Niolamia
Niolamia Temporal range:
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Skull and mandible | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Reptilia |
Clade: | Pantestudines |
Clade: | Testudinata |
Family: | †Meiolaniidae |
Genus: | †Niolamia Ameghino, 1899 |
Species | |
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Synonyms | |
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Niolamia is an
Niolamia is one of only two named meiolaniid turtles from South America, the other being Gaffneylania. Given that this family is primarily distributed throughout the Neogene and Quaternary of Australasia, this makes Niolamia an important piece in the evolutionary history and origin of this turtle family.
Remains attributed to Niolamia were seemingly first uncovered by Santiago Roth in 1889, with a well preserved skull found only shortly afterwards. The name Niolamia was however not coined by Roth or any of his contacts, but by Florentino Ameghino who briefly wrote about what was said to be another skull discovered by his brother. While Ameghino's name was retained for the animal, later publications were entirely based on Roth's material, as Ameghino neither diagnosed nor figured his fossil. The fact that Ameghino's material was never recovered after his initial communication has led some researchers to believe that the two specimens are one and the same.
History and naming
In 1898, British paleontologist
That same year Argentinian paleontologist Florentino Ameghino coined the name Niolamia argentina for a skull he claimed his brother Carlos discovered in the Guaranitic Formation.[3] However, little information was given by Ameghino at the time, as the researcher didn't established a holotype, didn't diagnose the species or even figured the fossil material. The imprecise information was not an accident, but the result of the rivalry between Ameghino and Moreno. Their rivalry, similar to the Bone Wars between Edward Drinker Cope and Othniel Charles Marsh in America, saw the two intentionally hide records from one another, eventually leading to the complete loss of some information.[1]
Around the same time Moreno authored a short communication describing the material Roth reportedly uncovered on his second expedition. Although various postcranial remains were also described, the skull (specimen MLP 26–40) was the only element that was figured. Two years later Arthur Smith Woodward published a more detailed paper, in which he illustrated the material more extensively and assigned it to the same species mentioned previously by Ameghino. However, rather than using the name Niolamia, Woodward created the combination Miolania argentina based on an accidental misspelling of Meiolania.
The naming issue was somewhat resolved when
While meiolaniid research received a considerable boost under Gaffney, the fossils of Niolamia remained in storage and publications released during this time were primarily based on the work of Woodward, rather than first hand observation of the skull.[8] The convoluted history and poor description of the early meiolaniid discoveries of South America eventually led to a full redescription of the neotype in 2011, authored by Juliana Sterli and Marcelo de la Fuente. Like Gaffney, they too argue that Crossochelys is simply a younger Niolamia individual, with the distinguishing features simply representing individual variation or traits that would change with age.[1][9]
In a later publication, Sterli would further comment on the complex history of this taxon and the impact of the intense rivalry between Ameghino and Moreno on the history of Niolamia. According to Sterli, this competition may explain the whereabouts or rather the absence of Ameghino's fossil, which had seemingly disappeared from research history following the initial short description. No subsequent authors figured, described or even compared the Ameghino skull to the Roth skull. Juliana Sterli offers two possible explanations for this. It is possible that Ameghino's skull was simply lost, however it is likewise a possibility that the Ameghino and Roth skulls are the same specimen and that Ameghino misattributed the discovery to his brother. This would explain the improbability of two well preserved skulls being discovered in such quick succession by different teams and also explain why no researcher ever figured the Ameghino skull or compared it to the Roth material. Subsequently, Sterli refers to the
Age and locality
The purposeful lack of information on the locality that the first Niolamia remains were found at and the general ambiguity around whether or not Ameghino's and Roth's skulls are distinct has led to a lot of confusion in subsequent years. Ameghino named the Guaranitic Formation in the
Description
Skull and horns
The best preserved fossil of Niolamia is a nearly completely preserved skull, which is the most diagnostic element in meiolaniid turtles. Like in its relatives, the skull is highly
Like other meiolaniids, the skull of Niolamia was covered in a multitude of horn or boss-like scales readily identifiable in the fossil material. These scales were described with varying nomenclature by different authors, with Woodward assigning them the numerals I - VII, while Simpson assigned letters to the individual scales. From Simpson's work, Gaffney refined and altered the nomenclature, which has since then become the standard nomenclature for the scale areas and horns of meiolaniid turtles. Most scales on the skull of Niolamia appear in pairs, the exception being scale A, X, Y and Z. Scale A in particular is part of the series of scales that form large horns and bosses along the back of the skull in all meiolaniids. While the region furthest back on the skull is covered by the A scale, which forms an "occipital crest". This crest appears as a large, upward directed frill with a deep notch along its middle. The B scales, which cover the horn cores formed by the
Postcrania
In addition to the skull, Niolamia is also known from a variety of postcranial remains. Elements of both
While no complete shell of Niolamia is known, researchers were nonetheless able to determine several aspects of its shape. Although the largest fossils of the shell are flattened, this is believed to have been caused by taphonomy, crushing the bone during preservation. Instead, it is suggested that Niolamia had a domed shell like modern tortoises, a hypothesis in part based on the angle between the dorsal process and the acromion of the shoulder blade, which resembles modern testudinids. The shell appears to be thickest towards the borders and thinnest towards the midline of the shell, reaching a maximum thickness of 1.5 cm (0.59 in). Of the ten recovered shell remains, only four have their position on the shell known. The largest fragment was likely located at the back edge of the carapace, correlating with the 8th costal plate (which cover the shell between its spine and edge) and the 10th & 11th peripheral plates (the bony plates forming the edge of the bony shell). In this region the shell appears to possess backwards directed spikes. Rather than holding its shape towards the back of the shell, the carapace of Niolamia forms a ditch at its rear-most point, creating an embayment bordered by the final supracaudal scale. The recovered pattern of sutures and scale sulci is similar to that of modern tortoises, but could not be compared to the shell of Meiolania, in which the number of scales is unknown.[1]
The carapace length of Niolamia has been estimated based on comparison between shell fragments and the general proportions of the related Meiolania as well as those of the stem-turtle Proganochelys. The result yielded an estimated shell length of 1.2 m (3 ft 11 in) for Niolamia, however this estimate is not a certain one due to the fact that only fragmentary shell remains were known. Subsequently, future discoveries could confirm or contradict these results.[1]
Like in other meiolaniids, the tail of Niolamia was covered in a series of protective bony rings that overlapped one another. These rings were roughly hexagonal in shape, with two pairs of spiky protrusions emerging. The top pair was more robust and directed upwards, while the smaller, lower pair was directed upwards and sideways. Unlike in Meiolania, and more like in Ninjemys, the tail rings of Niolamia were fully formed and lacked the opened underside seen in the more derived Meiolania.[1][11]
Phylogeny
Phylogenetic analysis consistently recover Niolamia in the same position relative to other meiolaniid turtles, which is as the basalmost member and sister to all Australasian taxa.[9][8] This placement is consistent with both its appearance in time and its place of origins, as this genus is older than all named species from Australia and the western pacific islands. The bridge between Niolamia and the physically much different Meiolania is somewhat bridged by Ninjemys, typically recovered as the basalmost of the Australasian forms, as it shares multiple traits including the laterally directed horns and large A scale area with Niolamia.[11] The later description of Gaffneylania, also from the Neogene of Argentina, did not change this placement much. However, this is in part due to the fragmentary nature of the later, as its position within the family is uncertain, being placed either alongside the derived Australasian forms or in a basal polytomy with Niolamia. The two phylogenetic trees below, after Gaffney, Archer & White[13] and Sterli, de la Fuenta & Krause[10] respectively, show the internal relationship of meiolaniids following the description of the two most recently named genera.[1]
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Paleobiogeography
Niolamia provides important insight into the
The presence of these two genera marks them as part of the Austral biogeogeographical kingdom, which includes southern South America, Antarctica, south Africa, Australia, Tasmania, New Zealand and New Guinea. Animals of this biogeographical kingdom were heavily affected by the breakup of Gondwana in their distribution, with Antarctica in particular serving as a connective bridge between the fauna of South America and Australasia. Meiolaniids derive from the primarily Gondwanan Meiolaniformes, which at one point would have been found in both southern South America and Antarctica. Given the fact that South Tasmanian Sea formed between the Cretaceous and late Eocene, meiolaniids must have arrived in Australia by this time, as is confirmed by the fossil record.[14] Antarctica and South America would remain connected until the opening of the Drake Passage during the Eocene-Oligocene, by which point the South American meiolaniids had not only dispersed into Patagonia but already gone extinct. The cause of the extinction of Niolamia and other South American meiolaniids was likely the gradually cooling and drying climate of middle Eocene Patagonia.[10]
Paleobiology
Lifestyle
Few studies directly deal with the specifics of Niolamia's lifestyle. One exception to this is a 2017 study dealing with the neuroanatomy of meiolaniids, which described the endocranial anatomy of Niolamia, Gaffneylania and Meiolania. Among the findings of the paper was that meiolaniids had an elongated vestibulum nasi. Although such a feature is associated with aquatic turtles today, correlating with a snorkel-like nose, it can also be interpreted as an adaptation for life in
Leaving aside the potential for additional uses, the enhanced sense of smell inferred by the large nasal cavity could have served several functions from foraging to intraspecific communication. Particular focus is placed on the later, with modern turtles displaying a wide variety of glands used to produce chemical secretions (musk glands, cloacal secretions and mental glands). While the presence of these glands cannot be observed in meiolaniid fossils, they may have been tied to the keen sense of smell inferred for them. Furthermore, chemical cues are associated with combat and aggression between modern turtles, especially during courtship, which may entail various shell-based maneuvers. This is congruent with prior studies suggesting a combat function for the highly armored bodies of meiolaniid turtles, which feature differently shaped horns, osteoderms and armored tails.[12]
The inner ear of Niolamia meanwhile is rather conservative and more in-line with what is seen in terrestrial turtles, which have a wide angle between the anterior and posterior semicircular canals. In modern tortoises this angle is approximately 100°, in more aquatic turtles like geoemydids and plesiochelydids 80-95° and in meiolaniids up to 115°. In terrestrial species, this wide angle between the semicircular canals servers to improve the stability of the head while walking. Given the similarities to modern turtles however, it is assumed that they were sensitive to low frequency sounds and not especially vocal animals, relying more heavily on smell. However as significant changes in the inner ear anatomy may often be subtle, a larger sample size would be needed to make more concrete observations for Niolamia and other meiolaniids.[12]
Generally, both the enhanced sense of smell and the anatomy of the inner ear support the idea that Niolamia was a terrestrial animal like modern tortoises.[12] This is the most commonly inferred lifestyle for meiolaniid turtles and generally favoured over the aquatic model, which is occasionally suggested but not widely accepted.[15]
Ontogeny
Comparison between the neotype of Niolamia and AMNH 3161, formerly the holotype of Crossochelys and now considered to be a juvenile Niolamia, makes it possible to identify several changes the animal underwent in the process of reaching adulthood. AMNH 3161 represents a very young individual, less than a fourth the size of the Niolamia neotype skull. Accordingly, the shape of the horns is also significantly downplayed compared to those of adult individuals. The large scales at the back of the skull for instance, Scales A, were initially interpreted as being separate in Crossochelys. Following the synonymty, this interpretation is either wrong or the scales would combine into a single element in adults, as exemplified by the neotype. The X scale, which is the scale placed atop the midline of the skull, extends between the G scales in Crossochelys, while no such split occurs in Niolamia. The fact that the X scales in derived meiolaniids, such as Ninjemys and Warkalania, forms an extension like that of Crossochelys suggests that this could be a
Among the features once considered to be diagnostic for Crossochelys was the presence of a "true temporal fenestra", however this is likely simply the result of the sutures not yet being closed in the juvenile.[1][7][8][6]
References
- ^ S2CID 83503956.
- ^ .
- ^ Ameghino, F. (1899). "Sinopsis geológico-paleontológica de la Argentina. Suplemento (adiciones y correcciones)". Imprenta la Libertad (Author Edition). La Plata, Argentina.
- ^ .
- ^ a b c d Simpson, G.G.; Williams, C.S. (1938). "Crossochelys, Eocene horned turtle from Patagonia" (PDF). Bulletin of the AMNH. 74 (5).
- ^ S2CID 83799914.
- ^ a b c d Simpson, G.G. (1937). "New reptiles from the Eocene of South America" (PDF). American Museum Novitates (927).
- ^ ISSN 0003-0090.
- ^ ISBN 978-3-319-00517-1.
- ^ hdl:11336/41594.
- ^ a b c d e Gaffney, E. S. (1992). "Ninjemys, a new name for "Meiolania" oweni (Woodward), a Horned Turtle from the Pleistocene of Queensland" (PDF). American Museum Novitates (3049): 1–10.
- ^ .
- ^ Gaffney, Eugene S.; Archer, Michael; White, Arthur (1992). "Warkalania, a New Meiolaniid Turtle from the Tertiary Riversleigh Deposits of Queensland, Australia" (PDF). The Beagle, Records of the Northern Territory Museum of Arts and Sciences. 9 (1): 35–48.
- S2CID 131795055.
- .