Secernosaurus
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''Secernosaurus'' (meaning "severed lizard") 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
herbivorous A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...
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 ...
. ''Secernosaurus'' was a
hadrosaur 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 incl ...
, a "duck-billed" dinosaur which lived during the Late
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
. ''Secernosaurus'' and its close relatives lived in South America, unlike most hadrosaurs, which lived in the
Laurasian Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pa ...
continents of Eurasia and North America. It has been suggested that the ancestors of ''Secernosaurus'' crossed into South America when a land bridge temporarily formed between North and South America during the Late Cretaceous and allowed
biotic interchange Biotic interchange is the process by which species from one Biota (taxonomy), biota invade another biota, usually due to the disappearance of a previously impassable barrier. These dispersal barriers can be physical, climatic, or biological and ca ...
between the two continents.


History of research

The holotype of ''Secernosaurus koeneri'' was collected in 1923 as part of an expedition by the Field Museum led by J. B. Abbott. However, the specimen was not studied until the 1970s. In 1979, Brett-Surman named ''Secernosaurus''. Though hadrosaurid specimens from South America had been described before, ''Secernosaurus koeneri'' was the first species of South American hadrosaurid to be formally named. The genus name ''Secernosaurus'' means "separated reptile" and comes from the Latin verb '' sēcernō'', meaning to sever or divide, in reference to its geographic location separated from Laurasian hadrosaurs. The species name ''S. koerneri'' honors Harold E. Koerner. The holotype of ''Secernosaurus koerneri'' is FMNH P13423, a partial skeleton from the Lago Colhué Huapi Formation of Chubut province, Argentina. In 2010, Albert Prieto-Marquez and Guillermo Salinas argued that ''
Kritosaurus ''Kritosaurus'' is an incompletely known genus of hadrosaurid (duck-billed) dinosaur. It lived about 74.5-66 million years ago, in the Late Cretaceous of North America. The name means "separated lizard" (referring to the arrangement of the cheek ...
australis'' was synonymous with ''Secernosaurus koeneri''. However, in 2015, Rodolfo Coria noted differences between the two, and suggested their taxonomy needed reevaluation. In 2022, the two species were recognized as separate and the genus '' Huallasaurus'' was established for ''Kritosaurus australis''.


Description

''Secernosaurus'' may have been small for a hadrosaurid. The type specimen pertains to an individual approximately 5 m long, but the maturity of the specimen is uncertain.


Classification

Phylogenetic analyses In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
have found ''Secernosaurus'' to be part of a
tribe The term tribe is used in many different contexts to refer to a category of human social group. The predominant worldwide usage of the term in English is in the discipline of anthropology. This definition is contested, in part due to confli ...
of hadrosaurid dinosaurs known as
Kritosaurini Kritosaurini is a tribe of saurolophine hadrosaurid dinosaurs from the Late Cretaceous. Discovery and naming The first member of the group discovered and named was ''Kritosaurus''; it was named by paleontologist Barnum Brown in 1910. Four year ...
, within the subfamily Saurolophinae. North American animals such as ''
Kritosaurus ''Kritosaurus'' is an incompletely known genus of hadrosaurid (duck-billed) dinosaur. It lived about 74.5-66 million years ago, in the Late Cretaceous of North America. The name means "separated lizard" (referring to the arrangement of the cheek ...
'' and ''
Gryposaurus ''Gryposaurus'' (meaning "hooked-nosed (Greek ''grypos'') lizard"; sometimes incorrectly translated as "griffin (Latin ''gryphus'') lizard") was a genus of duckbilled dinosaur that lived about 80 to 75 million years ago, in the Late Cretaceo ...
'' are also part of this clade. Specifically, within Kritosaurini, a 2022 paper by Sebastián Rozadilla and colleagues describing fossil material of South American hadrosaurs found those from the continent to all group together within a single clade, including ''Secernosaurus'', '' Huallasaurus'', '' Kelumapusaura'', and ''
Bonapartesaurus ''Bonapartesaurus'' (meaning "Bonaparte's lizard", named after José Bonaparte) is an extinct genus of herbivorous ornithopod dinosaur belonging to Hadrosauridae, which lived in the area of modern Argentina during the Campanian and Maastrichtia ...
''. The results of phylogenetic analysis of that study, modified from that of a 2020 study by Ryuji Takasaki and colleagues, is shown below:


Palaeoecology


Palaeoenvironment

The geologic layers of the Lago Colhué Huapi Formation, where ''Scernosaurus'' hails from, have proved difficult to interpret historically, their assignment shifting around between several different
geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...
s before finally being settled as its own unit of the Chubut Group, dating to the
Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval ...
. Additionally, ''Secernosaurus'' specifically had a very uncertain geologic provenance. Resultingly, the ecosystem ''Secernosaurus'' would've lived in was not strongly understood historically. A 2016 paper by Casal A. Gabriel and colleagues studied the climatic conditions of the region more in depth, finding evidence of climate change across geologic time. Fluvial systems and evidence of a floodplain environment were recognized, but the geologically lower parts of the formation also showed evidence of semi-arid conditions, large
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywal ...
deposits, and desiccation cracks, indicating intense aridization of the region compared to the very humid climate of the Bajo Barreal Formation's ecosystem that preceded Lago Colhué Huapi Formation. However,
palynological Palynology is the "study of dust" (from grc-gre, παλύνω, palynō, "strew, sprinkle" and ''-logy'') or of "particles that are strewn". A classic palynologist analyses particulate samples collected from the air, from water, or from deposit ...
data indicates that during the upper deposits of the formation at the very end of the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
and into the
Danian The Danian is the oldest age or lowest stage of the Paleocene Epoch or Series, of the Paleogene Period or System, and of the Cenozoic Era or Erathem. The beginning of the Danian (and the end of the preceding Maastrichtian) is at the Cretace ...
age of the
Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...
, the climate become more mild once again and returned to a balance wet and dry season. It is from this uppermost part of the formation that ''Secernosaurus'' it from. More recent discoveries of hadrosaur remains from other localities, which could belong to ''Secernosaurus'' might extend the range within the formation hadrosaurs are found in. Currently, however, they have not been research in depth. ''Secernosaurus'' is not the only dinosaur known from Lago Colhué Huapi. Remains of the sauropods '' Elaltitan lilloi'', ''
Argyrosaurus superbus ''Argyrosaurus'' ( ) is a genus of titanosaurian sauropod dinosaur that lived about 70 million years ago, during the Late Cretaceous Period (geology), Period of what is now Argentina. Discovery and naming The type species, ''Argyrosaurus superb ...
'' and '' Aeolosaurus colhuehuapensis'', as well as unidentified sauropod remains found in 2010, have been discovered in the formation. Very fragmentary remains of
dromaeosaurid 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 ...
and
megaraptoran Megaraptora is a clade of carnivorous tetanuran theropod dinosaurs with controversial relations to other theropods. Its derived members, the Megaraptoridae are noted for their elongated hand claws and proportionally large arms, which are usually ...
theropods have also been discovered, as is expected from other formations from a similar time and place. The enigmatic
ornithischian Ornithischia () is an extinct order 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 st ...
dinosaur ''
Notoceratops ''Notoceratops'' (meaning "southern horned face") is a dubious genus of extinct ornithischian dinosaur based on an incomplete, toothless left dentary (now lost) from the Late Cretaceous of Patagonia (in Argentina), probably dating to the Campani ...
'', based on a lost fragmentary specimen originally considered to belong to a
ceratopsian Ceratopsia or Ceratopia ( or ; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic ...
but now debated between that identity and that of a hadrosaur. Finally, scant fossils of chelonians,
crocodyliformes Crocodyliformes is a clade of crurotarsan archosaurs, the group often traditionally referred to as "crocodilians". They are the first members of Crocodylomorpha to possess many of the features that define later relatives. They are the only pse ...
, and
dipnoi Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
d fish have also been found.


Palaeobiogeography

''Secernosaurus'' lived in what is now Patagonia during the
Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval ...
age of the Cretaceous period. It was one of the few hadrosaurs to live in South America. Their presence in South America is likely to represent a dispersal event from North America during the
Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campani ...
, when the proto-
Antilles The Antilles (; gcf, label=Antillean Creole, Antiy; es, Antillas; french: Antilles; nl, Antillen; ht, Antiy; pap, Antias; Jamaican Patois: ''Antiliiz'') is an archipelago bordered by the Caribbean Sea to the south and west, the Gulf of Mex ...
may have formed an island chain that allowed land vertebrates to cross between the two continents. The arrival of hadrosaurids in South America may have caused the decline of the native ornithopods, the elasmarians.


See also

*
Timeline of hadrosaur research A timeline is a display of a list of events in chronological order. It is typically a graphic design showing a long bar labelled with dates paralleling it, and usually contemporaneous events. Timelines can use any suitable scale represent ...
* '' Alamosaurus'' – a
titanosaur Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of sauropod dinosaurs, including genera from all seven continents. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still thr ...
whose ancestors may have crossed into North America at the same time the ancestors of ''Secernosaurus'' crossed into South America * '' Huallasaurus'' * ''
Kritosaurus ''Kritosaurus'' is an incompletely known genus of hadrosaurid (duck-billed) dinosaur. It lived about 74.5-66 million years ago, in the Late Cretaceous of North America. The name means "separated lizard" (referring to the arrangement of the cheek ...
''


References

Saurolophines Maastrichtian life Late Cretaceous dinosaurs of South America Cretaceous Argentina Fossils of Argentina Los Alamitos Formation Golfo San Jorge Basin Fossil taxa described in 1979 Ornithischian genera {{ornithopod-stub