Ambondro (genus)
Encyclopedia
Ambondro is a genus
of mammal
from the middle Jurassic
(about 167 million years ago) of Madagascar
. The single species, A. mahabo, 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. The oldest known mammal with putatively tribosphenic teeth, at the time of its discovery it was the oldest mammal with such teeth 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 monotreme
s (the echidna
s, 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 in fact 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.
. 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
.
(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 long 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. 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.
In their paper, Flynn and colleagues described Ambondro as the oldest mammal with tribosphenic molars—the basic molar type of metatheria
n (marsupial
s and their extinct relatives) and eutheria
n (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 by 25 million years 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 monotreme
s, 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.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
from the middle Jurassic
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....
(about 167 million years ago) of Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
. The single species, A. mahabo, is known from a fragmentary lower jaw
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
with three teeth, interpreted as the last premolar
Premolar
The premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...
and the first two molars
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
. The premolar consists of a central cusp
Cusp (dentistry)
A cusp is an occlusal or incisal eminence on a tooth.Canine teeth, otherwise known as cuspids, each possess a single cusp, while premolars, otherwise known as bicuspids, possess two each. Molars normally possess either four or five cusps...
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
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
and placental mammals. The oldest known mammal with putatively tribosphenic teeth, at the time of its discovery it was the oldest mammal with such teeth by about 25 million years.
Upon its description in 1999, Ambondro was interpreted as a primitive relative of Tribosphenida
Tribosphenida
Tribosphenida is a group of mammals that is defined as including the ancestor of Hypomylos, Necrolestidae, Aegialodontia and supercohort Theria .-External links:* * * -Further reading:...
(marsupials, placentals, and their extinct tribosphenic-toothed relatives). In 2001, however, an alternative suggestion was published that united it with the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
Australian Ausktribosphenos
Ausktribosphenos
Ausktribosphenos is an extinct genus of Australosphenida from Early Cretaceous of Australia. The only recorded species, Ausktribosphenos nyktos, was found on Flat Rocks, Victoria.-References:...
and the monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s (the echidna
Echidna
Echidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
s, the platypus
Platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
, and their extinct relatives) into the clade Australosphenida
Australosphenida
The Australosphenida are a clade of mammals. Today, living specimens exist only in Australia and New Guinea with only five surviving species, but fossils have been found in Madagascar and Argentina...
, which would have acquired tribosphenic molars independently from marsupials and placentals. The Jurassic Argentinean Asfaltomylos
Asfaltomylos
Asfaltomylos is an extinct genus of Australosphenida from the middle Jurassic of Argentina. Only one species is recorded, Asfaltomylos patagonicus, from the Cañadon Asfalto Formation, Chubut Province, Patagonia.-References:...
and Henosferus
Henosferus
Henosferus is an extinct genus of Australosphenida from Middle Jurassic of Argentina. The only recorded species, Henosferus molus, was found on Cañadón Asfalto Formation, Chubut Province, Patagonia.-References:...
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 in fact 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.
Discovery and context
Ambondro mahabo was described by a team led by John Flynn in a 1999 paper in NatureNature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
. The scientific name derives from the village of Ambondromahabo, close to which the fossil was found. It is known from the Bathonian
Bathonian
In the geologic timescale the Bathonian is an age or stage of the Middle Jurassic. It lasted from approximately 167.7 Ma to around 164.7 Ma...
(middle Jurassic
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....
, 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
Group (stratigraphy)
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 divided into formations and are sometimes themselves grouped into "supergroups"....
". This unit has also yielded crocodyliform and plesiosaur
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...
teeth and remains of the sauropod Lapparentosaurus
Lapparentosaurus
Lapparentosaurus is a genus of macronarian sauropod dinosaur from the Middle Jurassic. Its fossils were found in Madagascar.In 1895 Richard Lydekker named a new species of Bothriospondylus, B. madagascariensis based on fossils found before 1894 by J.T. Last in the Majunga Basin in layers of the...
.
Description
Ambondro was described on the basis of a fragmentary right mandibleMandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
(lower jaw) with three teeth in it (Figure 1), interpreted as the last premolar
Premolar
The premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...
(p-last) and the first two molars
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
(m1 and m2). It is in the collection of the University of Antananarivo
University of Antananarivo
University of Antananarivo is the primary public university of Madagascar, located in the capital Antananarivo.The school traces its founding to 16 December 1955 and the formation of the Institute for Advanced Studies in Antananarivo. It quickly established itself as the main center for higher...
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
Cusp (dentistry)
A cusp is an occlusal or incisal eminence on a tooth.Canine teeth, otherwise known as cuspids, each possess a single cusp, while premolars, otherwise known as bicuspids, possess two each. Molars normally possess either four or five cusps...
. 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 long 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
Metacone
thumb|500px|left|Right upper molar showing the four main upper molars cusps.The metacone is a cusp on the molars of the upper dentition in hominids. It is found at the buccal distal area of the tooth...
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
Occlusion (dentistry)
Occlusion, in a dental context, means simply the contact between teeth. More technically, it is the relationship between the maxillary and mandibular teeth when they approach each other, as occurs during chewing or at rest....
). 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
Protocone
thumb|500px|right|Right upper molar showing the four main upper molars cusps.The protocone is a cusp of the molars of the upper dentition in Placental and Marsupial vertebrates .It is found at the mesiolingual area of the tooth...
(another cusp on the outer side of the tooth) on the upper molars. In a 2005 paper on Asfaltomylos
Asfaltomylos
Asfaltomylos is an extinct genus of Australosphenida from the middle Jurassic of Argentina. Only one species is recorded, Asfaltomylos patagonicus, from the Cañadon Asfalto Formation, Chubut Province, Patagonia.-References:...
, 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.
Interpretations
| 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 metatheria
Metatheria
Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.The earliest known...
n (marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s and their extinct relatives) and eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
n (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 by 25 million years into the past. Consequently, Flynn and colleagues argued against the prevailing view that tribosphenic mammals originated on the northern continents (Laurasia
Laurasia
In paleogeography, Laurasia was the northernmost of two supercontinents that formed part of the Pangaea supercontinent from approximately...
), and instead proposed that their origin lies in the south (Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
). 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
Ausktribosphenos
Ausktribosphenos is an extinct genus of Australosphenida from Early Cretaceous of Australia. The only recorded species, Ausktribosphenos nyktos, was found on Flat Rocks, Victoria.-References:...
, and the living monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s, which first appeared in the Cretaceous (united as Australosphenida
Australosphenida
The Australosphenida are a clade of mammals. Today, living specimens exist only in Australia and New Guinea with only five surviving species, but fossils have been found in Madagascar and Argentina...
). 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
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
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
Henosferus
Henosferus is an extinct genus of Australosphenida from Middle Jurassic of Argentina. The only recorded species, Henosferus molus, was found on Cañadón Asfalto Formation, Chubut Province, Patagonia.-References:...
, 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.
Literature cited
- Flynn, J.J., Parrish, J.M., Rakotosamimanana, B., Simpson, W.F. and Wyss, A.R. 1999. A Middle Jurassic mammal from Madagascar (subscription required). Nature 401:57–60.
- Luo, Z.-X., Cifelli, R.L. and Kielan-Jaworowska, Z. 2001. Dual origin of tribosphenic mammals (subscription required). Nature 409:53–57.
- Luo, Z.-X., Kielan-Jaworowska, Z. and Cifelli, R.L. 2002. In quest for a phylogeny of Mesozoic mammals. Acta Palaeontologica Polonica 47(1):1–78.
- Martin, T. and Rauhut, O.W.M. 2005. Mandible and dentition of Asfaltomylos patagonicus (Australosphenida, Mammalia) and the evolution of tribosphenic teeth (subscription required). Journal of Vertebrate Paleontology 25(2):414–425.
- Rauhut, O.W.M., Martin, T., Ortiz-Jaureguizar, E. and Puerta, P. 2002. A Jurassic mammal from South America (subscription required). Nature 416:165–168.
- Rougier, G.W., Martinelli, A.G., Forasiepi, A.M. and Novacek, M.J. 2007. New Jurassic mammals from Patagonia, Argentina: A reappraisal of australosphenidan morphology and interrelationships. American Museum Novitates 3566:1–54.
- Sigogneau-Russell, D., Hooker, J.J. and Ensom, P.C. 2001. The oldest tribosphenic mammal from Laurasia (Purbeck Limestone Group, Berriasian, Cretaceous, UK) and its bearing on the 'dual origin' of Tribosphenida (subscription required). Comptes Rendus de l'Académie des Sciences, Series IIA (Earth and Planetary Science) 333(2):141–147.
- Woodburne, M.O. 2003. Monotremes as pretribosphenic mammals (subscription required). Journal of Mammalian Evolution 10(3):195–248.
- Woodburne, M.O., Rich, T.H. and Springer, M.S. 2003. The evolution of tribospheny and the antiquity of mammalian clades (subscription required). Molecular Phylogeny and Evolution 28(2):360–385.