Ambondro mahabo

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Ambondro mahabo
Temporal range: Middle Jurassic Bathonian
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Ambondro lingual.jpg
Figure 1. Jaw of Ambondro, seen in lingual view (from the side of the tongue). Scale bar is 1 mm.
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Family: Henosferidae
Genus: Ambondro
Flynn et al., 1999
Species:
A. mahabo
Binomial name
Ambondro mahabo
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.

Mammal Class of tetrapods with milk-producing glands

Mammals are vertebrate animals constituting the class Mammalia, and characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals. The largest orders are the rodents, bats and Soricomorpha. The next three are the Primates, the Cetartiodactyla, and the Carnivora.

The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174 to 163 million years ago. Fossil-bearing rocks from the Middle Jurassic are relatively rare, but some important formations include the Forest Marble Formation in England, the Kilmaluag Formation in Scotland, the Daohugou Beds in China, Itat Formation in Russia, and the Isalo III Formation of western Madagascar.

In the geologic timescale the Bathonian is an age and stage of the Middle Jurassic. It lasted from approximately 168.3 Ma to around 166.1 Ma. The Bathonian age succeeds the Bajocian age and precedes the Callovian age.

Contents

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.

Tribosphenida infralegion of mammals

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.

The Cretaceous is a geologic period and system that spans from the end of the Jurassic Period 145 million years ago (mya) to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, and the longest period of the Phanerozoic Eon. The Cretaceous Period is usually abbreviated K, for its German translation Kreide.

Ausktribosphenos is an extinct genus of mammals from Early Cretaceous of Australia. The only recorded species, Ausktribosphenos nyktos, was found on Flat Rocks, Victoria.

Discovery and context

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 . [1]

<i>Nature</i> (journal) British multidisciplinary scientific journal

Nature is a British multidisciplinary scientific journal, first published on 4 November 1869. It is one of the most recognizable scientific journals in the world, and was ranked the world's most cited scientific journal by the Science Edition of the 2010 Journal Citation Reports and is ascribed an impact factor of 43.070, making it one of the world's top academic journals. It is one of the few remaining academic journals that publishes original research across a wide range of scientific fields.

Ambondromahabo is a village near Ambondromamy, Madagascar.

A group in stratigraphy is a lithostratigraphic unit, a part of the geologic record or rock column that consists of defined rock strata. Groups are generally divided into individual formations. Groups may sometimes be divided into "subgroups" and are themselves sometimes grouped into "supergroups".

Description

Figure 2. Generalized australosphenidan lower molar seen from above, illustrating major features. Abbreviations: ant, anterior (towards the front); pos, posterior (towards the back); ci, cingulum; pa, paraconid; pr, protoconid; me, metaconid; dm, distal metacristid; co, cristid obliqua; hy, hypoconid; hl, hypoconulid; ec, entocristid; tb, talonid basin. Australosphenidan molar labeled.png
Figure 2. Generalized australosphenidan lower molar seen from above, illustrating major features. Abbreviations: ant, anterior (towards the front); pos, posterior (towards the back); ci, cingulum; pa, paraconid; pr, protoconid; me, metaconid; dm, distal metacristid; co, cristid obliqua; hy, hypoconid; hl, hypoconulid; ec, entocristid; tb, talonid basin.

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. [2] 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. [3]

Mandible The lower jaw bone

The mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human face. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone of the skull.

Premolar

The premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered transitional teeth during chewing, or mastication. They have properties of both the canines, that lay anterior and molars that lay posterior, and so food can be transferred from the canines to the premolars and finally to the molars for grinding, instead of directly from the canines to the molars.

Molar (tooth) 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 mammal groups.

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". [2] The paraconid is higher than the metaconid. [4] At the front margin, a cingulum is present that is divided into two small cusps. [5] Unlike in various early tribosphenic mammals and close relatives, there is no additional cuspule behind the metaconid. [6] 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. [2]

The talonid, another group of cusps, makes up the back of the tooth. It is wider than long [4] and 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. [2] This entocristid is continuous with the lingual cingulum. [3]

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). [7] 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. [8] 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. [9]

Interpretations

Australosphenida

Asfaltomylos

Henosferus

Ambondro

Ausktribosphenos

Bishops

Monotremata

Boreosphenida

Metatheria

Eutheria

Monotremata

Metatheria

Eutheria

Ambondro

Asfaltomylos

Ausktribosphenos

Bishops

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. [Note 1] 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. [10] 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). [11] They cited the retention of a distal metacristid and an "open" trigonid as characters separating Ambondro from more modern tribosphenidans. [2]

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). [12] 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. [13]

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). [5]

The next year, Luo and colleagues published a more thorough analysis confirming their previous conclusion and adding the Cretaceous Australian Bishops to Australosphenida. [14] They mentioned the condition of the hypoconulid, which is inclined forward, rather than backward as in boreosphenidans, as an additional australosphenidan character [15] 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. [16] 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. [17]

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. [18] 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. [19] 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. [20] 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. [21]

Notes

  1. Compare similar trees in Luo et al. (2001, fig. 1), Luo et al. (2002, fig. 1), Rauhut et al. (2002, fig. 3), which included fewer australosphenidan species.

Related Research Articles

Metatheria subclass of mammals

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 slightly more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives.

Prototheria subclass of mammals

Prototheria is the subclass to which the orders Monotremata, Morganucodonta, Docodonta, Triconodonta and Multituberculata formerly belong.

<i>Steropodon</i> genus of mammals in the order Monotremata (fossil)

Steropodon galmani was a prehistoric species of monotreme, or egg-laying mammal, that lived about 105 million years ago (mya) in the Lower 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.

Docodonta order of mammals

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.

Australosphenida

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.

<i>Pseudotribos</i> genus of mammals (fossil)

Pseudotribos is an extinct genus of mammal that lived in Northern China during the Middle Jurassic some 165 million years ago, possibly was more closely related to monotremes than to placental or marsupial mammals, although inversely other studies recover shuotheres are closer to therians than monotremes. The only known specimen was found in the Daohygou Formation in Inner Mongolia.

Dryolestoidea is an extinct clade of Mesozoic mammals that only contains two orders. It has been suggested that this group is closely related to modern therian mammals.

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.

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.

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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 affinities of metatheria

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, 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.

References

  1. Flynn et al., 1999, pp. 57–58
  2. 1 2 3 4 5 Flynn et al., 1999, p. 58
  3. 1 2 Flynn et al., 1999, fig. 3
  4. 1 2 Rauhut et al., 2002, p. 167
  5. 1 2 Sigogneau-Russell et al., 2001, p. 146
  6. Flynn et al., 1999, fig. 2
  7. Luo et al., 2002, pp. 22, 29
  8. Flynn et al., 1999, p. 59
  9. Martin and Rauhut, 2005, pp. 422–423
  10. Flynn et al., 1999, p. 57; Rougier et al., 2007, p. 23
  11. Flynn et al., 1999, p. 60
  12. Luo et al., 2001, p. 56
  13. Luo et al., 2001, pp. 53, 56
  14. Luo et al., 2002, fig. 1
  15. Luo et al., 2002, p. 23
  16. Luo et al., 2002, p. 22
  17. Rauhut et al., 2002, p. 166
  18. Woodburne, 2003, pp. 233–235
  19. Woodburne, 2003, fig. 5; Woodburne et al., 2003, fig. 3
  20. Rougier et al., 2007, p. 31
  21. Rougier et al., 2007, pp. 24–25

Literature cited