TNM 02067
Encyclopedia
TNM 02067 is a fragmentary fossil dentary (lower jaw) from the Cretaceous
(between 146 and 66 million years ago) of Tanzania. The short, deep bone is about 19.5 mm (0.767716535433071 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 (less than 66 million years ago).
of southwestern Tanzania, which has also yielded remains of various other vertebrate
s, including birds and other saurischia
n dinosaurs. The discovery was reported in a 2003 paper by David Krause and colleagues. TZ-07 lies in the "Red Sandstone Unit" (RSU), an informal, poorly defined rock unit. Age estimates for the RSU have ranged from middle Jurassic
to Miocene
, but according to Krause and colleagues, part of this discrepancy is the result of confusion between two superficially similar rock units that outcrop nearby; the older one, where TZ-07 is located, is undoubtedly Mesozoic
and the younger is Cenozoic
. Based on the presence of non-avian dinosaurs and osteoglossomorph fishes, Krause and colleagues assigned TZ-07 to the Cretaceous
(146–66 million years ago). In 2007, Nancy Stevens and colleagues identified the unit that produced TNM 02067 as likely belonging to the middle part of the Cretaceous (around Aptian
to Cenomanian
). TNM 02067 is significant as one of the very few mammals from the Cretaceous of the southern continents (Gondwana
).
and cementum
, but what remains indicates that there was a large incisor
at the front and five cheekteeth further back, separated by a diastema
(gap) of about 2.5 mm (0.098 in). On the labial (outer) surface of the dentary, there is one large mental foramen
(opening). The mandibular symphysis, where the two halves of the lower jaw meet, is poorly preserved, but there is nothing to suggest that the left and right dentaries were fused. The lower margin of the bone is convex at the front, but concave further back, so that the depth of the dentary is 8.3 mm (0.33 in) below the diastema, but only 7.0 mm (0.28 in) below the third cheektooth. The origin of the coronoid process
, a projection at the back of the dentary, lies far to the front.
The large incisor is inclined forward, and its root—the only part of the tooth that is preserved—forms an angle of about 55° with the horizontal. At the tip of the alveolus, where the tooth projects out of the bone, it is 3.0 mm (0.12 in) high and 2.1 mm broad. The root extends through the dentary to a position below the third cheektooth. Only the roots of the first and second cheekteeth are preserved. Both are about 1.5 mm in diameter, but the first tooth may have been slightly smaller than the second. The third tooth, the largest of the cheekteeth, has the root slightly curved backward. This root is deeply anchored in the dentary, extending down through about three-fourths of the bone. The crown is preserved in the form of a stump of dentine, 2.3 mm (0.091 in) long and 1.9 mm (0.075 in) broad, that extends high above the dentary, indicating that the tooth was hypsodont
(high-crowned). High dentine stumps also remain of the fourth and fifth cheekteeth. The fourth is about as large as the first and second and the fifth is smaller, with a diameter of about 1.0 mm (0.039 in). The fourth cheektooth also has a long, curved root, which extends more than halfway through the dentary, and that of the fifth tooth is even smaller and shorter. The orientation of the roots and teeth indicates that all the teeth were single-rooted.
s, lagomorphs, hyrax
es, wombat
s, the aye-aye
, and the extinct apatemyids
, tillodonts
, and taeniodont
s—all of which are known only from the Cenozoic, less than 66 million years ago. Krause and colleagues could not exclude the possibility that TNM 02067 represents an early member of such a group or an otherwise unknown major group of mammals. However, only two groups of Mesozoic mammals resemble TNM 02067: gondwanatheres; and multituberculates in the superfamilies Djadochtatherioidea
and Taeniolabidoidea
. Djadochtatherioids and taeniolabidoids occur in the late Cretaceous
through Paleogene
of the northern continents (Laurasia
), and gondwanatheres, an enigmatic group of uncertain evolutionary affinities, are known from the late Cretaceous through Paleogene of Gondwana, with forms such as Gondwanatherium
, Sudamerica
(both from Argentina), Lavanify
(Madagascar), and Bharattherium
(India).
No multituberculates are known to have had hypsodont teeth, none has more than two molariform (molar-like) teeth in a single toothrow, and most have large, blade-like teeth. However, the form of the remains of TNM 02067's third cheektooth suggests it was not such a blade-like tooth and that at least the last three cheekteeth were likely molariform. In these respects, TNM 02067 does resemble gondwanatheres in the family Sudamericidae
, and Krause and colleagues tentatively identified it as representing that family, primarily on the basis of its hypsodont teeth. Krause and colleagues compared the dentary in detail to that of Sudamerica, the only other gondwanathere for which a substantial fragment of the jaw was known. Sudamerica has only four, not five, cheekteeth (all of which are molariform), a higher, narrower incisor with a root that extends further through the dentary, and a shorter diastema; in all these respects, TNM 02067 is more primitive. In addition, the mental foramen of the TNM 02067 is located lower and the cheekteeth vary more in size.
If truly a gondwanathere, TNM 02067 extends the known geographic range of the group to another part of Gondwana, the African mainland. The uncertain age of locality TZ-07 renders a precise assessment of the significance of TNM 02067 difficult. It may well be older than the oldest previously known gondwanathere, the Campanian
Gondwanatherium. It also has implications for the hypothesis that Africa was isolated from the rest of Gondwana from an early date—as early as the early Cretaceous
—and consequently had a fauna distinct from the rest of Gondwana for much of the Cretaceous, as it may be closely related to the sudamericids of South America, India, Madagascar, and Antarctica, a highly derived group.
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...
(between 146 and 66 million years ago) of Tanzania. The short, deep bone is about 19.5 mm (0.767716535433071 in) long, but the back part is broken off. It contains a large, forward-inclined incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
with a root that extends deep into the jaw, separated by a diastema
Diastema (dentistry)
Diastema is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars.-In humans:...
(gap) from five cheekteeth. Very little remains of the teeth, but enough to determine that they are hypsodont
Hypsodont
Hypsodont dentition is characterized by high-crowned teeth and enamel which extends past the gum line. This provides extra material for wear and tear. Some examples of animals with hypsodont dentition are cows, horses and deer; all animals that feed on gritty, fibrous material. The opposite...
(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
Sudamericidae
Sudamericidae is a family of gondwanathere mammals that lived during the late Cretaceous to Eocene. Its members include Lavanify from the Cretaceous of Madagascar, Bharattherium from the Cretaceous of India, Gondwanatherium from the Cretaceous of Argentina, Sudamerica from the Paleocene of...
—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
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
times (less than 66 million years ago).
Discovery and context
TNM 02067 was discovered in 2002 in the locality TZ-07 in the Mbeya RegionMbeya Region
Mbeya is one of Tanzania's 26 administrative regions. The regional capital is Mbeya. It is bordered to the northwest by Tabora Region, to the northeast by Singida Region, to the East by Iringa Region, to the South by Zambia and Malawi and to the West by Rukwa Region...
of southwestern Tanzania, which has also yielded remains of various other vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s, including birds and other saurischia
Saurischia
Saurischia meaning 'lizard' and ischion meaning 'hip joint') is one of the two orders, or basic divisions, of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two orders, based on their hip structure...
n dinosaurs. The discovery was reported in a 2003 paper by David Krause and colleagues. TZ-07 lies in the "Red Sandstone Unit" (RSU), an informal, poorly defined rock unit. Age estimates for the RSU have ranged from 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....
to Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
, but according to Krause and colleagues, part of this discrepancy is the result of confusion between two superficially similar rock units that outcrop nearby; the older one, where TZ-07 is located, is undoubtedly Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
and the younger is Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
. Based on the presence of non-avian dinosaurs and osteoglossomorph fishes, Krause and colleagues assigned TZ-07 to 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...
(146–66 million years ago). In 2007, Nancy Stevens and colleagues identified the unit that produced TNM 02067 as likely belonging to the middle part of the Cretaceous (around Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
to Cenomanian
Cenomanian
The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding...
). TNM 02067 is significant as one of the very few mammals from the Cretaceous of the southern continents (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,...
).
Description
TNM 02067 is a damaged, partial left dentary (lower jaw bone). It preserves much of the body of the bone, which is short and deep, but is broken along a vertical fracture behind the toothrow. There is another fracture in the front part of the jaw. The bone is 19.5 mm (0.77 in) long and 11.4 mm (0.45 in) deep. All the teeth are incomplete or absent, and lack both enamelTooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...
and cementum
Cementum
Cementum is a specialized calcified substance covering the root of a tooth. Cementum is excreted by cells called cementoblasts within the root of the tooth and is thickest at the root apex. These cementoblasts develop from undifferentiated mesenchymal cells in the connective tissue of the dental...
, but what remains indicates that there was a large incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
at the front and five cheekteeth further back, separated by a diastema
Diastema (dentistry)
Diastema is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars.-In humans:...
(gap) of about 2.5 mm (0.098 in). On the labial (outer) surface of the dentary, there is one large mental foramen
Mental foramen
The mental foramen is one of two holes located on the anterior surface of the mandible. It permits passage of the mental nerve and vessels. The mental foramen descends slightly in edentulous individuals.- Variations :...
(opening). The mandibular symphysis, where the two halves of the lower jaw meet, is poorly preserved, but there is nothing to suggest that the left and right dentaries were fused. The lower margin of the bone is convex at the front, but concave further back, so that the depth of the dentary is 8.3 mm (0.33 in) below the diastema, but only 7.0 mm (0.28 in) below the third cheektooth. The origin of the coronoid process
Coronoid process of the mandible
The mandible's coronoid process is a thin, triangular eminence, which is flattened from side to side and varies in shape and size....
, a projection at the back of the dentary, lies far to the front.
The large incisor is inclined forward, and its root—the only part of the tooth that is preserved—forms an angle of about 55° with the horizontal. At the tip of the alveolus, where the tooth projects out of the bone, it is 3.0 mm (0.12 in) high and 2.1 mm broad. The root extends through the dentary to a position below the third cheektooth. Only the roots of the first and second cheekteeth are preserved. Both are about 1.5 mm in diameter, but the first tooth may have been slightly smaller than the second. The third tooth, the largest of the cheekteeth, has the root slightly curved backward. This root is deeply anchored in the dentary, extending down through about three-fourths of the bone. The crown is preserved in the form of a stump of dentine, 2.3 mm (0.091 in) long and 1.9 mm (0.075 in) broad, that extends high above the dentary, indicating that the tooth was hypsodont
Hypsodont
Hypsodont dentition is characterized by high-crowned teeth and enamel which extends past the gum line. This provides extra material for wear and tear. Some examples of animals with hypsodont dentition are cows, horses and deer; all animals that feed on gritty, fibrous material. The opposite...
(high-crowned). High dentine stumps also remain of the fourth and fifth cheekteeth. The fourth is about as large as the first and second and the fifth is smaller, with a diameter of about 1.0 mm (0.039 in). The fourth cheektooth also has a long, curved root, which extends more than halfway through the dentary, and that of the fifth tooth is even smaller and shorter. The orientation of the roots and teeth indicates that all the teeth were single-rooted.
Identity
The dentary superficially resembles that of various other mammalian groups with enlarged incisors, such as rodentRodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s, lagomorphs, hyrax
Hyrax
A hyrax is any of four species of fairly small, thickset, herbivorous mammals in the order Hyracoidea. The rock hyrax Procavia capensis, the yellow-spotted rock hyrax Heterohyrax brucei, the western tree hyrax Dendrohyrax dorsalis, and the southern tree hyrax, Dendrohyrax arboreus live in Africa...
es, wombat
Wombat
Wombats are Australian marsupials; they are short-legged, muscular quadrupeds, approximately in length with a short, stubby tail. They are adaptable in their habitat tolerances, and are found in forested, mountainous, and heathland areas of south-eastern Australia, including Tasmania, as well as...
s, the aye-aye
Aye-aye
The aye-aye is a lemur, a strepsirrhine primate native to Madagascar that combines rodent-like teeth and a special thin middle finger to fill the same ecological niche as a woodpecker...
, and the extinct apatemyids
Apatemyidae
Apatemyidae is an extinct family of placental mammals that took part in the first placental evolutionary radiation together with other early mammals such as the leptictids....
, tillodonts
Tillodontia
Tillodontia is an extinct order of mammals that may be related to the pantodonts. They were widespread across North America and Eurasia during the late Paleocene and most of the Eocene. They went extinct in Europe during the early Eocene, while North American and Asian forms survived until the...
, and taeniodont
Taeniodont
The taeniodonts were an early group of mammals who lived from the Palaeocene to the Eocene. This group evolved quickly into highly specialized digging animals. Taeniodont species varied greatly in size, from rat-sized to species as large as a bear. Later species developed prominent front teeth and...
s—all of which are known only from the Cenozoic, less than 66 million years ago. Krause and colleagues could not exclude the possibility that TNM 02067 represents an early member of such a group or an otherwise unknown major group of mammals. However, only two groups of Mesozoic mammals resemble TNM 02067: gondwanatheres; and multituberculates in the superfamilies Djadochtatherioidea
Djadochtatherioidea
Djadochtatherioidea is a group of extinct mammals known from the upper Cretaceous of Central Asia. They were members of an also extinct order called Multituberculata. These were generally small, somewhat rodent-like creatures, who scurried around during the "age of the dinosaurs". Unusually for...
and Taeniolabidoidea
Taeniolabidoidea
Taeniolabidoidea is a group of extinct mammals known from North America and Asia. They were the largest members of the also extinct order Multituberculata. Lambdopsalis even provides direct fossil evidence of mammalian fur in a fairly good state of preservation for a 60-million-year-old animal...
. Djadochtatherioids and taeniolabidoids occur in the late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
through Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
of the northern continents (Laurasia
Laurasia
In paleogeography, Laurasia was the northernmost of two supercontinents that formed part of the Pangaea supercontinent from approximately...
), and gondwanatheres, an enigmatic group of uncertain evolutionary affinities, are known from the late Cretaceous through Paleogene of Gondwana, with forms such as Gondwanatherium
Gondwanatherium
Gondwanatherium is a genus of mammal from the extinct suborder Gondwanatheria that lived in Patagonia, South America during the "Age of Dinosaurs", specifically the Upper Cretaceous....
, Sudamerica
Sudamerica
Sudamerica, literally "South America" in Spanish, is a genus of mammal from the extinct suborder Gondwanatheria that lived in Patagonia, South America during the Paleocene, just after the end of the "Age of Dinosaurs"....
(both from Argentina), Lavanify
Lavanify
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...
(Madagascar), and Bharattherium
Bharattherium
Bharattherium is a mammal that lived in India during the Maastrichtian . 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...
(India).
No multituberculates are known to have had hypsodont teeth, none has more than two molariform (molar-like) teeth in a single toothrow, and most have large, blade-like teeth. However, the form of the remains of TNM 02067's third cheektooth suggests it was not such a blade-like tooth and that at least the last three cheekteeth were likely molariform. In these respects, TNM 02067 does resemble gondwanatheres in the family Sudamericidae
Sudamericidae
Sudamericidae is a family of gondwanathere mammals that lived during the late Cretaceous to Eocene. Its members include Lavanify from the Cretaceous of Madagascar, Bharattherium from the Cretaceous of India, Gondwanatherium from the Cretaceous of Argentina, Sudamerica from the Paleocene of...
, and Krause and colleagues tentatively identified it as representing that family, primarily on the basis of its hypsodont teeth. Krause and colleagues compared the dentary in detail to that of Sudamerica, the only other gondwanathere for which a substantial fragment of the jaw was known. Sudamerica has only four, not five, cheekteeth (all of which are molariform), a higher, narrower incisor with a root that extends further through the dentary, and a shorter diastema; in all these respects, TNM 02067 is more primitive. In addition, the mental foramen of the TNM 02067 is located lower and the cheekteeth vary more in size.
If truly a gondwanathere, TNM 02067 extends the known geographic range of the group to another part of Gondwana, the African mainland. The uncertain age of locality TZ-07 renders a precise assessment of the significance of TNM 02067 difficult. It may well be older than the oldest previously known gondwanathere, the Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
Gondwanatherium. It also has implications for the hypothesis that Africa was isolated from the rest of Gondwana from an early date—as early as the early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
—and consequently had a fauna distinct from the rest of Gondwana for much of the Cretaceous, as it may be closely related to the sudamericids of South America, India, Madagascar, and Antarctica, a highly derived group.
Literature cited
- Gurovich, Y. and Beck, R. 2009. The phylogenetic affinities of the enigmatic mammalian clade Gondwanatheria (subscription required). Journal of Mammalian Evolution 16:25–49.
- Krause, D.W., Gottfried, M.D., O'Connor, P.M., and Roberts, E.M. 2003. A Cretaceous mammal from Tanzania. Acta Palaeontologica Polonica 48(3):321–330.
- O'Connor, P.M., Gottfried, M.D., Stevens, N.J., Roberts, E.M., Ngasala, S., Kapilima, S. and Chami, R. 2006. A new vertebrate fauna from the Cretaceous Red Sandstone Group, Rukwa Rift Basin, Southwestern Tanzania. Journal of African Earth Sciences 44:277–288.
- Stevens, N.J., Gottfried, M.D., Roberts, E.M., Kapilima, S., Ngasala, S. and O'Connor, P.M. 2007. Paleontological exploration in Africa: A view from the Rukwa Rift Basin of Tanzania. Pp. 159–180 in Fleagle, J.G. and Gilbert, C.C. (eds.). Elwyn Simons: a search for origins. Springer, 460 pp. ISBN 9780387738956