Elasmosauridae is an extinct
family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
of
plesiosaurs
The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia.
Plesiosaurs first appeared i ...
, often called elasmosaurs. They had the longest necks of the plesiosaurs and existed from the
Hauterivian
The Hauterivian is, in the geologic timescale, an age in the Early Cretaceous Epoch or a stage in the Lower Cretaceous Series. It spans the time between 132.9 ± 2 Ma and 129.4 ± 1.5 Ma (million years ago). The Hauterivian is preceded by the ...
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 ...
stages 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 represented one of the two groups of plesiosaurs present at the end of the Cretaceous alongside
Polycotylidae
Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widespread during the early Late Cretaceous. ...
. Their diet mainly consisted of crustaceans and molluscs.
Description
![Thalassomedon haningtoni](https://upload.wikimedia.org/wikipedia/commons/6/63/Thalassomedon_haningtoni.jpg)
The earliest elasmosaurids were mid-sized, about . In the Late Cretaceous, elasmosaurids grew as large as , such as ''
Styxosaurus'', ''
Albertonectes
''Albertonectes'' is an extinct genus of elasmosaurid plesiosaur known from the Late Cretaceous (middle upper Campanian stage) Bearpaw Formation of Alberta, Canada. It contains a single species, ''Albertonectes vanderveldei''. ''Albertonectes'' ...
'', and ''
Thalassomedon''. Their necks were the longest of all the
plesiosaurs
The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia.
Plesiosaurs first appeared i ...
, with anywhere between 32 and 76 (''Albertonectes'')
cervical vertebrae. They weighed up to several tons.
Classification
Early three-family classification
Though Cope had originally recognized ''Elasmosaurus'' as a plesiosaur, in an 1869 paper he placed it, with ''
Cimoliasaurus
''Cimoliasaurus'' was a plesiosaur that lived during the Late Cretaceous (Maastrichtian) of New Jersey. It grew up to long and weighed up to .
Etymology
The name is derived from the Greek , meaning "white chalk", and , meaning "lizard", in ...
'' and ''
Crymocetus
''Crymocetus'' is an extinct genus of plesiosaur from the Cretaceous Chalk Group of Sussex, England. Since its description, it has been seldom examined by subsequent authors, except when mentioned in discussions of Cretaceous plesiosaurs
Taxono ...
'', in a new order of
sauropterygian
Sauropterygia (" lizard flippers") is an extinct taxon of diverse, aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria bec ...
reptiles. He named the group Streptosauria, or "reversed lizards", due to the orientation of their individual vertebrae supposedly being reversed compared to what is seen in other vertebrate animals.
He subsequently abandoned this idea in his 1869 description of ''Elasmosaurus'', where he stated he had based it on Leidy's erroneous interpretation of ''Cimoliasaurus''. In this paper, he also named the new family Elasmosauridae, containing ''Elasmosaurus'' and ''Cimoliasaurus'', without comment. Within this family, he considered the former to be distinguished by a longer neck with compressed vertebrae, and the latter by a shorter neck with square, depressed vertebrae.
In subsequent years, Elasmosauridae came to be one of three groups in which plesiosaurs were classified, the others being the
Pliosauridae
Pliosauridae is a family of plesiosaurian marine reptiles from the Latest Triassic to the early Late Cretaceous (Rhaetian to Turonian stages) of Australia, Europe, North America and South America. The family is more inclusive than the archetypal ...
and
Plesiosauridae
The Plesiosauridae are a monophyletic group of plesiosaurs.Ketchum, H. F., and Benson, R. B. J., 2010. "Global interrelationships of Plesiosauria (Reptilia, Sauropterygia) and the pivotal role of taxon sampling in determining the outcome of phylo ...
(sometimes merged into one group).
In 1874
Harry Seeley
Harry Govier Seeley (18 February 1839 – 8 January 1909) was a British paleontologist.
Early life
Seeley was born in London on 18 February 1839, the second son of Richard Hovill Seeley, a goldsmith, and his second wife Mary Govier. When his fat ...
took issue with Cope's identification of
clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the r ...
s in the shoulder girdle of ''Elasmosaurus'', asserting that the supposed clavicles were actually scapulae. He found no evidence of a clavicle or an
interclavicle
An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals (marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In t ...
in the shoulder girdle of ''Elasmosaurus''; he noted that the absence of the latter bone was also seen in a number of other plesiosaur specimens, which he named as new elasmosaurid genera: ''
Eretmosaurus
''Eretmosaurus'' is an extinct genus of plesiosaur.
Taxonomic history
''Eretmosaurus'' was coined by Harry Govier Seeley for ''Plesiosaurus rugosus'' Owen, 1840. Richard Owen had coined the name ''P. rugosus'' for numerous vertebrae from th ...
'', ''
Colymbosaurus
''Colymbosaurus'' is a genus of cryptoclidid plesiosaur from the Late Jurassic (Callovian-Tithonian) of the UK and Svalbard, Norway. There are two currently recognized species, ''C. megadeirus'' and ''C. svalbardensis''. Both species are relati ...
'', and ''
Muraenosaurus
''Muraenosaurus'' (from the Latin "'' Muraena''" meaning "eel" and "''Sauros''" meaning lizard) is an extinct genus of cryptoclidid plesiosaur reptile from the Oxford Clay of Southern England. The genus was given its name due to the eel-like ...
''.
Richard Lydekker
Richard Lydekker (; 25 July 1849 – 16 April 1915) was an English naturalist, geologist and writer of numerous books on natural history.
Biography
Richard Lydekker was born at Tavistock Square in London. His father was Gerard Wolfe Lydekker, ...
subsequently proposed that ''Elasmosaurus'', ''Polycotylus'', ''Colymbosaurus'', and ''Muraenosaurus'' could not be distinguished from ''Cimoliasaurus'' based on their shoulder girdles, and advocated their synonymization at the genus level.
Seeley noted in 1892 that the clavicle was fused to the coracoid by a suture in elasmosaurians, and was apparently "an inseparable part" of the scapula. Meanwhile, all plesiosaurs with two-headed neck ribs (the Plesiosauridae and Pliosauridae) had a clavicle made only of
cartilage, such that
ossification
Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in ...
of the clavicle would turn a "plesiosaurian" into an "elasmosaurian".
Williston doubted Seeley's usage of neck ribs to subdivide plesiosaurs in 1907, opining that double-headed neck ribs were instead a "primitive character confined to the early forms".
Charles Andrews elaborated on differences between elasmosaurids and pliosaurids in 1910 and 1913. He characterized elasmosaurids by their long necks and small heads, as well as by their rigid and well-developed scapulae (but atrophied or absent clavicles and interclavicles) for forelimb-driven locomotion. Meanwhile, pliosaurids had short necks but large heads, and used hindlimb-driven locomotion.
Refinement of plesiosaur taxonomy
Although the placement of ''Elasmosaurus'' in the Elasmosauridae remained uncontroversial, opinions on the relationships of the family became variable over subsequent decades. Williston created a revised taxonomy of plesiosaurs in a
monograph on the
osteology of reptiles (published posthumously in 1925). He provided a revised
diagnosis
Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine " cause and effect". In systems engin ...
of the Elasmosauridae; aside from the small head and long neck, he characterized elasmosaurids by their single-headed ribs; scapulae that meet at the midline; clavicles that are not separated by a gap; coracoids that are "broadly separated" in their rear half; short ischia; and the presence of only two bones (the typical condition) in the epipodialia (the "forearms" and "shins" of the flippers). He also removed several plesiosaurs previously considered to be elasmosaurids from this family due to their shorter necks and continuously meeting coracoids; these included ''Polycotylus'' and ''
Trinacromerum'' (the
Polycotylidae
Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widespread during the early Late Cretaceous. ...
), as well as ''Muraenosaurus'', ''
Cryptoclidus
''Cryptoclidus'' ( ) is a genus of plesiosaur reptile from the Middle Jurassic Period (geology), period of England, France, and Cuba.
Discovery
''Cryptoclidus'' was a plesiosaur whose specimens include adult and juvenile skeletons, and remai ...
'', ''
Picrocleidus'', ''
Tricleidus'', and others (the
Cryptoclididae
Cryptoclididae is a family (biology), family of medium-sized plesiosaurs that existed from the Middle Jurassic to the Early Cretaceous. They had long necks, broad and short skulls and densely packed teeth. They fed on small soft-bodied preys such ...
).
In 1940 Theodore White published a hypothesis on the interrelationships between different plesiosaurian families. He considered Elasmosauridae to be closest to the Pliosauridae, noting their relatively narrow coracoids as well as their lack of interclavicles or clavicles. His diagnosis of the Elasmosauridae also noted the moderate length of the skull (i.e., a mesocephalic skull); the neck ribs having one or two heads; the scapula and coracoid contacting at the midline; the blunted rear outer angle of the coracoid; and the pair of openings (fenestrae) in the scapula–coracoid complex being separated by a narrower bar of bone compared to pliosaurids. The cited variability in the number of heads on the neck ribs arises from his inclusion of ''
Simolestes'' to the Elasmosauridae, since the characteristics of "both the skull and shoulder girdle compare more favorably with ''Elasmosaurus'' than with ''Pliosaurus'' or ''Peloneustes''." He considered ''Simolestes'' a possible ancestor of ''Elasmosaurus''.
Oskar Kuhn
Oskar Kuhn (7 March 1908, Munich – 1990) was a German palaeontologist.
Life and career
Kuhn was educated in Dinkelsbühl and Bamberg and then studied natural science, specialising in geology and paleontology, at the University of Munich, f ...
adopted a similar classification in 1961.
Welles took issue with White's classification in his 1943 revision of plesiosaurs, noting that White's characteristics are influenced by both preservation and
ontogeny
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 s ...
. He divided plesiosaurs into two superfamilies, the
Plesiosauroidea
Plesiosauroidea (; Greek: 'near, close to' and 'lizard') is an extinct clade of carnivorous marine reptiles. They have the snake-like longest neck to body ratio of any reptile. Plesiosauroids are known from the Jurassic and Cretaceous period ...
and
Pliosauroidea
Pliosauroidea is an extinct clade of plesiosaurs, known from the earliest Jurassic to early Late Cretaceous. They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive t ...
, based on neck length, head size, ischium length, and the slenderness of the humerus and femur (the propodialia). Each superfamily was further subdivided by the number of heads on the ribs, and the proportions of the epipodialia. Thus, elasmosaurids had long necks, small heads, short ischia, stocky propodialia, single-headed ribs, and short epipodialia.
Pierre deSaint-Seine in 1955 and
Alfred Romer
Alfred Sherwood Romer (December 28, 1894 – November 5, 1973) was an American paleontologist and biologist and a specialist in vertebrate evolution.
Biography
Alfred Romer was born in White Plains, New York, the son of Harry Houston Romer an ...
in 1956 both adopted Welles' classification.
In 1962 Welles further subdivided elasmosaurids based on whether they possessed pelvic bars formed from the fusion of the ischia, with ''Elasmosaurus'' and ''
Brancasaurus
''Brancasaurus'' (meaning "Branca's lizard") is a genus of plesiosaur which lived in a freshwater lake in the Early Cretaceous of what is now North Rhine-Westphalia, Germany. With a long neck possessing vertebrae bearing distinctively-shaped "sh ...
'' being united in the subfamily Elasmosaurinae by their sharing of completely closed pelvic bars.
Persson, however, considered Welles' classification too simplistic, noting in 1963 that it would, in his opinion, erroneously assign ''Cryptoclidus'', ''Muraenosaurus'', ''Picrocleidus'', and ''Tricleidus'' to the Elasmosauridae. Persson refined the Elasmosauridae to include traits such as the crests on the sides of the neck vertebrae; the hatchet-shaped neck ribs at the front of the neck; the fused clavicles; the separation of the coracoids at the rear; and the rounded, plate-like pubis. He also retained the Cimoliasauridae as separate from the Elasmosauridae, and suggested, based on comparisons of vertebral lengths, that they diverged from the Plesiosauridae in the Late Jurassic or Early Cretaceous.
However, D.S. Brown noted in 1981 that the variability of neck length in plesiosaurs made Persson's argument unfeasible, and moved the aforementioned genera back into the Elasmosauridae; he similarly criticized Welles' subdivision of elasmosaurids based on the pelvic bar. Brown's diagnosis of elasmosaurids included the presence of five premaxillary teeth; the ornamentation of teeth by longitudinal ridges; the presence of grooves surrounding the
occipital condyles
The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra.
The condyles are oval or reniform (kidney-shaped) in shape, and their anteri ...
; and the broad-bodied scapulae meeting at the midline.
Modern phylogenetic context
Carpenter's 1997 phylogenetic analysis of plesiosaurs challenged the traditional subdivision of plesiosaurs based on neck length. He found that ''Libonectes'' and ''
Dolichorhynchops
''Dolichorhynchops'' is an extinct genus of polycotylid plesiosaur from the Late Cretaceous (early Turonian to late Campanian stage) of North America, containing three species, ''D. osborni'', ''D. bonneri'' and ''D. tropicensis'', as well as a ...
'' shared characteristics such as an opening on the palate for the
vomeronasal organ
The vomeronasal organ (VNO), or Jacobson's organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth (the hard palate) in various tetrapods. T ...
, the plate-like expansions of the
pterygoid bone
The pterygoid is a paired bone forming part of the palate of many vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fis ...
s, and the loss of the
pineal foramen
A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhyth ...
on the top of the skull, differing from the pliosaurs. While polycotylids had previously been part of the Pliosauroidea, Carpenter moved polycotylids to become the
sister group of the elasmosaurids based on these similarities, thus implying that polycotylids and pliosauroids evolved their short necks independently.
F. Robin O'Keefe likewise included polycotylids in the Plesiosauroidea in 2001 and 2004, but considered them more closely related to the Cimoliasauridae and Cryptoclididae in the
Cryptocleidoidea.
Some analyses continued to recover the traditional groupings. In 2008 Patrick Druckenmiller and Anthony Russell moved the Polycotylidae back into the Pliosauroidea, and placed ''
Leptocleidus
''Leptocleidus'' is an extinct genus of plesiosaur, belonging to the family (biology), family Leptocleididae.Smith AS, Dyke GJ. 2008. The skull of the giant predatory pliosaur ''Rhomaleosaurus cramptoni'': implications for plesiosaur phylogenetic ...
'' as their sister group in the newly named
Leptocleidoidea;
Adam Smith and Gareth Dyke independently found the same result in the same year.
However, in 2010 Hilary Ketchum and Roger Benson concluded that the results of these analyses were influenced by inadequate sampling of species. In the most comprehensive phylogeny of plesiosaurs yet, they moved the Leptocleidoidea (renamed the Leptocleidia) back into the Plesiosauroidea as the sister group of the Elasmosauridae;
subsequent analyses by Benson and Druckenmiller recovered similar results, and named the Leptocleidoidea–Elasmosauridae grouping as
Xenopsaria.
The content of Elasmosauridae also received greater scrutiny. Since its initial assignment to the Elasmosauridae, the relationships of ''Brancasaurus'' had been considered well supported, and it was recovered by O'Keefe's 2004 analysis
and Franziska Großmann's 2007 analysis.
However, Ketchum and Benson's analysis instead included it in the Leptocleidia,
and its inclusion in that group has remained consistent in subsequent analyses.
Their analysis also moved ''Muraenosaurus'' to the Cryptoclididae, and ''
Microcleidus'' and ''Occitanosaurus'' to the Plesiosauridae;
Benson and Druckenmiller isolated the latter two in the group
Microcleididae
Microcleididae is an extinct family of basal plesiosauroid plesiosaurs from the Early Jurassic (middle Sinemurian to late Toarcian stages) of France, Germany, Portugal and the United Kingdom. Currently, the oldest and the most known microcleidi ...
in 2014, and considered ''Occitanosaurus'' a species of ''Microcleidus''.
These genera had all previously been considered to be elasmosaurids by Carpenter, Großmann, and other researchers.
Within the Elasmosauridae, ''Elasmosaurus'' itself has been considered a "wildcard taxon" with highly variable relationships.
Carpenter's 1999 analysis suggested that ''Elasmosaurus'' was more
basal (i.e. less specialized) than other elasmosaurids with the exception of ''Libonectes''.
[ In 2005 Sachs suggested that ''Elasmosaurus'' was closely related to ''Styxosaurus'',] and in 2008 Druckenmiller and Russell placed it as part of a polytomy
An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...
with two groups, one containing ''Libonectes'' and '' Terminonatator'', the other containing ''Callawayasaurus
''Callawayasaurus'' is a genus of plesiosaur from the family Elasmosauridae. When the holotype was first described by Samuel Paul Welles in 1962, it was described as ''Alzadasaurus colombiensis'' before being moved into its current genus by Ken ...
'' and '' Hydrotherosaurus''. Ketchum and Benson's 2010 analysis included ''Elasmosaurus'' in the former group. Benson and Druckenmiller's 2013 analysis (below, left) further removed ''Terminonatator'' from this group and placed it as one step more derived
Derive may refer to:
* Derive (computer algebra system), a commercial system made by Texas Instruments
* ''Dérive'' (magazine), an Austrian science magazine on urbanism
*Dérive, a psychogeographical concept
See also
*
*Derivation (disambiguatio ...
(i.e., more specialized). In Rodrigo Otero's 2016 analysis based on a modification of the same dataset (below, right), ''Elamosaurus'' was the closest relative of ''Albertonectes
''Albertonectes'' is an extinct genus of elasmosaurid plesiosaur known from the Late Cretaceous (middle upper Campanian stage) Bearpaw Formation of Alberta, Canada. It contains a single species, ''Albertonectes vanderveldei''. ''Albertonectes'' ...
'', forming the Styxosaurinae with ''Styxosaurus'' and ''Terminonatator''. Danielle Serratos, Druckenmiller, and Benson could not resolve the position of ''Elasmosaurus'' in 2017, but they noted that Styxosaurinae would be a synonym of Elasmosaurinae if ''Elasmosaurus'' did fall within the group. In 2021 a new topology placed '' Cardiocorax'' as a sister taxon of Libonectes, representing an older lineage of elasmosaurids in the Maastrichtian.
Topology A: Benson ''et al.'' (2013)
Topology B: Otero (2016)
The family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
Elasmosauridae was erected by Cope
The cope (known in Latin as ''pluviale'' 'rain coat' or ''cappa'' 'cape') is a liturgical vestment, more precisely a long mantle or cloak, open in front and fastened at the breast with a band or clasp. It may be of any liturgical colour.
A c ...
in 1869, and anchored on the genus ''Elasmosaurus
''Elasmosaurus'' (;) is a genus of plesiosaur that lived in North America during the Campanian stage of the Late Cretaceous period, about 80.5million years ago. The first specimen was discovered in 1867 near Fort Wallace, Kansas, US, and was se ...
''.
References
External links
"Lepidosauromorpha: Elasmosauridae." Palaeos.com
{{Taxonbar, from=Q134303
Plesiosaurs
Cretaceous plesiosaurs
Maastrichtian extinctions
Prehistoric reptile families