Lesothosaurus

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Lesothosaurus
Temporal range:
Ma
MNHN LES 17, a skull
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: †Ornithischia
Clade: †
Saphornithischia
Clade: †Genasauria
Genus: †Lesothosaurus
Galton, 1978
Type species
†Lesothosaurus diagnosticus
Synonyms
  • Fabrosaurus australis
    Ginsburg, 1964
  • Stormbergia dangershoeki
    Butler, 2005

Lesothosaurus is a

omnivorous in some parts of the year.[4]

Discovery and naming

Illustration of the Fabrosaurus holotype dentary.

Fossils referrable to Lesothosaurus may have been known from as early as 1959, when a right

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.[1]

Map of where Lesothosaurus fossils have been found in Lesotho and South Africa.

British paleontologist Peter Galton named Lesothosaurus diagnosticus in 1978, with NHMUK PV RU B17 and NHMUK PV RU B23 as the syntypes (the series of fossils that diagnose a species).[11] 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.[11] The specific name diagnosticus is derived from the Greek root diagnostikos meaning “distinguished” in reference to Lesothosaurus being a distinct member of Fabrosauridae.[11] In the material referred to Lesothosaurus, Galton stated that some of it was instead from a “large fabrosaurid”.[11] 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.[3] Stormbergia was named for the Stormberg Series of rocks in southern Africa, which includes 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.[7] 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.[3] A study published in 2017 by Baron, Norman & Barrett demonstrated that 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.[3] 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.[12] Redescription of the syntypes came in 2015 and 2017, including the integration of CT technology.[3][2]

Description

Life restoration (reconstructed without filaments) and size compared to a human

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

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.[2] Analysis of its teeth has shown that Lesothosaurus sliced up its food with its beak and was not able to chew its food.[14] Studies of 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.[4]

The small skull of Lesothosaurus was narrow and pointed, with large eye sockets. It had large cavities for 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.[2] Its teeth were pointed with grooved edges. The skull was mounted on a short but flexible neck.[13]

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.[15]

Skull and dentition

Skull diagram

The skull and teeth of Lesothosaurus are more generalized than the heavily specialized and unusual anatomies of the contemporary

joint (where the skull contacts the jaw bone) is depressed relative to 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 the area where the neurovascular supply accesses the dental lamina.[2] 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.[16][17] 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 is widest at the midshafts of the quadrates in occipital view.[2][18]

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 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 the tall, distinct processes (projection of bone) like in advanced Ornithischians like Triceratops and Zalmoxes.[19][20] 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 is preserved.[18][2]

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

tooth replacement in these teeth.[18][2]

Postcrania

Although many specimens are known, some elements of Lesothosaurus’ postcranial anatomy are poorly known, especially in the axial skeleton which is incomplete in all specimens. The

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.[18][3] The chevrons are Y-shaped in anterior and posterior view and attached to the ventral side of the caudal centra, with larger attachment points on the proximal caudals.[3]

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 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.[3]

The forelimbs were small relative to 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 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.[7][1][3]

The hindlimbs were long and slender, similar to those of most other small basal ornithischians. 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 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 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.[7][18][3]

Life restoration (reconstructed with filaments) of Lesothosaurus.

Classification

Peter Galton considered Lesothosaurus to be 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 the two main orders of Dinosauria (the other being Saurischia).[1] 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.[3] Alternatively, this dinosaur may be a very early thyreophoran, a member of the group including the armored stegosaurians and ankylosaurians.[22]

The Butler et al., 2005 analysis placed Lesothosaurus at the base of Neornithischia:

Euparkeria

Basal neornithischians like Lesothosaurus are known from several time periods and regions, with Nanosaurus fossils coming from the Upper Jurassic Morrison Formation (155-148 mya) of the western United States,[23] Hexinlusaurus from the Shaximiao Formation, dating to the Middle Jurassic (170-168 mya), in southern China,[24] and Hypsilophodon from the Early Cretaceous (130-125 mya) of England.[25] 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.[26][23][3]

Paleoenvironment

Lesothosaurus is known from fossils found in formations of the

cynodonts.[28][29]

Other dinosaurs from these formations include the heterodontosaurid Heterodontosaurus, the basal sauropodomorph Massospondylus, and the theropod

niche partitioning).[30][31]

References

  1. ^
    ISSN 0272-4634
    .
  2. ^
    S2CID 510709
    .
  3. ^
    ISSN 0024-4082
    .
  4. ^
    S2CID 17069570
    .
  5. ^ a b c Ginsburg, L., (1964), "Decouverte d’un Scelidosaurien (Dinosaure ornithischien) dans le Trias superieur du Basutoland", Comptes Rendus de l'Académie des Sciences de Paris, 258; 2366–2368.
  6. ISSN 1012-0750
    .
  7. ^
    ISSN 1096-3642
    .
  8. ISSN 0952-8369
    .
  9. ^ Thulborn, R. A. (1972). The postcranial skeleton of the Triassic ornithischian dinosaur Fabrosaurus australis. Palaeontology, 15(1), 29-60.
  10. ^ Thulborn, R. A. (1970). The skull of Fabrosaurus australis, a Triassic ornithischian dinosaur. Palaeontology, 13(3), 414-432.
  11. ^
    S2CID 84613826
    .
  12. ISSN 0016-6995
    .
  13. ^
    ISBN 1-84028-152-9
    .
  14. ISBN 978-0-7566-9910-9
    .
  15. ISSN 0078-8554
    .
  16. ^ Knoll, F. (2008). Buccal soft anatomy in Lesothosaurus (Dinosauria: Ornithischia). Neues Jahrbuch fur Geologie und Palaontologie-Abhandlungen, 248(3), 355-364.
  17. S2CID 52986589
    .
  18. ^ a b c d e Norman, D. B., Witmer, L. M., & Weishampel, D. B. (2004). Basal ornithischia. The dinosauria, 2, 325-334.
  19. OCLC 801843269
    .
  20. S2CID 86339025
    .
  21. OCLC 11886969
    .
  22. S2CID 86728076
    .
  23. ^
    S2CID 73691452
    .
  24. S2CID 85892620
    .
  25. ^ Galton, P. M. (1974). The ornithischian dinosaur Hypsilophodon from the Wealden of the Isle of Wight.
  26. S2CID 90008574
    .
  27. ^ Sereno, P.C. (2012). pp. 4–17.
  28. ^
    S2CID 4198113
    .
  29. doi:10.2113/176.1.81
    .
  30. S2CID 128636991. Archived from the original
    (PDF) on 2017-08-11. Retrieved 2015-12-30.
  31. PMID 22509242
    .

Bibliography

  • P. M. Galton. 1978. Fabrosauridae, the basal family of ornithischian dinosaurs (Reptilia: Ornithischia). Paläontologische Zeitschrift 52(1/2):138-159
  • Butler, R.J., 2005. "The 'fabrosaurid' ornithischian dinosaurs of the Upper Elliot Formation (Lower Jurassic) of South Africa and Lesotho." Zoological Journal of the Linnean Society 145: 175–218.
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