Masiakasaurus knopfleri
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''Masiakasaurus'' is a
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of small predatory noasaurid
theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s from the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...
of
Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...
. In Malagasy, ''masiaka'' means "vicious"; thus, the genus name means "vicious lizard". The
type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...
, ''Masiakasaurus knopfleri'', was named after the musician
Mark Knopfler Mark Freuder Knopfler (born 12 August 1949) is a British singer-songwriter, guitarist, and record producer. Born in Scotland and raised in England, he was the lead guitarist, singer and songwriter of the rock band Dire Straits. He pursued a s ...
, whose music inspired the expedition crew. It was named in 2001 by Scott D. Sampson, Matthew Carrano, and Catherine A. Forster. Unlike most theropods, the front teeth of ''M. knopfleri'' projected forward instead of straight down. This unique dentition suggests that they had a specialized diet, perhaps including fish and other small prey. Other bones of the skeleton indicate that ''Masiakasaurus'' were bipedal, with much shorter forelimbs than hindlimbs. ''M. knopfleri'' was a small theropod, reaching long and weighing . ''Masiakasaurus'' lived around 70 million years ago, along with animals such as ''
Majungasaurus ''Majungasaurus'' (; ) is a genus of abelisaurid theropod dinosaur that lived in Madagascar from 70 to 66 million years ago, at the end of the Cretaceous Period, making it one of the last known non-avian dinosaurs that went extinct during the ...
'', ''
Rapetosaurus ''Rapetosaurus'' ( ) is a genus of titanosaurian sauropod dinosaur that lived in Madagascar from 70 to 66 million years ago, at the end of the Cretaceous Period. Only one species, ''Rapetosaurus krausei'', has been identified. Like other sauropo ...
'', and ''
Rahonavis ''Rahonavis'' is a genus of bird-like theropods from the Late Cretaceous (Maastrichtian, about 70 mya) of what is now northwestern Madagascar. It is known from a partial skeleton ( UA 8656) found by Catherine Forster and colleagues in Maevaran ...
''. ''Masiakasaurus'' was a member of the group
Noasauridae Noasauridae is an extinct family of theropod dinosaurs belonging to the group Ceratosauria. They were closely related to the short-armed abelisaurids, although most noasaurids had much more traditional body types generally similar to other th ...
, small predatory
ceratosaurs Ceratosaurs are members of the clade Ceratosauria, a group of dinosaurs defined as all theropods sharing a more recent common ancestor with ''Ceratosaurus'' than with birds. The oldest known ceratosaur, ''Saltriovenator'', dates to the earliest ...
found primarily in
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
.


History

Remains of ''Masiakasaurus'' have been found in the Late Cretaceous
Maevarano Formation The Maevarano Formation is a Late Cretaceous sedimentary rock formation found in the Mahajanga Province of northwestern Madagascar. It is most likely Maastrichtian in age, and records a seasonal, semiarid environment with rivers that had greatly v ...
in northwestern Madagascar and were first described in the journal ''
Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
'' in 2001. Fragmentary bones comprising around 40% of the skeleton were collected near the village of Berivotra. Several parts of the skull, including the distinctive teeth, were found. The humerus (upper arm bone), pubis, hindlimbs, and several vertebrae were also collected. In 2011, additional specimens of ''Masiakasaurus'' were described. The braincase,
premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...
, facial bones, ribcage, portions of the hands and pectoral girdle (
coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...
), and much of the cervical and dorsal vertebral column were described for the first time. The discovery of this new material clarified many aspects of noasaurid anatomy and made the genus among the best known dinosaurs. The new finds did however not allow for a detailed study of its evolutionary relationships among
ceratosaurs Ceratosaurs are members of the clade Ceratosauria, a group of dinosaurs defined as all theropods sharing a more recent common ancestor with ''Ceratosaurus'' than with birds. The oldest known ceratosaur, ''Saltriovenator'', dates to the earliest ...
. With the new material, around 65% of the skeleton is currently known.


Description


Skull

The most distinctive characteristic of ''Masiakasaurus'' is the forward-projecting, or procumbent, front teeth. The teeth are heterodont, meaning that they have different shapes along the jaw. The first four
dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...
teeth of the lower jaw project forward, with the first tooth angled only 10° above the horizontal. These teeth are long and spoon-shaped with hooked edges. They have carinae, or sharp edges, that are weakly serrated. Serrations are more evident along the rear edge the posterior teeth in the back of the jaw, which are also recurved and laterally compressed (flattened from the side), resembling the less unusual teeth of other carnivorous dinosaurs. The margin of the dentary curves downward so that the alveoli (tooth sockets) of the front teeth are directed forward. In fact, the alveolus of the first tooth is actually situated lower than the bottom edge of the rest of the lower jaw. The lower part of the rear edge of the dentary has a long prong, known as a ventral process. This differs from the situation in abelisaurids, which have a much shorter ventral process. On the other hand, the upper part of the rear edge of the dentary is very similar to that of abelisaurids such as ''Majungasaurus'' and ''
Carnotaurus ''Carnotaurus'' (; ) is a genus of theropod dinosaur that lived in South America during the Late Cretaceous period, probably sometime between 71 and 69 million years ago. The only species is ''Carnotaurus sastrei''. Known from a single well-p ...
''. This part of the bone possesses an array of four small structures, three of which line a socket which connects to the
surangular The suprangular or surangular is a jaw bone found in most land vertebrates, except mammals. Usually in the back of the jaw, on the upper edge, it is connected to all other jaw bones: dentary, angular, splenial and articular. It is often a mu ...
bone at the back of the lower jaw. Although the surangular bone is not preserved, several other bones of the lower jaw are, including a triangular angular bone, a gently curving prearticular bone, and a damaged yet notably concave
articular bone The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two ...
. The angular and prearticular formed the lower edge of a large and rounded in the lower jaw (known as a mandibular fenestra) while the articular bone formed the lower part of the jaw joint. A long and tapering
hyoid The hyoid bone (lingual bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies between the base of the mandible and the third cervical vertebra. ...
(tongue bone) has also been preserved.
/ref> The front teeth of the upper jaw are also procumbent, and the margin of the premaxilla curves slightly upward to direct them outward. Unlike the skulls of abelisaurids, which are very deep, the skull of ''Masiakasaurus'' is long and low. The lacrimal bone, lacrimal and
postorbital bone The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some v ...
s around the eye are textured with bumpy projections. Not including the highly modified jaws and teeth, the skull of ''Masiakasaurus'' possesses many general ceratosaurian characteristics. Overall, its morphology is intermediate between abelisaurids and more basal ceratosaurs.


Vertebrae

The neck is relatively narrow in comparison to abelisaurids and bear stout neck ribs. While many theropods have s-shaped necks, the ribs would make the neck rather stiff in ''Masiakasaurus'', and the back of the neck is positioned almost horizontally, giving it only a slighter curve. Like those of other abelisauroids, the vertebrae are heavily pneumaticized, or hollowed, and have relatively short neural spines. Pneumaticity is limited to the neck and foremost back vertebrae, however. Pneumatic cavities are also present in the braincase.


Forelimbs

As in other ceratosaurs, the shoulder blade (
scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...
) and shoulder girdle fuse into a single bone, the
scapulocoracoid The scapulocoracoid is the unit of the pectoral girdle that contains the coracoid and scapula. The coracoid itself is a beak-shaped bone that is commonly found in most vertebrates with a few exceptions. The scapula is commonly known as the ''shoulde ...
. This bone is very large and broad, even compared to the condition in other ceratosaurs. The scapula portion (above the glenoid, or arm socket) tapers towards the back while the coracoid portion (below the glenoid) is expanded into a curved blade-like structure. While abelisaurids have arms that are extremely reduced in size, ''Masiakasaurus'' and other noasaurids had longer forelimbs. The humerus is slender and known bones of the hand are relatively short. The related genus ''Noasaurus'' has a large and curved raptorial
ungual An ungual (from Latin ''unguis'', i.e. ''nail'') is a highly modified distal toe bone which ends in a hoof, claw, or nail. Elephants and ungulates have ungual phalanges, as did the sauropod Sauropoda (), whose members are known as sauropods (; ...
(claw) which was originally interpreted as a sickle-like foot claw as in
dromaeosaurids Dromaeosauridae () is a family of feathered theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek ('), meaning ...
such as ''
Velociraptor ''Velociraptor'' (; ) is a genus of small dromaeosaurid dinosaur that lived in Asia during the Late Cretaceous epoch, about 75 million to 71 million years ago. Two species are currently recognized, although others have been assigned in the p ...
''. More recently, this has been re-evaluated as a claw of the hand. The penultimate
phalanx The phalanx ( grc, φάλαγξ; plural phalanxes or phalanges, , ) was a rectangular mass military formation, usually composed entirely of heavy infantry armed with spears, pikes, sarissas, or similar pole weapons. The term is particularly ...
, the finger bone that immediately precedes the raptorial ungual in ''Noasaurus'', is also known in ''Masiakasaurus'' and has a similar appearance. The enlarged ungual, however, is unknown in ''Masiakasaurus''. It is assumed that members of this genus had four fingers, with the middle two fingers being the longest as in other ceratosaurians.


Classification

In its initial 2001 description, ''Masiakasaurus'' was classified as a basal abelisauroid related to ''
Laevisuchus ''Laevisuchus'' (, "light crocodile") is a genus of theropod dinosaur from the Late Cretaceous. Its remains were discovered by Charles Alfred Matley near Jabalpur in Maastrichtian deposits in the Lameta Formation in India, and were named and de ...
'' and ''Noasaurus'', two poorly known genera named in 1933 and 1980, respectively. In the following year, Carrano ''et al.'' (2002) placed ''Masiakasaurus'' along with ''Laevisuchus'' and ''Noasaurus'' in the family Noasauridae. They conducted a phylogenetic analysis of abelisauroids using characteristics from ''Masiakasaurus''. Below is a
cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
from an updated version of their analysis showing the phylogenetic placement of ''Masiakasaurus''.Rauhut, O.W.M., and Carrano, M.T. (2016). The theropod dinosaur ''Elaphrosaurus bambergi'' Janensch, 1920, from the Late Jurassic of Tendaguru, Tanzania. ''Zoological Journal of the Linnean Society'', (advance online publication)


Paleobiology

Carrano ''et al.'' (2002) distinguished two forms of ''Masiakasaurus'', a robust form and a gracile form. The robust morph includes specimens with thicker bones and more pronounced projections for the attachment of ligaments and muscles. The gracile form includes specimens that are more slender and have less pronounced muscle attachments. It also has unfused
tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...
e, unlike the fused tibiae of the robust form. These two varieties may be an indication of
sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...
in ''Masiakasaurus'', but they may also represent two distinct populations. One specimen of ''Masiakasaurus'', a right scapulocoracoid, bears holes that may be puncture marks from predation or scavenging. ''
Majungasaurus ''Majungasaurus'' (; ) is a genus of abelisaurid theropod dinosaur that lived in Madagascar from 70 to 66 million years ago, at the end of the Cretaceous Period, making it one of the last known non-avian dinosaurs that went extinct during the ...
'', a large abelisaurid from the Maevarano Formation, may have preyed upon ''Masiakasaurus''. The holes may also have been the result of an infection.


Diet

The procument front teeth of ''Masiakasaurus'' were likely an adaptation for grasping small prey. They would have been unsuitable for tearing larger food apart. In the front of the jaws, carinae are restricted to the base of the teeth and would not have been used to tear prey. The back teeth, however, share the same general characteristics as those of most other theropods, suggesting that they served a similar function in ''Masiakasaurus'' such as cutting and slicing. Several feeding behaviors have been proposed for ''Masiakasaurus'' on the basis of its unusual dentition. Because the front teeth would have been well suited for grasping, ''Masiakasaurus'' may have consumed small vertebrates, invertebrates, and possibly even fruits.


Growth

In 2013, Lee and O'Connor observed that ''Masiakasaurus'' would be a good subject for an analysis of theropod growth, considering that there is an abundance of fossil material to examine from a broad range of
ontogenetic Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the st ...
stages. The study showed that ''Masiakasaurus'' grew determinately, and reached full maturity at a small body size. Competing theories that ''Masiakasaurus'' specimens represent the juvenile form of a larger-bodied theropod were not supported by the data. ''Masiakasaurus'' took 8 to 10 years to grow the size of a large dog. This indicates a rate of growth that is 40% slower than that of comparably sized non-avian theropods, a finding that is supported by the unusual prominence of parallel-fibered bone which is known to be associated with relatively slow growth. However, individuals in this genus grew 40% faster than crocodylians. Lee and O'Connor noted that the evolution of slow growth gave this dinosaur the advantage of minimizing the nutritional investment allocated toward structural growth while living in a semiarid and seasonally stressful environment.Andrew H. Lee & Patrick M. O’Connor (2013) Bone histology confirms determinate growth and small body size in the noasaurid theropod Masiakasaurus knopfleri. Journal of Vertebrate Paleontology 33(4): 865-87

/ref>


See also

*
Timeline of ceratosaur research This timeline of ceratosaur research is a chronological listing of events in the history of paleontology focused on the ceratosaurs, a group of relatively primitive, often horned, predatory theropod dinosaurs that became the apex predators of ...


References


External links

* '' The Geological Society of London'' (25 January 2001)
"Palaeontologists in dire straits name dinosaur for the Sultan of Swing"
Retrieved 7 October 2005. {{Taxonbar, from=Q131197 Late Cretaceous dinosaurs Abelisaurs Dinosaurs of India and Madagascar Maevarano fauna Fossil taxa described in 2001 Prehistoric animals of Madagascar Maastrichtian life Taxa named by Scott D. Sampson Taxa named by Catherine Forster