Deep Down In The Abyss: The prehistoric 'Sea Monster' of the Mesozoic era

Introduction                                                                                                                                                                                             'She sells seashells by the seashore'. This tongue twister was based on Mary Anning, a pioneering palaeontologist and fossil collector (Strauss, 2019). In the early 19th century, she was the first to discover what is now named Ichthyosaurus communis, meaning 'fish-lizard' (Mary-Claire Eylott, n.d). However, this genus was neither a fish nor a lizard. Scientists classify Ichthyosaurus communis as a marine reptile (Marek, 2015). Ichthyosaurus communis is a part of the order Ichthyosauria, which sits in the superorder of Ichthyopterygia (Fig.1). Ichthyosauria holds many records: possessing the largest eyes out of all living things ever to have existed (Motani, Rothschild and Wahl, 1999), being the first air breathing tetrapods with a fish-like body (Marek, 2015) and even laying claim to being the largest animal on earth of all time (Lomax et al., 2018).

Evolutionary History                                                                                                                                                                                The history of all life started int he waters. Some of the living species acquired lungs and were able to breathe air above the surface of water. Eventually these species were able to exit the seas and live on land. However, these early terrestrial dwellers re-entered the waters. Most likely due to the competition on land. It is these land dwellers which are thought to be the ancestry of the Ichthyosauria. Thus, the ichthyosaurs were known to be the secondary aquatic animals (Lomax, 2018; Marek, 2015). Ichthyosauria ruled most of the Mesozioc era (Motani, 2009: p.7). From the Early Triassic to Late Cretaceous, around 250 millions years to 90 million years ago, there were three evolutionary grades of Ichthyosauria (Moon, 2017: p.36).

Grade I: basal                                                                                                                                                                                           The basal grade of ichthyosaurs encompassed all the early undeveloped forms. It was in the early Triassic when a huge burst in diversity occurred within the Ichthyosauria (Sander, 2021). The 3 focal early genera were Chaohusaurus from China, Utatsusaurus from Japan and Grippia from Canada. All these early forms ranged from 1 to 3 metres in length (Lomax, 2018). Every genera had a long narrow body, a small caudal fin and a high presacral vertebrae count. These features pointed towards an anguilliform swimming mode (Motani et al., 1996). The swimming mode is similar to how modern-day eels locomote. The basal ichthyosaurs wiggled their long narrow bodies to move around. Due to this swimming mode these early forms could not swim fast, deep or long distances (Marek, 2015). 

Grade II: intermediate                                                                                                                                                                      A few million years later, during the mid-Triassic, the species Mixosaurus cornalianus, developed 5 distinct rows of finger bones in the front paddle (Bassani, 1886). In addition, the Mixosaurus genus was also the first to show evidence of vivaparity in the Ichthyosauria (Motani, 2009: p. 9). Essentially vivaparity signified that ichthyosaurs gave birth to live young. Since the ichthyosaurs had their fins modified, they were unable to walk on land (Lomax, 2018). Consequently, they had to stay in the waters. The isolation from terrain prevented the marine reptiles from laying eggs on land. As a result the intermediate grade of Ichthyosauria adapted and gave birth to live young, without having to exit the water. A fossil from Holzmaden, Germany of a female Stenopterygius quadriscissus clearly shows 3 unborn young inside her body. A fourth baby is seen to come out of the mother tail first. The reason for the young to exit tail first was so that they could have time to acclimate and to ultimately prevent themselves from drowning (Strauss, 2019). Another adaptation the intermediate grade acquired was the ability to engage in hemispheric sleep. Hemispheric sleep describes one side of the brain resting at a time. As the ichthyosaurs were always in water, hemispheric sleep allowed them to be constantly on the move (Motani, 2009: p. 8). Not too long before the Triassic - Jurassic boundary, approximately 210 million years ago, a fourth mass extinction event had occurred. Close to 76% of all land and aquatic species went extinct, while the order Ichthyosauria survived (Britannica, 2021). Nevertheless, the vast diversity that was present in the Triassic stagnated and started to plummet. Only 3 to 4 lineages remained. This fragment in time was known as the evolutionary bottleneck (Thorne et al., 2011). 

Grade III: fish shaped                                                                                                                                                                                   In the early Jurassic, approximately 201 million years ago, the ichthyosaur genera evolved even further by changing their appearance to look like the stereotypical version of the Ichthyosaur images we observe online. Parvipalvia, meaning ‘small pelvic girdle’ was the main group that demonstrated this iconic appearance (Motani, 2009: p.7). The fish-shaped grade of Ichthyosauria adopted a new thunniform body plan. Namely, a fusiform (streamlined) body shape, long thin snout, and paired flipper-like appendages (Motani et al., 1996). The eye size grew larger in conjunction with the body (Motani, 2009: p.8). In addition to this the new fish shaped forms evolved discoidal vertebrae, retaining their high vertebrae count. They also gained highly modified dorsal and tail fins, which the early forms didn’t possess. These new ‘upgrades’ in the body plan indicated a pelagic swimming mode (Motani et al., 1996). Ichthyosaurs moved their crescent-like tail fin side to side in order to swim faster and greater distances than their earlier forms. The paired appendages were most likely present for maneuvering while

swimming at great speeds, approximately 21Mph (Lomax, 2018). Scientists and collectors also noticed that the lower lobe of the vertebral column only went down into the ventral part of the tail fin, leaving the dorsal part with no hard tissue (Motani, 2009: p. 7). Hence, the dorsal part is not commonly found in fossils. As well as being agile swimmers, Jurassic Ichthyosaurs were commendable divers. Ichthyosaurs could swim down to about 600m deep. The Jurassic forms adapted for deep dives by growing bigger eyes and having their skeleton lightened (Marek, 2015). Evidence for this can be found by looking at the absurdly large eye sockets. It was estimated that the largest eyes were greater than 25cm in diameter (Motani, 2009: p.7), which is bigger than a soccer ball. The colossal eyes could absorb a large amount of light indicating that the Ichthyosaurs could have roamed in low light environments. Another piece of evidence is Caisson disease, colloquially known as ‘the bends’ (Marek, 2015). The Caisson disease was caused from swimming up too quickly from a great depth causing the body to experience a drastic change in pressure. This disease was identified in a number of ichthyosaurs by finding gas bubbles on the interior. As ichthyosaurs were air breathing tetrapods, they had to resurface in order to fill their system with oxygen. Ichthyosaurs that swam up too quickly in order to escape a predator or because they urgently needed more air would have received this disease.                                  During the Early Cretaceous there was only a single genus called Platypterigius. Very few Ichthyosauria fossils were found in the Cretaceous beds, showing a massive drop in diversity and a path to ichthyosaur extinction (Fischer et al., 2014). Ichthyosaurs disappeared from the fossil record in the Late Cretaceous, about 90 million years ago, 25 million years before the 5th mass extinction when all the dinosaurs were wiped off planet earth (Bardet, 1992). There is no definitive answer as to how the ichthyosaur empire ceased to exist. Dale (2018) and Fischer et al. (2016) hypothesise that there was a great ocean anoxic event. The rest of the majority propose it was the steady decline in diversity and numbers that led to ichthyosaur extinction. Even though the ichthyosaurs became extinct they lived the 2nd most out of all the marine reptiles during the Mesozoic (Fig.3) implying that the Ichthyosauria were a hugely successful clade of marine reptiles (Motani, 2009: e4). 

Homology and Homoplasy in the Ichthyosauria                                                                                                                       Both homology and homoplasy are single anatomical features of species in a phylogeny. Homology concerns itself with traits that are derived from a single common ancestor.

Forefin                                                                                                                    The order Ichthyosauria have paddle like limbs, known as forefins. Those limbs were greatly modified throughout the evolution of the Ichthyosauria. The Jurassic period was undoubtedly the one in which forefin morphology was extremely diverse and complex. An example of a homologous feature within the Ichthyosauria is owning an extra zeugopodial element anterior to the radius and in contact with the humerus. Genera such as Ophthalmosaurus, Brachypterygius, Caypullisaurus and Platypterigius all share this feature in the forefin (Motani, 1999: p. 39; Fernandez, 2010: p.517). This zeugopodial element is seen to be in contact with the humeri of all species mentioned prior. Furthermore the different elements
in contact with the humerus is another homologous feature. There are 3 different types of contact. No.1, the humerus is in contact with the radius, ulna and extra zeugopodial element. This first type of contact is seen in Ophthalmosaurus, Caypullisaurus and in the species Platypterigius longmani. No.2, the humerus is in contact with the radius, ulna and intermedium. This contact with the humerus can be seen in Brachypterygius and Aegirosaurus. No.3, the humerus contacts the same radius, ulna, extra zeugopodial element as well as a pisiform. Most Tithonian ichthyosaurs have this type of contact (Fernandez, 2001: p. 519).

Another example of homology can also be found in the forefin. For instance, notching. A notch is essentially a cell to cell signaling pathway. Notching in the digits of the forefin occur from the loss of perichondral bone. This process is known as a transition series, part of an evolutionary pattern (Maxwell et al., 2013). Forefin elements, at the anterior surface, of Stenopterygius uniter and Temnodintosaurus trigonodon both exhibited notching. The process of loss of perichondral bone in hindfins was delayed compared to the forefin. From this, two patterns of reduction of notches were mentioned. The first pattern being a constant linear reduction in the leading-edge digit. This first pattern is seen in Excalibosaurus, Eurhinosaurus and Temnodontosaurus platyodon. The second pattern involved the loss of notches from phalanges as well as from carpals and the radius. This pattern is seen in Stenopterigius, excluding S. aaleniensis and also seen in Ichthyosaurus, excluding I. conybeari. Even though the amount of notches vary within genera, the strong relationship with body size implies that the morphology was caused by the delay in perichondral ossification (Maxwell et al., 2013). 

Homoplasy concerns itself with features of species not derived from a single common ancestor. The order Ichthyosauria provides us with many cases of homoplasy. For instance, the different skin layers of an Ichthyosaur. Soft parts are not preserved as well as hard parts and so are a rare find in the fossil record. However, an unusual preservation of Stenopterygius soft tissue remnants were discovered in Holzmaden, Germany dating back 178 million years ago. Those same tissue remnants provided us with many homoplasy examples in the Ichthyosauria.

Skin shading                                                                                                                                                                                          From analysing the soft tissue remnants, traces of pigment carrying cells known as chromatophores were found (Black, 2018). Namely, melanophores containing the pigment melanin (Greshko, 2018). Scientists used chromatophores to explore what colours the ichthyosaur possessed and uncovered that adult species showed countershading. Countershading is a camouflaging technique. Ichthyosaurs had a dark shading on the dorsal side and light shading on the ventral side. Thus, the dark side would blend in with the dark water from the above perspective and the light side would mix in with the bright sky from below (Lindgren et al., 2018; Buchholz, n.d.). The order Lamniformes to which the mackerel sharks belong also show countershading. Dolphins, in the order Cetacea, also exhibit the same colouration pattern of the outer skin. Even though sharks, dolphins and ichthyosaurs have some similarity in their overall appearance, as seen in (Fig.6), they do not have a common ancestor. 

Blubber                                                                                                                                         After describing the homoplasy of the outer skin it made sense to do the same thing on the interior. Blubber is a fatty layer deep beneath the skin. This fatty layer was found in the Stenopterygius at Holzmaden. The possession of this insulating layer ties ichthyosaurs to elevated metabolism, implying that they were endotherms (Black, 2018; Greshko,2018). This meant that ichthyosaurs were able to stay warmer than the surrounding environment, which would explain how they could swim in cold deep waters. A different group are also known for their internal body heating. The order Cetacea in which dolphins, porpoises, whales lie (Black, 2018). The extinct Ichthyosauria and extant Cetacea do not have the same ancestor. However, they independently evolved similarities on the inside.  

Scientific significance of Ichthyosaurus communis in the order Ichthyosauria                                                                    Ichthyosaurus communis from Greek translates to common fish lizard. Furthermore, I. communis is the type species for the genus Ichthyosaurus, which in turn is the type genus for the order Ichthyosauria (Lomax et al., 2017; Massare and Lomax, 2017). That is the primal reason why the Ichthyosauria is named as it is. The total amount of species within the genus was over 50, when new ichthyosaur fossils were found every day. At present Ichthyosaurus has stopped being the ‘wastebasket’ for all found ichthyosaurs and now only holds 6 species. The method used for redefining and categorising the species, involved using the neotype of Ichthyosaurus communis (Massare and Lomax, 2017). In the remote past the neotype of I. communis was thought to be the neotype of I. intermedius. However, McGowan (1974) suggests that both I. communis and I. intermedius can be identified as one species, as the majority of the features in both neotypes, of each species, are similar. And since the name I. communis was announced earlier it held the priority.                                                                      The original stratigraphic range for I. communis was between the Rhaetian and Sinemurian (Bennett, 2012). This changed when a new specimen of Ichthyosaurus communis [NHMUK R15907] was found in Stonebarrow Marl at Charmouth, Dorset. 

Enough evidence was voiced to solidify that the newly found specimen lived in the Pliensbachian. This news extended the stratigraphic range of I. communis. Since Ichthyosaurus specimens were a rare find from the Pliensbachian, the newly found species became an important find. Up until the discovery, ichthyosaurs from the Pliensbachian were poorly known because all the remains were fragmented. Thus, the discovery of [NHM R15907] became significant for the scientific community (Bennett, 2012).                                              Another way the I. communis enhances our understanding about Ichthyosauria is what their diet was. A small specimen [BU 5289] was reported on by Lomax et al. (2017) and assigned to I. communis. Identified as a neonate, the specimen had a total length of ~590mm. In the report the neonate specimen’s stomach content was described. A gross number of black cephalopod hooklets were located in between the ribs (Lomax et al., 2017). This indicated that the Ichthyosaurus ate cephalopods. However, in a different specimen of Stenopterygius, studied by Dick et al. (2016), they found fish scales in the stomach contents and no hooklets. Hence, stating that the majority of ichthyosaurs had a cephalopod diet would be inaccurate. More evidence was acquired via a different Ichthyosaurus specimen [OUMNH J.13593] that showed both cephalopod and fish scales in its stomach contents. Therefore, smaller fish than ichthyosaurs themselves and cephalopods were part of their diet (Bennett, 2012; Lomax et al., 2017). The same research conducted by Lomax et al. (2017) mentioned that the osteological comparison between small examples of Ichthyosaurus, such as [BU 5289] and other large examples of Ichthyosaurus can be used to identify specimens that are not only in the Ichthyosaurus group. As a result, many other unknown genera in the order Ichthyosauria have a chance to become known. 

Conclusion                                                                                                                                                                                          This research on the order of Ichthyosauria highlights the evolutionary history of the group. Specifically how the underdeveloped basal forms with unpronounced appendages and tail evolved a fish-like body plan and modified its limbs. The forefin was the source of most homologous traits due to its highly modified elements and diversity within the group. The similarity and convergent evolution between the Lamniformes, Cetacea and Icthyopterygia was discussed. And incontrovertibly, Ichthyosaurus communis demonstrated many points on why it is significant to the scientific community as the type species of the order Ichthyosauria.

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