian (a member of the group that includes marsupials, placentals, and their closest extinct relatives) more derived than the early Cretaceous Vincelestes of Argentina, but in 2006 he and colleagues instead refused to place it in any existing higher-order mammalian group and claimed that "it represents a major new nontherian clade". [12]

Footnotes

  1. In his 2000 conference abstract on this skeleton, Krause mentioned the absence of the septomaxilla as a derived character, [13] but in the 2006 paper he and his colleagues wrote on the Cretaceous vertebrates of Madagascar, the "septomaxilla with prominent septomaxillary canal" was mentioned as a primitive feature of the skeleton. [12] Here the more recent interpretation is followed.

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<span class="mw-page-title-main">Multituberculata</span> Extinct order of mammals

Multituberculata is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, and reached a peak diversity during the Late Cretaceous and Paleocene. They eventually declined from the mid-Paleocene onwards, disappearing from the known fossil record in the late Eocene. They are the most diverse order of Mesozoic mammals with more than 200 species known, ranging from mouse-sized to beaver-sized. These species occupied a diversity of ecological niches, ranging from burrow-dwelling to squirrel-like arborealism to jerboa-like hoppers. Multituberculates are usually placed as crown mammals outside either of the two main groups of living mammals—Theria, including placentals and marsupials, and Monotremata—but usually as closer to Theria than to monotremes. They are considered to be closely related to Euharamiyida and Gondwanatheria as part of Allotheria.

<span class="mw-page-title-main">Molar (tooth)</span> Large tooth at the back of the mouth

The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name molar derives from Latin, molaris dens, meaning "millstone tooth", from mola, millstone and dens, tooth. Molars show a great deal of diversity in size and shape across the mammal groups. The third molar of humans is sometimes vestigial.

<span class="mw-page-title-main">Gondwanatheria</span> Extinct group of Mammaliaformes that lived during the Upper Cretaceous through the Miocene

Gondwanatheria is an extinct group of mammaliaforms that lived in parts of Gondwana, including Madagascar, India, South America, Africa, and Antarctica during the Upper Cretaceous through the Miocene. Until recently, they were known only from fragmentary remains. They are generally considered to be closely related to the multituberculates and likely the euharamiyidians, well known from the Northern Hemisphere, with which they form the clade Allotheria.

Ferugliotherium is a genus of fossil mammals in the family Ferugliotheriidae from the Campanian and/or Maastrichtian period of Argentina. It contains a single species, Ferugliotherium windhauseni, which was first described in 1986. Although originally interpreted on the basis of a single brachydont (low-crowned) molar as a member of Multituberculata, an extinct group of small, rodent-like mammals, it was recognized as related to the hypsodont (high-crowned) Sudamericidae following the discovery of additional material in the early 1990s. After a jaw of the sudamericid Sudamerica was described in 1999, these animals were no longer considered to be multituberculates and a few fossils that were previously considered to be Ferugliotherium were assigned to unspecified multituberculates instead. Since 2005, a relationship between gondwanatheres and multituberculates has again received support. A closely related animal, Trapalcotherium, was described in 2009 on the basis of a single tooth.

Lavanify is a mammalian genus from the late Cretaceous of Madagascar. The only species, L. miolaka, is known from two isolated teeth, one of which is damaged. The teeth were collected in 1995–1996 and described in 1997. The animal is classified as a member of Gondwanatheria, an enigmatic extinct group with unclear phylogenetic relationships, and within Gondwanatheria as a member of the family Sudamericidae. Lavanify is most closely related to the Indian Bharattherium; the South American Sudamerica and Gondwanatherium are more distantly related. Gondwanatheres probably ate hard plant material.

<span class="mw-page-title-main">Metatheria</span> Clade of marsupials and close relatives

Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives. It is one of two groups placed in the clade Theria alongside Eutheria, which contains the placentals. Remains of metatherians have been found on all of Earth’s continents.

<i>Steropodon</i> Extinct genus of monotremes

Steropodon is a genus of prehistoric platypus-like monotreme, or egg-laying mammal. It contains a single species, Steropodon galmani, that lived about 100.2–96.6 million years ago during the Cretaceous period, from early to middle Cenomanian. It is one of the oldest monotremes discovered, and is one of the oldest Australian mammal discoveries. Several other monotremes are known from the Griman Creek Formation, including Dharragarra, Kollikodon, Opalios, Parvopalus, and Stirtodon.

Ferugliotheriidae is one of three known families in the order Gondwanatheria, an enigmatic group of extinct mammals. Gondwanatheres have been classified as a group of uncertain affinities or as members of Multituberculata, a major extinct mammalian order. The best-known representative of Ferugliotheriidae is the genus Ferugliotherium from the Late Cretaceous epoch in Argentina. A second genus, Trapalcotherium, is known from a single tooth, a first lower molariform, from a different Late Cretaceous Argentinean locality. Another genus known from a single tooth, Argentodites, was first described as an unrelated multituberculate, but later identified as possibly related to Ferugliotherium. Finally, a single tooth from the Paleogene of Peru, LACM 149371, perhaps a last upper molariform, and a recent specimen from Mexico, may represent related animals.

<span class="mw-page-title-main">Australosphenida</span> Subclass of mammals

The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. Although they have often been suggested to have acquired tribosphenic molars independently from those of Tribosphenida, this has been disputed. Fossils of australosphenidans have been found from the Jurassic of Madagascar and Argentina, and Cretaceous of Australia and Argentina. Monotremes have also been considered a part of this group in its original definition and in many subsequent studies, but its relationship with other members has been disputed by some scholars.

<span class="mw-page-title-main">Monotreme</span> Order of egg-laying mammals

Monotremes are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified by structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts, compared to the more common mammalian types. Although they are different from almost all mammals in that they lay eggs, like all mammals, the female monotremes nurse their young with milk.

<i>Ambondro mahabo</i> Species of small mammal from the middle Jurassic of Madagascar

Ambondro mahabo is a mammal from the Middle Jurassic (Bathonian) Isalo III Formation of Madagascar. The only described species of the genus Ambondro, it is known from a fragmentary lower jaw with three teeth, interpreted as the last premolar and the first two molars. The premolar consists of a central cusp with one or two smaller cusps and a cingulum (shelf) on the inner, or lingual, side of the tooth. The molars also have such a lingual cingulum. They consist of two groups of cusps: a trigonid of three cusps at the front and a talonid with a main cusp, a smaller cusp, and a crest at the back. Features of the talonid suggest that Ambondro had tribosphenic molars, the basic arrangement of molar features also present in marsupial and placental mammals. It is the oldest known mammal with putatively tribosphenic teeth; at the time of its discovery it antedated the second oldest example by about 25 million years.

UA 8699 is a fossil mammalian tooth from the Cretaceous of Madagascar. A broken lower molar about 3.5 mm (0.14 in) long, it is from the Maastrichtian of the Maevarano Formation in northwestern Madagascar. Details of its crown morphology indicate that it is a boreosphenidan, a member of the group that includes living marsupials and placental mammals. David W. Krause, who first described the tooth in 2001, interpreted it as a marsupial on the basis of five shared characters, but in 2003 Averianov and others noted that all those are shared by zhelestid placentals and favored a close relationship between UA 8699 and the Spanish zhelestid Lainodon. Krause used the tooth as evidence that marsupials were present on the southern continents (Gondwana) as early as the late Cretaceous and Averianov and colleagues proposed that the tooth represented another example of faunal exchange between Africa and Europe at the time.

<i>Galulatherium</i> Fossil taxon

Galulatherium is an extinct genus of possibly gondwanathere mammal, from the Late Cretaceous (Turonian-Campanian)-aged Galula Formation of Tanzania. It is known solely from the type specimen TNM 02067 a fragmentary fossil dentary. The short, deep bone is about 19.5 mm (0.77 in) long, but the back part is broken off. It contains a large, forward-inclined incisor with a root that extends deep into the jaw, separated by a diastema (gap) from five cheekteeth. Very little remains of the teeth, but enough to determine that they are hypsodont (high-crowned). The third cheektooth is the largest and the roots of the teeth are curved. First described in 2003, TNM 02067 has been tentatively identified as a sudamericid—an extinct family of high-crowned gondwanathere mammals otherwise known from South America, Madagascar, India, and Antarctica. If truly a gondwanathere, it would be the only African member of the group and may be the oldest. The describers could not exclude other possibilities, such as that the jaw represents some mammalian group known only from younger, Cenozoic times. In 2019 the fossil was CT scanned, which revealed additional details of the specimen.

LACM 149371 is an enigmatic fossil mammalian tooth from the Paleogene of Peru. It is from the Santa Rosa fossil site, which is of uncertain age but possibly late Eocene or Oligocene. The tooth is poorly preserved and may have been degraded by acidic water or because it passed through a predator's digestive tract. Its largest dimension is 2.65 mm. It is triangular in shape and bears six cusps that surround the middle of the tooth, where there are three basins (fossae). Crests connects the cusps and separate the fossae. The microscopic structure of the enamel is poorly preserved.

Trapalcotherium is a fossil mammal from the Cretaceous of Argentina in the family Ferugliotheriidae. The single species, T. matuastensis, is known from one tooth, a first lower molar. It is from the Allen Formation, which is probably Maastrichtian in age, and was first described in 2009. The tooth bears two rows of cusps, one at the inner (lingual) side and the other at the outer (labial) side, which are connected by transverse ridges separated by deep valleys. This pattern is reminiscent of Ferugliotherium, a gondwanathere mammal from similarly aged deposits in Argentina, and Trapalcotherium is therefore recognized as a member of the same family Ferugliotheriidae. Ferugliotheriidae is one of two families of gondwanatheres, an enigmatic group without close relationships to any living mammals.

Bharattherium is a mammal that lived in India during the Maastrichtian and possibly the Paleocene. The genus has a single species, Bharattherium bonapartei. It is part of the gondwanathere family Sudamericidae, which is also found in Madagascar and South America during the latest Cretaceous. The first fossil of Bharattherium was discovered in 1989 and published in 1997, but the animal was not named until 2007, when two teams independently named the animal Bharattherium bonapartei and Dakshina jederi. The latter name is now a synonym. Bharattherium is known from a total of eight isolated fossil teeth, including one incisor and seven molariforms.

<span class="mw-page-title-main">Paratheria (mammals)</span> Former taxonomic group including xenarthran and similar mammals

Paratheria is an obsolete term for a taxonomic group including the xenarthran mammals and various groups thought to be related to them. It was proposed by Oldfield Thomas in 1887 to set apart the sloths, anteaters, armadillos, and pangolins, usually classified as placentals, from both marsupial and placental mammals, an arrangement that received little support from other workers. When teeth of the extinct gondwanathere mammals were first discovered in Argentina in the 1980s, they were thought to be related to xenarthrans, leading to renewed attention for the hypothesis that xenarthrans are not placentals. However, by the early 1990s, gondwanatheres were shown to be unrelated to xenarthrans, and xenarthrans are still considered to be placentals.

<span class="mw-page-title-main">Yinotheria</span> Subclass of mammals

Yinotheria is a proposed basal subclass clade of crown mammals uniting the Shuotheriidae, an extinct group of mammals from the Jurassic of Eurasia, with Australosphenida, a group of mammals known from the Jurassic to Cretaceous of Gondwana, which possibly include living monotremes. Today, there are only five surviving species of monotremes which live in Australia and New Guinea, consisting of the platypus and four species of echidna. Fossils of yinotheres have been found in Britain, China, Russia, Madagascar and Argentina. Contrary to other known crown mammals, they retained postdentary bones as shown by the presence of a postdentary trough. The extant members (monotremes) developed the mammalian middle ear independently.

Groeberiidae is a family of strange non-placental mammals from the Eocene and Oligocene epochs of Patagonia, Argentina and Chile, South America. Originally classified as paucituberculate marsupials, they were suggested to be late representatives of the allothere clade Gondwanatheria. However, the relationship of the type genus, Groeberia, to Gondwanatheria has been firmly rejected by other scholars.

<i>Holoclemensia</i> Extinct genus of mammals

Holoclemensia is an extinct genus of mammal of uncertain phylogenetic placement. It lived during the Early Cretaceous and its fossil remains were discovered in Texas.

References

  1. Flynn et al., 1999; Luo et al., 2001, 2002; Sigogneau-Russell et al., 2001; Woodburne, 2003; Woodburne et al., 2003; Rougier et al., 2007
  2. Krause et al., 2006, p. 178
  3. Krause et al., 2006, p. 187
  4. Krause et al., 2006, table 3
  5. Krause et al., 1997; Prasad et al., 2007, p. 23
  6. Krause, 2000; Krause et al., 2006, pp. 186–187
  7. Krause and Grine, 1996; Krause et al., 2006, p. 187
  8. Rich et al., 2009, p. 1
  9. Krause, 2001; Averianov et al., 2003
  10. Archibald, 2003, p. 352
  11. 1 2 Krause, 2000; Krause et al., 2006, p. 188
  12. 1 2 3 Krause et al., 2006, p. 188
  13. Krause, 2000

Literature cited