User:NTown31/Thalassiodracon

Original Article

Thalassiodracon hawkinsi is known from a number of complete skeletons (lectotype: NHMUK 2018) acquired by the fossil collector Thomas Hawkins in Somerset, England during the early 1830s, before 1834. Hawkins, an eccentric Pre-Adamite who had his fossils heavily restored and illustrated by distinguished artists in expensive editions to propagate his ideas, named these Plesiosaurus triotarsostinus in 1834 and Hezatarostinus in 1840 but these names are generally disregarded. Thalassiodracon lived in the Late Triassic (Rhaetian) to the Early Jurassic (Hettangian) of Europe (age range: 201.6 to 196.5 million years ago). It was named as Plesiosaurus Hawkinsii in 1838 by Richard Owen and it was made the type species of the genus Thalassiodracon in 1996 by Storrs & Taylor.

A replica of the holotype specimen, replica catalogued as LDUCZ-X227, has been in the collection of the Grant Museum of Zoology and Comparative Anatomy in London, England since 1890 after being catalogued by Ray Lankester, and it is a cast of a specimen created in 1862 by Henry Augustus Ward. The original fossil, catalogued as NHMUK 2018 and also bought by Hawkins in the 1830s, can be seen on display at the Natural History Museum, also in London, England.

The genus name Thalassiodracon means "sea dragon", while the specific name hawkinsi honours Hawkins.

Description

Thalassiodracon was a small plesiosaur, measuring approximately 1.5 m (4.9 ft) long. It has a skull length of 17 cm (6.7 in), neck length of 75.5 cm (2 ft 5.7 in), and trunk length of 68 cm (2 ft 3 in).

The neck of Thalassiodracon, however, was slightly shorter than that of subsequent Plesiosaurs. The cervical vertebrae were between 27 and 31, while those of Plesiosaurus were 35 - 37. Moreover, the skull was unusually short and equipped with long teeth. The orbits were very large, with sclerotic rings. Like all plesiosaurs, this animal had legs like paddles for swimming in the sea.

A computed tomography of an exceptionally preserved skull, and examination of other specimens (Benson et al., 2011) yields new anatomical data. Thalassiodracon had a dorsomedian ridge on the premaxilla, a squamosal bulb, four premaxillary teeth, and a heterodont maxillary dentition.

Quotes (Not mine):

"Plesiosaur skulls such as that of Thalassiodracon hawksini (Fig. 5) have notably enlarged orbits and large flattened sclerotic rings, suggesting its eyes were well adapted for seeing underwater and in low-light conditions. Additionally, the elongation of the stapes and their fusion to the exoccipital bones appear to be adaptations for hearing underwater. These adaptations are especially pronounced in cryptoclidids such as Abyssosaurus nataliae, which had an extremely shortened skull with wide orbits and huge exoccipital bones, which would have aided in navigating through the dark and withstanding the pressure of deep marine environments."

"Plesiosaurians thrived in saltwater and freshwater habitats, from open marine to brackish and fluvial environments. Relative flipper size was likely linked to feeding strategies and local ecology. Plesiosauromorphs generally have larger foreflippers than hindflippers and are regarded as having been ambush predators and efficient, moderately fast long-distance swimmers. Pliosauromorphs usually have larger hindflippers than foreflippers at large body sizes and are generally considered to have been fast, powerful, and agile predators that pursued their prey."

"All plesiosaurians were faunivorous but showed various dietary preferences, as indicated by their dentition and fossil gastric contents. Dentition is associated with functional morphology, where the shape of a tooth influences its purpose. Plesiosauromorphs overall had piercing and general-use teeth that are thin to moderately conical (Fig. 4A). Some aristonectid elasmosaurids and cryptocleidids are suggested to have been filter-feeders, supported by trace fossils interpreted as plesiosaur sieve marks through soft sediments to draw out elusive prey."

"Regurgitalites (fossilised stomach contents) in plesiosaurians have been found with ammonite jaw parts (Fig. 4C), indicating cephalopod consumption (among other invertebrates). A small ichthyosaur neonate found in the gut region of a plesiosaur suggests they were also opportunists. Larger plesiosaurians, particularly large-headed pliosaurs, were able to feed on larger prey such as fish and other large marine reptiles. Plesiosaurian skeletons are also often associated with gastroliths, which would have influenced the animal’s buoyancy, aided in food processing, or both.

"They were endothermic, with a high metabolism, allowing for survival in high latitudes and deep water. These lines of evidence have vastly improved our understanding of the evolution and adaptations of plesiosaurians."

"The plesiosaurian tail was reduced, and unlike basal members like nothosaurs that likely relied on their tails for forward locomotion, derived plesiosaurian tails had tail flukes for largely maintained streamlining. As for necks, thicker-necked pliosauromorphs were likely greater pursuit hunters than their longer necked contemporaries, which likely employed different foraging strategies." (1)

https://www.darwinsdoor.co.uk/feed/plesiosaurian-evolution-and-adaptations.html

"hindlimbs slightly longer than forelimbs in adult." (2)

https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=100053&is_real_user=1

"Although possibly a primitive pliosaur,‭ ‬the small size of Thalassiodracon meant that it was still probably a hunter of fish and‭ ‬maybe smaller marine reptiles,‭ ‬including small juveniles of larger genera.‭ ‬Later pliosaurs such as Pliosaurus and Brachauchenius would grow‭ ‬to much larger sizes and fulfil niches amongst the apex predators of the Mesozoic oceans." (3)

https://www.prehistoric-wildlife.com/species/t/thalassiodracon.html

"We measured and analysed the degree of tooth variation along the jaws of the basal pliosaurid Thalassiodracon, and two derived genera, Peloneustes and Pliosaurus (figure 4). Thalassiodracon hawkinsii is a small basal pliosaurid, with a long-necked, ‘plesiosauromorph’ [20] body plan. Its pliosaurid affinity was established by Benson et al. [19] (see also the electronic supplementary material, figure S1). The skull is relatively small and proportionally short (table 2), housing numerous slender teeth. All other specimens in this study had a more typical ‘pliosauromorph’ body plan with higher head to neck ratios and longer skulls. Our analysis assessed whether dentition is uniform (excluding variation in size) or if there is a distinct morphological variation.

"Tooth replacement patterns are described for four pliosaurid taxa, Thalassiodracon, P. carpenteri, P. kevani and Peloneustes. μCT scans were available for Thalassiodracon and the two Pliosaurus specimens. Tooth replacement schedules for Peloneustes were inferred by detailed observation of both upper and lower jaws. Only the lower jaw had sufficiently well preserved teeth to infer tooth cycle patterns.

"The replacement takes place in cycles as proposed by Edmund [4], which produce repeating wave-like patterns along the jaw. The green coloured lines show the waves for Thalassiodracon (figure 5e). They are illustrated here as four-stage waves, with a period of 3 (in other words, at every third tooth, a new wave begins). In Thalassiodracon, the pattern continues smoothly across the premaxillary–maxillary boundary." (4)

https://royalsocietypublishing.org/doi/full/10.1098/rsos.150384

"These casts include the holotype of Thalassiodracon hawkinsi (Owen, 1838); a ‘sternum‘ and ‘scapula’ illustrated by Thomas Hawkins and now referable to Eurycleidus arcuatus (Owen, 1840);" (5)

https://www.researchgate.net/profile/Patrick-Wyse-Jackson/publication/264383865_Thomas_Hawkins_Lord_Cole_William_Sollas_and_all_casts_of_Lower_Jurassic_marine_reptiles_in_the_Geological_Museum_Trinity_College_Dublin_Ireland/links/5cfbc406a6fdccd1308d1744/Thomas-Hawkins-Lord-Cole-William-Sollas-and-all-casts-of-Lower-Jurassic-marine-reptiles-in-the-Geological-Museum-Trinity-College-Dublin-Ireland.pdf

Name	Status	Common Name(s)	Fossil Occurrences	Oldest	Youngest
Plesiosaurus hawkinsii ✝ species	accepted (GBIF) listed (PBDB)		8

"viviparous (based on Plesiosauria)" (6)

https://www.mindat.org/taxon-4821530.html

"ABSTRACT—We provide a complete description of one of the oldest plesiosaurians, Stratesaurus taylori from the earliest Hettangian of the United Kingdom. At least 25 apomorphies distinguish S. taylori from the sympatric Thalassiodracon hawkinsii, to which all three specimens of S. taylori were originally referred. Several features of the skull of S. taylori suggest specialization on small prey items, or sieve feeding. In particular, it has anteriorly inclined premaxillary and mesial maxillary teeth and an only weakly heterodont maxillary dentition. This indicates niche partitioning among sympatric small-bodied plesiosaurians: T. hawkinsii has a pronouncedly heterodont dentition. With a body length estimated around 2 m, 5. taylori is one of the smallest plesiosaurians, comparable to T. hawkinsii. Our anatomical review of S. taylori suggests difficulty determining its precise phylogenetic affinities. This is consistent with a general lack of phylogenetic resolution among earliest Jurassic plesiosaurians, which may result from missing data on their Triassic ancestry. However, due to its plesiomorphic morphology and well-characterized anatomy, we recommend S. taylori as an ingroup representative of Plesiosauria for future cladistic analyses of Triassic sauroptery" (7)

https://www.jstor.org/stable/pdf/24524184.pdf

GBIF (8) 31 total specimens all in uk

https://www.gbif.org/occurrence/search?taxon_key=4821530

More proof Thalassiodracon is a basal form (9)

https://onlinelibrary.wiley.com/doi/full/10.1111/j.1475-4983.2007.00654.x

"Thalassiodracon “sea dragon” was a genus of small pliosaur placed in the family Pliosauridae within the order Plesiosauria. Carnivorous marine reptiles, they measured around 1.5 – 2 metres in length with the skull being quite large in relation to the body – possibly for hunting smaller marine reptiles as well as fish. T. hawkinsi is the only species in the genus and is known from several nearly complete specimens found in the Blue Lias and White Lias geological formations in the UK. They lived during the late Triassic to early Jurassic periods around 195-200 million years ago.

Ward’s fossil casts made every attempt to be true to the original, as did Hawkins himself. Hawkins not only carefully reassembled the fossil skeletons he collected, but he included detailed illustrations of specimens in his publications – notably Memoirs of Icthyosaurii and Plesiosaurii (1835) and The Book of the Great Sea Dragons (1840).

T. hawkinsi is the only species in the genus and is known from several nearly complete specimens found in the Blue Lias and White Lias geological formations in the UK. They lived during the late Triassic to early Jurassic periods around 195-200 million years ago." (10)

https://blogs.ucl.ac.uk/museums/2015/09/14/specimen-of-the-week-205-the-plesiosaur-cast/#more-43529

Original notes stored at museum (11)

https://data.nhm.ac.uk/dataset/collection-specimens/resource/05ff2255-c38a-40c9-b657-4ccb55ab2feb?view_id=6ba121d1-da26-4ee1-81fa-7da11e68f68e&filters=higherClassification%3AAnimalia%3B+Chordata%3B+Reptilia%3B+Sauropterygia%3B+Plesiosauria%3B+Plesiosauroidea

Reworked:

The genus name Thalassiodracon means "sea dragon", while the species name, hawkinsi, honours Thomas Hawkins, who first studied the creature.

The original fossil, designated as NHMUK 2018 was bought by Hawkins in the 1830s, and is currently on display at the Natural History Museum in London, England. A cast of the holotype fossil was created in 1862 by Henry Augustus Ward. This replica was catalogued as LDUCZ-X227 by Ray Lankester and is also located in London, at the Grant Museum of Zoology and Comparative Anatomy.

My new stuff:

Thalassiodracon hawkinsi is a basal species in Pliosauridae, found within the order Plesiosauria. The species hawkinsi is currently the lone member contained in the Thalassiodracon genus.

Thalassiodracon hawkinsi lived in the late Triassic period to the early Jurrassic period in the Mesozoic era, from about 200 million to 195 million years ago.

In Memoirs of Icthyosaurii and Plesiosaurii (1835) and The Book of the Great Sea Dragons (1840), Hawkins published his own illustrations after reconstructing the fossils he had obtained. Some of Hawkins original notes are being stored at the Natural History Museum in London.

Another cast of Thalassiodracon hawkinsi is being displayed at the Geological Museum of Trinity College in Dublin, Ireland. Three fossils found in Somerset, England originally identified as Thalassiodracon were redescribed as another small Plesiosaur, Stratesaurus taylori. According to the Global Biodiversity Information Facility, there is a total of 31 fossil occurrences of Thalassiodracon hawkinsi all located in the United Kingdom.

Plesiosaurs lived in both saltwater and freshwater areas, and specific adaptations determined in what kind of environment each member would be able to thrive in. In Thalassiodracon hawkinsi, the orbits of the skull were enlarged, containing likewise large, flat Sclerotic ringsThese rings are theorized to help the eyes of the creature to be well adjusted for a low-light environment found in its habitat. Other features like elongated staples and fused exoccipital bones seem to suggest adapted underwater hearing. Like some modern reptiles, the teeth inside the skull had a constant replacement cycle as they wore out during the animal's lifetime. The cycle appears to have occurred in a wave shaped pattern, with cycle repeating every third tooth. Thalassiodracon was compared to other pliosaurs with the standard body model to determine if this feature was specific or continued in its lineage.

Separating them from earlier ancestors, Thalassiodracon and following members had their tails shortened. In adulthood, the forelimbs of Thalassiodracon hawkinsi are shorter than its hindlimbs. Plesiosaurs with this characteristic are speculated to be pursuit predators, using speed and strength to hunt down their prey. In general, Plesiosaurs had three primary ways to traverse through the water with their flippers: rowing, flying, and rowing flight. Unlike the reptiles of today, Thalassiodracon hawkinsi and other plesiosaurs were warm-blooded with a high metabolism, enabling them to have an active lifestyle.

All Plesiosaurs, including Thalassiodracon were faunivorous, but widely ranged in their diets. Animals such as ammonites, cephalopods, and other invertebrates were often found in plesiosaur remains. Thalassiodracon is also speculated to hunt fish, small marine reptiles, and juveniles of other species that shared its environment. Plesiosaurs are also known to feast on ichthyosaurs, which may have been true for Thalassiodracon. With the bones of dinosaurs and pterosaurs being found in Plesiosaur remains, these animals are also possibilities for their diets.

Based off of other Plesiosaurs, Thalassiodracon hawkinsi was viviparous, meaning it gave birth to live young.

Bibliography