Temporal range: Middle Jurassic Bathonian
|Figure 1. Jaw of Ambondro, seen in lingual view (from the side of the tongue). Scale bar is 1 mm.|
|Genus:||† Ambondro |
Flynn et al., 1999
Flynn et al., 1999
Ambondro mahabo is a mammal from the Middle Jurassic (Bathonian) Isalo III Formation (about 167 million years ago) 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.
Upon its description in 1999, Ambondro was interpreted as a primitive relative of Tribosphenida (marsupials, placentals, and their extinct tribosphenic-toothed relatives). In 2001, however, an alternative suggestion was published that united it with the Cretaceous Australian Ausktribosphenos and the monotremes (the echidnas, the platypus, and their extinct relatives) into the clade Australosphenida, which would have acquired tribosphenic molars independently from marsupials and placentals. The Jurassic Argentinean Asfaltomylos and Henosferus and the Cretaceous Australian Bishops were later added to Australosphenida, and new work on wear in australosphenidan teeth has called into question whether these animals, including Ambondro, did have tribosphenic teeth. Other paleontologists have challenged this concept of Australosphenida, and instead proposed that Ambondro is not closely related to Ausktribosphenos plus monotremes, or that monotremes are not australosphenidans and that the remaining australosphenidans are related to placentals.
Ambondro mahabo was described by a team led by John Flynn in a 1999 paper in Nature . The scientific name derives from the village of Ambondromahabo, close to which the fossil was found. It is known from the Bathonian (middle Jurassic, about 167 million years ago) of the Mahajanga Basin in northwestern Madagascar, in the Isalo III unit, the youngest of the three rock layers that make up the Isalo "Group". This unit has also yielded crocodyliform and plesiosaur teeth and remains of the sauropod Lapparentosaurus .
Ambondro was described on the basis of a fragmentary right mandible (lower jaw) with three teeth in it (Figure 1), interpreted as the last premolar (p-last) and the first two molars (m1 and m2). It is in the collection of the University of Antananarivo as specimen UA 10602. Relative to other primitive mammals, it is small. Each of the teeth has a prominent cingulum (shelf) on the inner (lingual) side. The p-last has a strong central cusp. There is a cuspule (small cusp) on the back of the tooth and probably another on the inner front corner. This tooth resembles the molars of symmetrodonts, a group of primitive mammals, but the back cusp is smaller than the metaconid of symmetrodonts.
The front half of the m1 and m2 consists of the trigonid, a group of three cusps forming a triangle: the paraconid at the front on the inner side, protoconid in the middle on the outer (labial) side, and metaconid at the back on the inner side (see Figure 2). The three cusps form a right angle with each other at the protoconid, so that the trigonid is described as "open".The paraconid is higher than the metaconid. At the front margin, a cingulum is present that is divided into two small cusps. Unlike in various early tribosphenic mammals and close relatives, there is no additional cuspule behind the metaconid. At the back of the trigonid, the crest known as the distal metacristid is located relatively close to the outer side of the tooth and is continuous with another crest, the cristid obliqua, which is in turn connected to the back of the tooth.
The talonid, another group of cusps, makes up the back of the tooth. It is wider than longand contains a well-developed cusp, the hypoconid, on the outer side and a depression, the talonid basin, in the middle. The cristid obliqua connects to the hypoconid. The smaller hypoconulid cusp is present towards the inner side of the tooth, and the hypoconid and hypoconulid are connected by a cutting edge which is suggestive of the presence of a metacone cusp on the upper molars. Further towards the inner side, a crest, the entocristid, rims the talonid basin; on m1, it is swollen and on m2, it contains two small cuspules, but a distinct entoconid cusp is absent. This entocristid is continuous with the lingual cingulum.
Wear facets are areas of a tooth that show evidence of contact with a tooth in the opposing jaw when the teeth are brought together (known as occlusion).Flynn and colleagues identified two wear facets at the front and back margins of the talonid basin; they argue that these wear facets suggest the presence of a protocone (another cusp on the outer side of the tooth) on the upper molars. In a 2005 paper on Asfaltomylos , a related primitive mammal from Argentina, Thomas Martin and Oliver Rauhut disputed the presence of these wear facets within the talonid basin in Ambondro and instead identified wear facets on the cusps and crests surrounding the basin. They proposed that wear in the australosphenidan talonid occurs mainly on the rims, not in the talonid basin itself, and that australosphenidans may not have had a functional protocone.
|Figure 3. Alternative views of the relationships of Ambondro. Top, Rougier et al., (2007, fig. 9): australosphenidans, including monotremes and Ambondro, are distinct from boreosphenidans. Bottom, Woodburne et al. (2003, fig. 3): australosphenidans, including Ambondro but excluding monotremes, are closely related to placentals. Many taxa are omitted from both trees for clarity.|
In their paper, Flynn and colleagues described Ambondro as the oldest mammal with tribosphenic molars—the basic molar type of metatherian (marsupials and their extinct relatives) and eutherian (placentals and their extinct relatives) mammals, characterized by the protocone cusp on the upper molars contacting the talonid basin on the lower molars in chewing. The discovery of Ambondro was thought to extend the known temporal range of tribosphenic mammals 25 million years further into the past.Consequently, Flynn and colleagues argued against the prevailing view that tribosphenic mammals originated on the northern continents (Laurasia), and instead proposed that their origin lies in the south (Gondwana). They cited the retention of a distal metacristid and an "open" trigonid as characters separating Ambondro from more modern tribosphenidans.
In 2001, Zhe-Xi Luo and colleagues alternatively proposed that a tribosphenic molar pattern had arisen twice (compare Figure 3, top)—once giving rise to the marsupials and placentals (Boreosphenida), and once producing Ambondro, the Cretaceous Australian Ausktribosphenos , and the living monotremes, which first appeared in the Cretaceous (united as Australosphenida).They characterized Australosphenida by the shared presence of a cingulum on the outer front corner of the lower molars, a short and broad talonid, a relatively low trigonid, and a triangulated last lower premolar.
Also in 2001, Denise Sigogneau-Russell and colleagues in their description of the earliest Laurasian tribosphenic mammal, Tribactonodon , agreed with the relationship between Ausktribosphenos and monotremes, but argued that Ambondro was closer to Laurasian tribosphenidans than to Ausktribosphenos and monotremes. As evidence against the integrity of Australosphenida, they cited the presence of lingual cingula in various non-australosphenidan mammals; the presence of two cusps in the anterior cingulum in Ambondro as well as some boreosphenidans; the different appearance of the premolar in Ambondro (flat) and Ausktribosphenos (squared); and the contrast between the talonids of Ambondro (with a well-developed hypoconid on the labial side) and Ausktribosphenos (squared).
The next year, Luo and colleagues published a more thorough analysis confirming their previous conclusion and adding the Cretaceous Australian Bishops to Australosphenida.They mentioned the condition of the hypoconulid, which is inclined forward, rather than backward as in boreosphenidans, as an additional australosphenidan character and noted that Ausktribosphenos and monotremes were united, to the exclusion of Ambondro, by the presence of a V-shaped notch in the distal metacristid. In the same year, Asfaltomylos was described from the Jurassic of Argentina as another australosphenidan. In contrast to Ambondro, this animal lacked a distal metacristid and did not have as well-developed a lingual cingulum.
However, in 2003 Michael Woodburne and colleagues revised the phylogenetic analysis published by Luo and colleagues, making several changes to the data, particularly in the monotremes.Their results (Figure 3, bottom) challenged the division between Australosphenida and Boreosphenida, as proposed by Luo et al. Instead, they excluded monotremes from Australosphenida and placed the remaining australosphenidans close to Eutheria, with Ambondro most closely related to Asfaltomylos. In 2007, Guillermo Rougier and colleagues described another australosphenidan, Henosferus , from the Jurassic of Argentina; they argued against a relationship between Eutheria and Australosphenida (Figure 3, top), but were ambivalent about the placement of monotremes within Australosphenida. Based in part on Martin and Rauhut's earlier work on wear facets in australosphenidans, they questioned the presence of a true functional protocone on the upper molars of non-monotreme australosphenidans—none of which are known from upper teeth—and consequently suggested that australosphenidans may not, after all, have had truly tribosphenic teeth.
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 mammal groups.
Prototheria is a paraphyletic subclass to which the orders Monotremata, Morganucodonta, Docodonta, Triconodonta and Multituberculata have been assigned, although the validity of the subclass has been questioned.
Steropodon galmani was a prehistoric species of monotreme, or egg-laying mammal, that lived about 105 million years ago (mya) in the Late Cretaceous period. It is one of the oldest monotremes discovered, and is one of the oldest Australian mammal discoveries.
Teinolophos trusleri is a prehistoric species of monotreme, or egg-laying mammal. It is known from four specimens, each consisting of a partial lower jawbone collected from Flat Rocks, Victoria, Australia. It lived during the Aptian age of the Lower Cretaceous.
Tribosphenida is a group (infralegion) of mammals that includes the ancestor of Hypomylos, Aegialodontia and Theria. Its current definition is more or less synonymous with Boreosphenida.
Docodonta is an order of extinct mammaliaforms that lived during the Mesozoic, from the Middle Jurassic to Early Cretaceous. They are distinguished from other early mammaliaforms by their relatively complex molar teeth, from which the order gets its name. Until recently, Docodonta were represented primarily by teeth and jaws found across former Laurasia,. However, recent discoveries in China include some exceptionally well preserved, almost complete body fossils.
The Australosphenida are a proposed infraclass of mammals within subclass Yinotheria. Today, there are only five surviving species, which live in Australia and New Guinea, but fossils have been found in Madagascar and Argentina. The surviving species consist of the platypus and four species of echidna.
Ausktribosphenidae is a family of mammals from the Early Cretaceous of Australia that are closely related to monotremes.
Shuotherium is a fossil mammal known from Middle-Late Jurassic of the Forest Marble Formation of England, and the Shaximiao Formation of Sichuan, China.
Asfaltomylos is an extinct genus of the primitive mammal subclass Australosphenida from the middle Jurassic of Argentina. The type and only species is Asfaltomylos patagonicus, recovered from and named after the Cañadón Asfalto Formation, Cañadón Asfalto Basin of Chubut Province, Patagonia.
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.
Nesomys narindaensis is an extinct rodent that lived in northwestern Madagascar. It is known from subfossil skull bones and isolated molars found in several sites during field work that started in 2001. First described in 2010, it is placed in the genus Nesomys together with three smaller living species, which may differ in some details of molar morphology. The presence of N. narindaensis, a rare element in the local rodent fauna, suggests that the region was previously more humid.
Several mammals are known from the Mesozoic of Madagascar. The Bathonian 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, 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 has been interpreted as either a marsupial or a placental.
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.
Agathaeromys is an extinct genus of oryzomyine rodents from the Pleistocene of Bonaire, Netherlands Antilles. Two species are known, which differ in size and some details of tooth morphology. The larger A. donovani, the type species, is known from hundreds of teeth, found in four localities that are probably 900,000 to 540,000 years old. A. praeuniversitatis, the smaller species, is known from 35 teeth found in a single fossil site, which is probably 540,000 to 230,000 years old.
Dermotherium is a genus of fossil mammals closely related to the living colugos, a small group of gliding mammals from Southeast Asia. Two species are recognized: D. major from the Late Eocene of Thailand, based on a single fragment of the lower jaw, and D. chimaera from the Late Oligocene of Thailand, known from three fragments of the lower jaw and two isolated upper molars. In addition, a single isolated upper molar from the Early Oligocene of Pakistan has been tentatively assigned to D. chimaera. All sites where fossils of Dermotherium have been found probably developed in forested environments and the fossil species probably were forest dwellers like living colugos, but whether they already had the gliding adaptations of the living species is unknown.
Afrasia djijidae is a fossil primate that lived in Myanmar approximately 37 million years ago, during the late middle Eocene. The only species in the genus Afrasia, it was a small primate, estimated to weigh around 100 grams (3.5 oz). Despite the significant geographic distance between them, Afrasia is thought to be closely related to Afrotarsius, an enigmatic fossil found in Libya and Egypt that dates to 38–39 million years ago. If this relationship is correct, it suggests that early simians dispersed from Asia to Africa during the middle Eocene and would add further support to the hypothesis that the first simians evolved in Asia, not Africa. Neither Afrasia nor Afrotarsius, which together form the family Afrotarsiidae, is considered ancestral to living simians, but they are part of a side branch or stem group known as eosimiiforms. Because they did not give rise to the stem simians that are known from the same deposits in Africa, early Asian simians are thought to have dispersed from Asia to Africa more than once prior to the late middle Eocene. Such dispersals from Asia to Africa also were seen around the same time in other mammalian groups, including hystricognathous rodents and anthracotheres.
Yinotheria is a proposed basal subclass clade of crown mammals that contains a few fossils of the Mesozoic and the extant monotremes. Today, there are only five surviving species, which live in Australia and New Guinea, but fossils have been found in England, China, Russia, Madagascar and Argentina. The surviving species consist of the platypus and four species of echidna.
Yingabalanara is an extinct mammal from the Miocene of Australia. Known only from a few teeth, its affinities with other mammal groups remain unresolved.
Tikitherium is an extinct genus of Mammaliaformes from the Late Triassic. It is thought to be a insectivore and a close relative to Docodonta. Tikitherium refers to Tiki, the village located near the Tiki Formation where the specimen was found and therium is Greek for “Beast”. The species was named copei in honor of Edward Drinker Cope for his pioneering discoveries towards understanding mammalian molars.
|Wikimedia Commons has media related to Ambondro .|
|Wikispecies has information related to Ambondro|