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89-509: Lesothosaurus is a monospecific genus of ornithischian dinosaur that lived during the Early Jurassic in what is now South Africa and Lesotho . It was named by paleontologist Peter Galton in 1978, the name meaning " lizard from Lesotho". The genus has only one valid species, Lesothosaurus diagnosticus . Lesothosaurus is one of the most completely-known early ornithischians, based on numerous skull and postcranial fossils from

178-602: A common ancestor ) or paraphyletic (excluding some descendants), these concepts do not apply to monotypic taxa because they contain only a single member. Monotypic taxa are part of a broader challenge in biological classification known as aphyly – situations where evolutionary relationships are poorly supported by evidence. This includes both monotypic groups and cases where traditional groupings are found to be artificial. Understanding how monotypic taxa fit into this bigger picture helps identify areas needing further research. The German lichenologist Robert Lücking suggests that

267-482: A hierarchical system. When taxonomists identify a monotypic taxon, this often reflects uncertainty about its relationships rather than true evolutionary isolation . This uncertainty is evident in many cases across different species. For instance, the diatom Licmophora juergensii is placed in a monotypic genus because scientists have not yet found clear evidence of its relationships to other species. Some taxonomists argue against monotypic taxa because they reduce

356-592: A basal ornithopod in the family Fabrosauridae, which included several other ornithischians such as Nanosaurus (from the Late Jurassic of North America), Echinodon (from the Lower Cretaceous of England), and Fabrosaurus (which Galton considered distinct from Lesothosaurus but only included the holotype). However, a 1991 redescription by Paul Sereno suggested that Lesothosaurus and many other "fabrosaurids" were actually basal members of Ornithischia, one of

445-633: A distinct group from stegosaurs in the 1920s despite many members being known for decades before, with the group now encompassing a broad array of heavy, quadrupedal ornithischians with extensive armour covering their body and skull. The fifth recognized major subgroup of ornithischians is Pachycephalosauria , which was first named in 1974 after being confused for a long time with the theropod Troodon on account of their similarly omnivorous and unique teeth. Pachycephalosaurians are unique for their tall, thickened skulls and small, bipedal bauplan, suggesting that their domes were for sexual display or combat in

534-680: A family. Some examples of monotypic groups are: Ornithischia Ornithischia ( / ˌ ɔːr n ə ˈ θ ɪ s k i . ə / ) is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds . The name Ornithischia , or "bird-hipped", reflects this similarity and is derived from the Greek stem ornith- ( ὀρνιθ- ), meaning "bird", and ischion ( ἴσχιον ), meaning "hip". However, birds are only distantly related to this group, as birds are theropod dinosaurs. Ornithischians with well known anatomical adaptations include

623-459: A keratinous beak . The premaxillary teeth and the first lower tooth in Heterodontosaurus are enlarged into sizeable canines. In later ornithopods, the skulls are more elongate and sometimes fully rectangular, with a very large nasal opening, and a thin, elongate palpebral that can extend the entire way across the orbit. Teeth are almost always absent from the premaxilla, the antorbital fossa

712-525: A member of the group including the armored stegosaurians and ankylosaurians . The Butler et al ., 2005 analysis placed Lesothosaurus at the base of Neornithischia: Euparkeria Marasuchus Herrerasaurus Pisanosaurus Heterodontosauridae Eocursor Thyreophora Lesothosaurus Agilisaurus Hexinlusaurus Othnielosaurus Cerapoda Basal neornithischians like Lesothosaurus are known from several time periods and regions, with Nanosaurus fossils coming from

801-436: A natural Dinosauria, which has been supported since. The first cladistic studies on Ornithischia were published simultaneously in 1984 by David B. Norman , Andrew R. Milner, and Paul C. Sereno . These studies differed somewhat in their results, but found that Iguanodon was closer to hadrosaurs than other ornithopods, followed by Dryosaurus , Hypsilophodon and then Lesothosaurus and its relatives. While

890-553: A natural classification. From a cladistic perspective, which focuses on shared derived characteristics to determine evolutionary relationships, the theoretical status of monotypic taxa is complex. Some argue they can only be justified when relationships cannot be resolved through synapomorphies (shared derived characteristics); otherwise, they would necessarily exclude related species and thus be paraphyletic. However, others contend that while most taxonomic groups can be classified as either monophyletic (containing all descendants of

979-442: A sacrodorsal with a short sacral rib. The sacrals notably had large transverse processes that were muscle attachments to the sacral ribs. The number of caudal (tail) vertebrae is unknown, but the proximal caudals are well preserved. The centra of these caudals became more cylindrical as they became located distally, in contrast to the spool-shaped dorsal centra. The chevrons are Y-shaped in anterior and posterior view and attached to

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1068-429: A triangular skull that had large circular orbits on the sides. This suggests that early ornithischians had relatively huge eyes that faced laterally. The forelimbs of early ornithischians are considerably shorter than their hindlimbs. A small forelimb such as those present in early ornithischians would not have been useful for locomotion, and it is evident that early ornithischians were bipedal dinosaurs. The entire skeleton

1157-443: A unique skull anatomy that is unlike any other ornithischian. The bones of the top of the skull are thickened and in many taxa expanded significantly to form round bony domes as the top of the head, as well as possessing small nodes or elongate spikes along the back edge of the skull. Many taxa are only known from these thick skull domes, which are fused from the frontal and parietal bones. As in many other ornithischians,

1246-409: Is box-like, while the snout tapers to a point. The nasal opening is small, the antorbital fossa that opens from the side of the skull into the palate is large, shallow and triangular, the orbit is large and round and has a palpebral creating a brow, and the lower jaw has a large mandibular fenestra . The skulls of Emausaurus and Scelidosaurus , two early members of

1335-500: Is disc-shaped and subcircular in lateral and medial views. No sternal ribs are preserved in Lesothosaurus, but based on related taxa, the sternal plates were connected to the rib cage by elements known as sternal ribs. The pelvis was long and expanded dorsally on the ilium, with a long pubis that had a stub-like prepubis connected to it. The ischium had a large proximal end with a curved, thin shaft. The forelimbs were small relative to

1424-402: Is known as "Gregg's Paradox": if a single species is the only member of multiple hierarchical levels (for example, being the only species in its genus, which is the only genus in its family), then each level needs a distinct definition to maintain logical structure. Otherwise, the different taxonomic ranks become effectively identical, which creates problems for organizing biological diversity in

1513-775: Is known from fossils found in formations of the Karoo Supergroup , including the Upper Elliot Formation and the Clarens Formation, which date to the Hettangian and Sinemurian ages of the Lower Jurassic , around 200–190  million years ago . Originally, Lesothosaurus was thought to be from the Upper Triassic period. The Upper Elliot Formation consists of red/purple mudstone and red/white sandstone , whereas

1602-549: Is mostly straight except for transversely expanded proximal and distal ends, the proximal end having two small, rounded condyles. The ulna is also straight, but is slightly longer with a ovoid cross-section. There is only a single, incomplete manus known from Lesothosaurus. The manus has 5 metacarpals and 5 phalanxes, though only the second phalanx is fully preserved. The unguals are small and triangular in dorsal view. The hindlimbs were long and slender, similar to those of most other small basal ornithischians. The femur (thigh bone)

1691-472: Is much debate over whether these filaments found in specimens of Tianyulong , Psittacosaurus , and Kulindadromeus may have been primitive feathers . Ornithischia is a very large and diverse group of dinosaurs, with members known from all continents, habitats, and a very large range of sizes. They are primarily herbivorous browsers or grazers, but some members may have also been opportunistic omnivores. Ornithischians are united by multiple features of

1780-562: Is now seen as a nomen dubium , though the holotype is likely from an individual of Lesothosaurus . The holotype was all that was known until expeditions by the London University College to the same site in Lesotho from 1963 to 1964 recovered scores of fossils from Lesothosaurus , including a partial skeleton including a skull and another isolated partial skull (NHMUK PV RU B17 & NHMUK PV RU B23). These specimens were described in

1869-407: Is particularly associated with island species. Among 25 documented extinct monotypic genera studied, 22 occurred on islands, with flightless animals being particularly vulnerable to human impacts. Just as the term monotypic is used to describe a taxon including only one subdivision, the contained taxon can also be referred to as monotypic within the higher-level taxon, e.g. a genus monotypic within

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1958-438: Is preserved. Lesothosaurus has two types of teeth preserved: long, curved, sharp premaxillary teeth at the front of the mouth; wide, short, robust maxillary and dentary teeth. There are 6 premaxillary teeth on the left and right sides of the premaxilla which are preceded by a small edentulous (tooth-lacking) section that shows signs of preserving a large rhampthotheca (beak) made of keratin . A neck and slight swelling divides

2047-508: Is reduced and round to slit-like, the tip of the snout is sometimes flared to form a broad beak. Members of the ornithopod family Hadrosauridae show further adaptations, including the formation of dental batteries where teeth are continuously replaced, and in many genera the development of prominent cranial crests formed by multiple different bones of the skull. Pachycephalosauria , at one time thought to be close to ornithopods and now known to be related instead to ceratopsians , show

2136-399: Is widest at the midshafts of the quadrates in occipital view. The mandible (lower jaw) of Lesothosaurus has a nearly straight ventral margin and bears only a slight upturn at its anterior tip. The mandible is made up mostly of the dentary , which is 50% of its length in lateral view. Characteristically of Ornithischians, there is a small beak-like bone at the tip of the dentary known as

2225-427: The Early Jurassic (199(?)-190 million years ago). The dentary was described as the holotype of a new genus and species, Fabrosaurus australis , by paleontologist Leonard Ginsburg in 1964. Ginsburg placed it in the family Scelidosauridae and diagnosed it based on its unusual tooth morphology when compared to the only other contemporary ornithischian Heterodontosaurus . Due to its fragmentary nature, Fabrosaurus

2314-668: The Elliot Formation , and the location (Dangerhoek Farm) in South Africa at which the type specimen was found. The type specimen consists of a partial postcranial skeleton, with two additional referred specimens assigned to the species. Fossils from Elliot Formation sites in South Africa outside of Lesotho in Jamestown were described in the 2000s, including a nearly complete skeleton of an adult preserved in articulation. A study published in 2017 by Baron, Norman & Barrett demonstrated that

2403-662: The Late Jurassic , encompassing a diverse array of bodyforms from the small, bipedal Psittacosaurus up to the very large, quadrupedal, horned and frilled ceratopsids like Torosaurus , which has the longest skull of any terrestrial vertebrate. Ornithopods, which range from the Early Jurassic in some studies until the end of the Cretaceous with continuous diversity, are generally bipedal and unarmoured, though some later groups like Hadrosauridae evolved complex dental anatomy in

2492-558: The Upper Elliot Formation . It had a simpler tooth and jaw anatomy than later ornithischians, and may have been omnivorous in some parts of the year. Fossils referrable to Lesothosaurus may have been known from as early as 1959, when a right dentary (lower jawbone) fragment bearing three teeth was collected by French geologist Jean Fabre from the Red Beds of the Upper Elliot Formation near Mapheteng in Lesotho , Southern Africa, dating to

2581-711: The Upper Jurassic Morrison Formation (155-148 mya) of the western United States, Hexinlusaurus from the Shaximiao Formation , dating to the Middle Jurassic (170-168 mya), in southern China, and Hypsilophodon from the Early Cretaceous (130-125 mya) of England. However, the phylogenetic status of basal neornithischians is constantly in a flux and some analyses have recovered these taxa as basal ornithopods or in other groups. Lesothosaurus

2670-493: The ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops ), the pachycephalosaurs or "thick-headed" dinosaurs, the armored dinosaurs ( Thyreophora ) such as stegosaurs and ankylosaurs , and the ornithopods . There is strong evidence that certain groups of ornithischians lived in herds, often segregated by age group, with juveniles forming their own flocks separate from adults. Some were at least partially covered in filamentous (hair- or feather- like) pelts, and there

2759-403: The infratemporal fenestrae are sub-rectangular in lateral view and extend for most of the skull’s height. The infratemporal fenestra is oblong with an oblique axis, while the supratemporal fenestra is oval in outline. The anterior naris (nostril) had several small, sub-ovate openings along its length. The craniomandibular joint (where the skull contacts the jaw bone ) is depressed relative to

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2848-487: The postzygapophyses . The 3rd cervical is also amphicoelus but has a trapezoidal centrum shape. The neural arch is expanded dorsoventrally, but has a small neural spine. As for the dorsal (back) vertebrae, Lesothosaurus has no complete dorsal columns preserved but likely had 12-15 dorsal (back) vertebrae . The dorsals also had spool-shaped centra, ventral keels (though they are lost in more caudal centra), and neural spines that are short and rectangular. The neural spines of

2937-470: The predentary . The predentary is shaped like an arrowhead in ventral view, with one elongated central keel with smaller lateral processes jutting off the sides. The oral margin is smooth and straight in lateral view, with an uncurved anterior tip. Two foramina are preserved on and near the lateral processes, suggesting that this element was well-supplied with blood and nerves. There is a well-developed coronoid eminence, but it does not expand dorsally into

3026-583: The syntypes (the series of fossils that diagnose a species). The generic name Lesothosaurus is derived from the Kingdom of Lesotho, where the fossils were discovered, and the Latin root sauros meaning “lizard”, a root commonly used in dinosaur names. The specific name diagnosticus is derived from the Greek root diagnostikos meaning “distinguished” in reference to Lesothosaurus being a distinct member of Fabrosauridae . In

3115-625: The 1970s as belonging to Fabrosaurus by geologist Richard A. Thulborn . A joint expedition between the NHMUK , London University College , Yale University , and the South African Museum collected many additional specimens of Lesothosaurus from the same site in 1967-68. This included very well preserved cranial material, some of the best known, that was described in the 1991. British paleontologist Peter Galton named Lesothosaurus diagnosticus in 1978, with NHMUK PV RU B17 and NHMUK PV RU B23 as

3204-496: The addition of horns above each orbit and on the top of the snout, as well as substantial elongation of the frill and in many genera the development of two large parietal fenestrae forming holes in the frill. The skull and frill elongation makes the skulls of Torosaurus and Pentaceratops the largest of any known terrestrial vertebrate, at over 2 m (6.6 ft) long. Early ornithischians were relatively small dinosaurs, averaging about 1–2 meters in body length, with

3293-503: The anterior dorsals are also larger than those of the posterior ones. Ossified tendons are preserved attached to the neural spines of anterior dorsals, suggesting they were arranged longitudinally as in Heterodontosaurus, Scelidosaurus, & Hypsilophodon. This feature probably countered stress caused by bending forces acting on the spine during bipedal locomotion. The sacral vertebrae series had 5 vertebrae with sacral ribs and

3382-420: The area where the neurovascular supply accesses the dental lamina . This condition is similar to that in the extant Lepidosaurs, which have lizard lips, leading some paleontologists to suggest that Lesothosaurus had cheeks and lips covering its teeth. The cranium is widest across the postorbitals in dorsal view. It tapers anteriorly to the premaxillae, which creates a short, strongly pointed muzzle. The skull

3471-545: The armoured group Thyreophora , show similarities in the box-like skull that tapers to the front. The antorbital fossa is smaller and forming an elongate oval in both taxa, and the palpebral which is elongate and slender in Lesothosaurus is widened in Emausaurus and completely incorporated into the skull as a flat bone in Scelidosaurus . Skulls in members of the thyreophoran group Stegosauria are much longer and lower, with

3560-455: The axial skeleton which is incomplete in all specimens. The cervical (neck) vertebrae were only 9 in number, but no full cervical series are known. The cervical series of the syntype NHMUK PV R11004 preserves the 2nd cervical, known as the axis , in articulation with the 3rd cervical vertebra. The centrum of the axis is spool-shaped without a ventral keel. The neural arch is very large and well-developed, greatly extending posterodorsally past

3649-441: The basal skull length i. e. from the premaxilla tip to the posterior margin of the basioccipital ). The skulls bears a relatively small, sub-triangular antorbital fenestra (a large gap of bone) with an apex pointing dorsally and a length that is circa 13% of basal skull length. The supratemporal fenestrae are anteroposteriorly longer than mediolaterally wide, with a sub-ovary to sub-triangular outline in dorsal view. In contrast,

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3738-403: The base of the tooth crown from the long, subcylindrical roots . The lingual surface of the more medial premaxillary teeth have a vertical furrow and an adjacent sharp ride that extends towards the crown’s mesial edge. The last two teeth crowns in the series acquire distal and medial denticles . The posterior process of the premaxilla lacks alvelovi, creating a small diastema (gap) inbetween

3827-745: The common application of the term monotypic is frequently misleading, "since each taxon by definition contains exactly one type and is hence "monotypic", regardless of the total number of units", and suggests using "monospecific" for a genus with a single species, and "monotaxonomic" for a taxon containing only one unit. Species in monotypic genera tend to be more threatened with extinction than average species. Studies have found this pattern particularly pronounced in amphibians , where about 6.56% of monotypic genera are critically endangered , compared to birds and mammals where around 4.54% and 4.02% of monotypic genera face critical endangerment respectively. Studies have found that extinction of monotypic genera

3916-414: The cranium is just behind the orbit (eye socket) in lateral view and the skull roof ( frontals , parietals ) are gently rounded in lateral view. The snout is smoothly tapered to the premaxilla (snout tip bone). The skull lacks a break in slope along the snout anterior to the orbit, as in the contemporary Heterodontosaurus . The orbits are round and large relative to the skull size (making up 36% of

4005-423: The differences between Stormbergia and Lesothosaurus are most likely related to the animal's growth. The authors argued that Stormbergia is a junior subjective synonym of Lesothosaurus and should be regarded as invalid. Several other skull and postcranial specimens have been discovered since, including the description of two partial skulls in 2002, which preserved signs of individual variation. Redescription of

4094-442: The drier climate at this time in southern Africa. Both formations are famous for their abundant vertebrate fossils, including temnospondyl amphibians , turtles, lepidosaurs , aetosaurs , crocodylomorphs , and non-mammal cynodonts . Other dinosaurs from these formations include the heterodontosaurid Heterodontosaurus , the basal sauropodomorph Massospondylus , and the theropod Megapnosaurus . The Upper Elliot Formation shows

4183-1094: The early evolution of ornithopods considerably, and showing that the evolution of ornithischians was far from definitive. Below are the cladograms of Sereno, Butler and colleagues, and Dieudonné and colleagues, restricted to the major clades of Ornithischia, Heterodontosauridae, Lesothosaurus and Pisanosaurus . Sereno, 1986 Lesothosaurus Stegosauria [REDACTED] Ankylosauria [REDACTED] Pachycephalosauria [REDACTED] Ceratopsia [REDACTED] Heterodontosauria Ornithopoda [REDACTED] Butler et al., 2008 Pisanosaurus Heterodontosauridae Lesothosaurus Stegosauria [REDACTED] Ankylosauria [REDACTED] Ornithopoda [REDACTED] Pachycephalosauria [REDACTED] Ceratopsia [REDACTED] Dieudonné et al., 2021 Lesothosaurus Stegosauria [REDACTED] Ankylosauria [REDACTED] Ornithopoda [REDACTED] Ceratopsia [REDACTED] Pachycephalosauria (incl. heterodontosaurids ) [REDACTED] When Ornithischia

4272-542: The eye and jaw muscles. The tip of the snout likely ended in a small beak, based on a blade-like predentary bone (at the tip of the lower jaw) and a roughly-texture front end of the cranium. Its teeth were pointed with grooved edges. The skull was mounted on a short but flexible neck. A bonebed of Lesothosaurus described in 2016 includes material from three large individuals. This association suggest that this early ornithischian dinosaur may have lived in groups. The skull and teeth of Lesothosaurus are more generalized than

4361-415: The femur and more slender. The fibula has an expansion at the proximal end and a thin shaft that is sub-oval in cross-section. The astragalus and calcaneum are small, though the latter is the smaller of the two, and only preserved in one specimen. The metatarsals are thin, long, and tightly fit together. The metatarsals have great expansions at the distal ends where the pedal phalanges would articulate with

4450-512: The form of batteries of teeth. Stegosaurs are comparatively limited, restricted to a primarily Jurassic group of moderate to large, quadrupedal herbivores with two rows of vertical plates ornamenting their spine, which possibly did not go extinct until the Late Cretaceous, though at the time of Marsh Stegosauria was used for all armored and quadrupedal taxa, many of which are now separated into Ankylosauria . Ankylosaurs were only recognized as

4539-452: The form of head-butting or flank-butting. Some taxa, particularly those at one point groupt together in the ornithopod family Hypsilophodontidae , are now recognized to not fall within any of the major ornithischian groups, and either be outside Genasauria, or on the basal stem of Neornithischia outside Cerapoda. Following the publication of the PhyloCode to provide rules and regulations on

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4628-468: The group Predentata to unite ornithopods, stegosaurs, and Ceratopsia within Dinosauria, but with additional work and new discoveries the unnatural nature of Dinosauria came to be accepted, and the names Seeley proposed found common use. After further decades, in 1974 Robert T. Bakker and Peter M. Galton provided new evidence in support of the grouping of ornithischians and saurischians together within

4717-440: The heavily specialized and unusual anatomies of the contemporary scelidosaurid and heterodontosaurid ornithischians, which exhibit traits like osteoderms and extreme heterodonty . The best preserved skull is NHMUK PV RU B23, though it is missing some elements. The skull was unlike the triangular skull of Heterodontosaurus in that the caudal half was boxy while the anterior half was tapered and elongated. The height apex of

4806-427: The information content of biological classifications. As taxonomists Backlund and Bremer explain in their critique, "'Monotypic' taxa do not provide any information about the relationships of the immediately subordinate taxon". When monotypic taxa are sister to a single larger group, they might be merged into that group; however, when they are sister to multiple other groups, they may need to remain separate to maintain

4895-445: The largest horned ornithischians were around 8.5 m (28 ft) and 11 t (11 long tons; 12 short tons), and the largest crested ornithischians were around 15 m (49 ft) and 13.5 t (13.3 long tons; 14.9 short tons). Much of the knowledge of early ornithischian anatomy comes from Lesothosaurus , which is a taxon known from multiple skulls and skeletons from the Early Jurassic of Lesotho . The rear of its skull

4984-619: The largest known heterodontosaurid diversity of any rock unit; besides Heterodontosaurus , it contained Lycorhinus , Abrictosaurus , and Pegomastax . Yet another member of the family, Geranosaurus , is known from the Clarens Formation. The high heterodontosaurid diversity have led researchers to conclude that different species might have fed on separate food sources in order to avoid competition ( niche partitioning ). [REDACTED] [REDACTED] [REDACTED] Monotypic taxon Monotypic taxa present several important theoretical challenges in biological classification . One key issue

5073-530: The margin of the maxillary alveoli. There are 15-16 tooth positions in the maxilla, in contrary to the 11 preserved in Pisanosaurus ’ (a Late Triassic dinosauriform that may be an ornithischian). There are 20 tooth positions in the dentary of Lesothosaurus, but only 15 are preserved in Pisanosaurus. The alveolar foramina are on the medial wall of the maxilla and dentary, one per tooth position, and represent

5162-490: The material referred to Lesothosaurus , Galton stated that some of it was instead from a “large fabrosaurid”. This “large fabrosaurid” was finally named in 2005, dubbed Stormbergia dangershoeki , on the basis of the partial postcranial skeleton SAM-PK-K1105. This species almost certainly represents the adult form of Lesothosaurus . Stormbergia was named for the Stormberg Series of rocks in southern Africa, which includes

5251-458: The opisthopubic pelvis evolved a fourth time, in the clade Dromaeosauridae , but this is controversial, as other authors argue that dromaeosaurids are mesopubic. It has also been argued that the opisthopubic condition is basal to maniraptorans (including among others birds, therizinosauroids and dromaeosaurids), with some clades having later experienced a reversal to the propubic condition. The first recognition of an herbivorous group of dinosaurs

5340-474: The origins of the group and the relationships of primitive taxa like Pisanosaurus and members of Silesauridae may sometimes be found to be ornithischians outside this core grouping. Madzia and colleagues also provided a composite cladogram of Ornithischia to illustrate the consensus of internal divisions, which can be seen below. Ornithischia has been defined as all taxa closer to Iguanodon than Allosaurus or Camarasaurus . Genasauria has been defined as

5429-420: The palate. The skulls are known from many early ornithopods and some heterodontosaurids , showing similar general features. Skulls are relatively tall with shorter snouts, but the snout is elongated in some later taxa like Thescelosaurus . The orbit and antorbital fossa are large, but the nasal opening is small, and while teeth are present in the premaxilla, there is a toothless front tip that likely formed

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5518-743: The phylogenetics of Ornithischia was published in 2008 by Richard J. Butler and colleagues, including many primitive ornithischians and members from all of the major subgroups, to test some of the hypotheses given previously about ornithischian evolution and the relationships of the groups. Thyreophora was found to be a supported group, as well as the clade of pachycephalosaurs and ceratopsians that Sereno named Marginocephalia in 1986. Some taxa considered earlier to be ornithopods, like heterodontosaurids, Agilisaurus , Hexinlusaurus and Othnielia , were instead found to be outside of both Ornithopoda and Ceratopsia, but still closer to those two groups than thyreophorans. The early Argentinian taxon Pisanosaurus

5607-460: The premaxillary and maxillary tooth rows. The maxillary and dentary teeth are low, triangular, and “leaf-shaped” with a distinct neck and cingulum . The denticles are coarse on the medial and distal tooth borders, with sporadically developed high-angled marginal tooth wearing . This suggests rapid tooth replacement in these teeth. Although many specimens are known, some elements of Lesothosaurus’ postcranial anatomy are poorly known, especially in

5696-560: The relationships within Ornithischia with greater detail was that of Sereno in 1986 , who provided features that supported the evolution of all ornithischian groups and shared similarities with earlier studies. Sereno found that Lesothosaurus was the most primitive ornithischian, with all other ornithischians united within the clade Genasauria, which has two subgroups. The first subgroup, Thyreophora , unites ankylosaurs and stegosaurs along with more primitive taxa like Scelidosaurus , while

5785-453: The rest of the body. The humerus was elongate and straight in anterior and lateral views, with expanded proximal and distal ends linked by a long, slender shaft. The proximal end had a large deltopectoral crest which was asymmetrical and C-shaped in dorsal view. The humeral shaft has a rounded, transverse cross-section. The humerus length was only 63.3% as long as the femur length, which is much shorter than other basal ornithischians. The radius

5874-438: The rest of the leg. Metatarsal III is the longest of them all and has the greatest transverse width distally and at the mid-shaft. The metatarsal I is truncated and less than half the length of metatarsal II. Notably, Lesothosaurus lacks a metatarsal V, a distinct trait of the taxon. The pedal digits (toes) are long, with pedal phalanges (toe bones) that are spool-shaped, with large proximal and distal ends but thin shafts. Digit I

5963-420: The second subgroup, Cerapoda , contained ornithopods, ceratopsians, pachycephalosaurs, and small primitive forms. One group of the small primitive forms considered to be cerapodans by Sereno, Heterodontosauridae , has since been found to be a group of very early ornithischians of similar evolutionary status as Lesothosaurus , although this result is not definitive. The first large-scale numerical analysis of

6052-411: The skull, limbs, and hip, were unrelated to other dinosaurs, and so he proposed that Dinosauria was an unnatural grouping of two independently-evolved suborders , Saurischia and Ornithischia. It is from the anatomy of the hip that Seeley chose the name Ornithischia, referencing the bird-like anatomy of the ischium bone. Many researchers did not follow the division of Seeley at first, with Marsh naming

6141-471: The skull, teeth, and skeleton, including especially the presence of a predentary and palpebral , an increased number of sacral vertebrae , the absence of gastralia , and an opisthopubic pubis . Early ornithischians ranged around 1–2 m (3.3–6.6 ft) in length, with them increasing in size over time so that the largest armoured ornithischians were around 7.5 m (25 ft) and 9 t (8.9 long tons; 9.9 short tons),

6230-546: The slightly younger Clarens Formation consists of white/cream-coloured sandstone. The Clarens Formation is less rich in fossils than the Upper Elliot Formation; its sediments also often form cliffs, restricting accessibility for fossil hunters. The Upper Elliot Formation is characterised by animals that appear to be more lightly built than those of the Lower Elliot Formation, which may have been an adaptation to

6319-429: The snout is short and tapering, the nasal opening is small, the antorbital fossa is sometimes absent, and there are premaxillary teeth, though only three. The two palpebrals are also incorporated into the skull roof as in thyreophorans, rather than free. Ceratopsians, the sister group to pachycephalosaurs, also display many cranial adaptations, most importantly the evolution of a bone called the rostral that forms

6408-438: The study of Norman placed ceratopsians between Hypsilophodon and more derived ornithopods, the study of Sereno placed ceratopsians with ankylosaurs and stegosaurs. It has since been recognized by that ceratopsians are closer to ornithopods than the armoured ankylosaurs and stegosaurs, but the relationships of some groups are still in states of change, with some more consistent results than others. An early study that looked at

6497-420: The sutures separating skull bones are almost completely obliterated by surface texturing, and there is bony armour above the orbits, and at the top and bottom corners of the back of the skull. Teeth are sometimes absent from the premaxilla, and both the upper and lower jaws have deeply inset teeth creating large cheeks. Ankylosaurs also have very extensive and complicated network of sinuses, formed by bone growth in

6586-506: The syntypes came in 2015 and 2017, including the integration of CT technology. Due to the great quantity and quality of specimens known from Lesothosaurus , information about its anatomy is known in detail. Lesothosaurus was a lightly built, bipedal animal that varied between 1 (3.3 ft) to 2 meters (6.6 ft) long. It was one of the earliest ornithischians . Its long slender legs, small arms with hands that would not have been able to grasp properly, and slender tail all suggest that it

6675-538: The tall, distinct processes (projection of bone) like in advanced Ornithischians like Triceratops and Zalmoxes . The jaw joint is slightly depressed relative to the alveolar bar that takes up most of the mandible. The mandible preserves an anteroposteriorly elongated fenestra between the dentary, angular , and surrangular , similar to the ones in the skull, that would make the mandible lighter. The mandible differentiates from those of other Ornithischians greatly in that an inturned, ‘spout-like’ mandibular symphysis

6764-420: The tooth wear have shown much less abrasion on the teeth than would be expected of a plant-eater feeding mainly on tough, arid-climate plants, and concluded that Lesothosaurus was probably an opportunistic omnivore , feeding primarily on small animals during seasons when softer plants were not available. The small skull of Lesothosaurus was narrow and pointed, with large eye sockets. It had large cavities for

6853-565: The top beak opposite the predentary. The jugal bones flare to the sides to create a pentagonal skull seen from above, the nasal opening is closer to the top of the snout than the teeth, and while the snout tapers in some taxa, it is very deep and short in Psittacosaurus . The ceratopsian palpebral is generally triangular, and the back edge of the skull roof forms a flat frill that is enlarged in more derived ceratopsians. The ceratopsian family Ceratopsidae progresses on these features with

6942-413: The two main orders of Dinosauria (the other being Saurischia). This opinion has been supported by later cladistic studies of basal Ornithischia, which have also found it as the basalmost member of Neornithischia (a group that includes pachycephalosaurs, ceratopsians, and ornithopods) and related to Agilisaurus, Hexinulsaurus, and Nanosaurus. Alternatively, this dinosaur may be a very early thyreophoran ,

7031-401: The use of taxonomic names for groups, the internal classification of Ornithischia was revised by Daniel Madzia and colleagues in 2021 to provide a framework of definitions and taxa for other studies to follow and modify from. They names the new clade Saphornithischia to unite heterodontosaurids with more derived ornithischians to encompass the concept of the well-supported clear ornithischians, as

7120-420: The ventral side of the caudal centra, with larger attachment points on the proximal caudals. The scapula (shoulder blade) is not fused to the coracoid and is longer than the humerus (upper arm bone). The dorsal surface develops a large, bar-like acromion process that extends further dorsally than in many other ornithischians. The distal end of the scapula is greatly expanded and has a convex margin. The coracoid

7209-661: The width at the back being greater than the height in Stegosaurus . The snout and lower jaw are long and deep, and in some genera the premaxilla does not have any teeth. As in Scelidosaurus , the palpebral forms the top border of the orbit as a flat brow bone, but the antorbital fossa is reduced to the point of absence in some genera. Ankylosaurs , the other group of armoured ornithischians, have very robust, immobile skulls, with three significant features that separate them from other groups. The antorbital fossa, supratemporal fenestra and mandibular fenestra are all closed,

7298-588: Was "opisthopubic", meaning that the pubis pointed down and backwards ( posterior ), parallel with the ischium (Figure 1a). Additionally, the ilium had a forward-pointing process (the preacetabular process) to support the abdomen. This resulted in a four-pronged pelvic structure. In contrast to this, the saurischian pelvis was "propubic", meaning the pubis pointed toward the head ( anterior ), as in ancestral reptiles (Figure 1b). The opisthopubic pelvis independently evolved at least three times in dinosaurs (in ornithischians, birds and therizinosauroids ). Some argue that

7387-432: Was a fast runner. Like all ornithischians, the tips of Lesothosaurus upper and lower jaws were horny, forming a beaklike structure. Behind the beak were leaf-shaped teeth that lined the jaws. The teeth of the premaxillae (six per side) are more slender and curved than the maxillary teeth. Analysis of its teeth has shown that Lesothosaurus sliced up its food with its beak and was not able to chew its food. Studies of

7476-521: Was a hallux, with one small phalange ending in a large ungual (claw). Digit II had two large, thick phalanges ending in a large, wide ungual. Digit III was the longest digit by length and had a combined length of 57.7 millimetres (2.27 in). It was composed of 3 pedal phalanges and an ungual. Digit IV had the most pedal phalanges, with 4 preserved and a small ungual. The ungual bones of the toes were claw-like, and not hoof-like as in more advanced ornithischians. Peter Galton considered Lesothosaurus to be

7565-459: Was first named, Seeley united the orders Ornithopoda and Stegosauria of Marsh's taxonomy within the new group. Ceratopsia was then recognized as a unique group related to ornithopods and stegosaurs by Marsh by 1894, with each of the three suborders still being recognized as distinct groups today. Ceratopsians are recognized as group that grew in diversity later in the Cretaceous after evolving in

7654-494: Was found to be the most primitive ornithischian, but while overall results agreed with earlier studies and showed some stability, areas of the evolutionary tree were found to be problematic, and with potential for later change. In 2021 , a new phylogenetic study was published authored by Paul-Emile Dieudonné and colleagues that instead found Heterodontosauridae to nest alongside Pachycephalosauria within Marginocephalia, changing

7743-500: Was lightly built, with a largely fenestrated skull and a very stout neck and trunk. The tail is nearly half of the dinosaurs' overall length. The long tail presumably acted as a counterbalance and as a compensating mechanism for shifts in the creature's center of gravity. The hindlimbs of early ornithischians show that the tibia is considerably longer than the femur, a feature that suggests that early ornithischians were adapted for bipedality, and were fast runners. The ornithischian pelvis

7832-399: Was long and bowed anteriorly, with a large 4th trochanter for muscle attachments. The femoral head at the proximal end was large, while the distal end terminated with two condyles where the tibia would interlock. The largest known femur is from NMQR 3076, which measures 273.70 millimetres (10.776 in) in length. The tibia (shin bone) has a very similar morphology, but it is 25% longer than

7921-517: Was named Orthopoda in 1866 by Edward Drinker Cope , a name that is now recognized as a synonym of Ornithischia. Discussions on the taxonomy of dinosaurs by Othniel Charles Marsh identified two major groups of herbivorous dinosaurs, Ornithopoda and Stegosauria , containing genera from a broad geographic and stratigraphic distribution. While often these groups were placed within Dinosauria, Harry Govier Seeley suggested instead in 1888 that ornithopods and stegosaurs, which shared many features in

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