''Siamodon'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus of iguanodontian ornithopod dinosaur from

Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pro ...

deposits of northeastern Thailand.

Discovery

''Siamodon'' is known from the holotype PRC-4, a well-preserved left maxilla and from the referred materials PRC-5, an isolated maxillary tooth and PRC-6, a

braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calvaria or skul ...

. It was collected in the Ban Saphan Hin site, Nakhon Ratchasima Province, from the

Khok Kruat Formation The Khok Kruat Formation () is a rock formation found in northeastern Thailand. It is the uppermost formation of the Khorat Group. It is dated to the Aptian stage of the Early Cretaceous period, and is notable for its fossils of dinosaurs. It ...

, dating to the

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 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...

stage of the late

, about 125-113 million years ago.

Description

''Siamodon'' shows a combination of plesiomorphic and apomorphic features, including a maxilla shaped like an

isosceles triangle In geometry, an isosceles triangle () is a triangle that has two sides of equal length. Sometimes it is specified as having ''exactly'' two sides of equal length, and sometimes as having ''at least'' two sides of equal length, the latter versio ...

, with the dorsal process located at about mid-length of the bone; a strong longitudinal bulge on the medial surface of the maxilla; at least 25 maxillary teeth, which bear a prominent median primary ridge, and one short weak subsidiary ridge or no subsidiary ridge at all, and mamillated denticles on the crown margins. The maxilla is 230 millimeters long, and has a height of 100 millimeters. The height of the isolated tooth is about 25-28 millimeters, and the width is about 14-17 millimeters. ''Siamodon'' differs from more basal iguanodontians, such as '' Iguanodon'' and closely related forms, in the morphology of its maxillary teeth, which are narrower and bear a strong median primary ridge, sometimes accompanied by a weak subsidiary ridge, instead of a distally displaced primary ridge and several subsidiary ridges, and the apex of the maxilla is in a more posterior position. Its maxilla differs from that of

hadrosaurids Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which includ ...

in the articular area for the jugal, with forms a tab-like jugal process. Whereas in hadrosaurids, the expanded anterior end of the jugal contacts and overlaps a large sutural area on the maxilla. The combination of characters seen in the maxilla of ''Siamodon nimngami'' indicate that it belongs to a group of iguanodontians more derived than ''Iguanodon'' but basal to Hadrosauridae. It may be closely related to ''

Probactrosaurus ''Probactrosaurus'' (meaning "before ''Bactrosaurus''") is an early herbivorous hadrosauroid iguanodont dinosaur. It lived in China during the Late Cretaceous period. Discovery and species In 1959 and 1960 a Soviet-Chinese expedition uncovered ...

'' from China but they differ in the number of tooth positions.

Etymology

''Siamodon'' was first named by Éric Buffetaut and Varavudh Suteethorn in

2011 File:2011 Events Collage.png, From top left, clockwise: a protester partaking in Occupy Wall Street heralds the beginning of the Occupy movement; protests against Libyan dictator Muammar Gaddafi, who was killed that October; a young man celebrate ...

. The type species is ''Siamodon nimngami''. The generic name is derived from Siam, the ancient name of Thailand; and ', Greek for "tooth". The

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

is in honour of Witaya Nimngam, who donated the specimens to the authors.

References

{{Taxonbar, from=Q1705962 Iguanodonts Aptian life Early Cretaceous dinosaurs of Asia Cretaceous Thailand Fossils of Thailand Fossil taxa described in 2011 Taxa named by Éric Buffetaut Taxa named by Varavudh Suteethorn Ornithischian genera