Introduction
D. belli (Dromilites belli) is a species of extinct decapod crustacean from the Eocene period belonging to the family Dromiidae. While D. belli is an early example of Dromiid, there are many different species of the family still alive today.
The family Dromiidae are also known as sponge crabs because of their unique behaviour of fastening sea sponges to the back of their carapace. They then hold the sponge in place by folding their two most back legs behind them. This newly added décor acts as natural camouflage against potential predation and the incredible thing about this behaviour is the sea sponge continues to grow while mounted on the back of an individual. It is unknown whether or not this is a symbiotic relation between the crab and the sponge as we do not yet know if the sponge benefits at all. We also do not know if this behaviour would have been present in D. belli. But, it's highly probable that it either started here and carried on through to modern extant species, or was already present and had already been established in earlier species. The majority of species also feature fine, bristle like hairs that covered the most of the carapace and pereopods. due to the nature of fossilisation it is known if these hairs were present in D. belli. As of yet no fossil evidence on the presence of these bristles on the species has been discovered.
It's unknow for certain which extant Dromiid is the closest relative of D belli, if it even has one. But, from my research into the Dromiidae family, I speculate it could be the species Lauridromia dehaani (shown below.) This is due to the familiar carapace shape and the presence of four bumps on the dorsal side with relative bilateral symmetry in most cases. In the following post I will be detailing specifics about a D. belli specimen that was found on the foreshore London Clay sediments of Seasalter, located in North Kent.
[Figure 1. L. dehaani, a possible relative of D. belli]
Geography
The specimen I will be detailing in this article was found in London Clay sediment on the foreshore of Seasalter, located in North Kent, England. This would date the specimen back to the Eocene period, specifically the Ypresian age, roughly 56-49 MYA.
The rock groups in and around Seasalter are comprised of Thames Groups and Lambeth Groups, with the former overlying the latter. The top of the Thames Group is the top of the London Clay Formation and the base of the group is the base of the Harwich Formation. Thames Group is mainly silty clays and clays, some being sandy or gravelly, with the addition of some silts, sands, gravels and calcareous mudstones. The Thames Group was deposited in environments ranging from marine shore faces ranging out to outer marine shelfs. The location also features pockets of Alluvium deposits.
Seasalter beach has many London Clay formations and fossils are somewhat regularly found on the foreshore, having been broken off from the surrounding cliff faces by storms and high tides.
(Figure 2. Geographical location of where the specimen was discovered & it's bedrock geology)
Specimen Description
This particular specimen has been labelled as DBE12022 in my collection catalogue and is approximately 50 millimetres in horizontal length, 25 millimetres in vertical height and has a front-to-back depth of 32 millimetres. This puts this particular individual at about the same size as the extant Dromiid called Psuedodromia latens, also known as the furred sponge crab due to it's body being covered in fine fur like bristles. However, as touched on below, this may not have been D. belli's maximum size due to this individual not being fully mature and could more than likely get bigger after moulting.
(Figure 3. Dorsal view of DBE12022.)
The specimen is dark in colour, due to the presence of pyrite which made preparing it somewhat difficult. The chelipeds are present up to the carpus, but both are completely missing their chelae (claws). The carapace itself is in relatively good condition with some clear weathering spots. The entire carapace is covered in bumps and mounds. However, the presence of four particular bumps on the dorsal side of the carapace, just in front of the centre, may suggest that D. belli shares some relation to the extant Dromiid with a similarly structured carapace called Lauridromia dehaani as mentioned above. The anterior section of the carapace also features two points that protrude forward and is present and common in most Dromiid species.
(Figure 4. Ventral view of the DBE12022.)
As for the ventral side of the specimen, the abdominal region is on show quite nicely despite having a few weathering spots. From inspecting it I could tell that this particular individual was an immature female due to the rounded triangle shaped tip of the abdomen. This in turn shows me that this individual was probably not fully grown either as it would have been due to moult it's exoskeleton, evident by the incomplete morph of the abdomen. Female abdomens start out narrow and pointy towards the tip and as they mature, the region starts to round itself out until it's completely rounded and wide enough to contain and hold their eggs.
Macro-Inspection of DBE12022
While inspecting the dorsal side of the carapace utilising a 50X-1000X digital microscope, I noticed a multitude of small rounded craters that were located in specific collections across the carapace. This could be possible evidence that D. belli could have possessed the fine, fur-like bristles that are common in many species of Dromiids, although I cannot conclude that this is the case.
(Figure 5. Dorsal side of the carapace at 65X, featuring small rounded craters in pockets across the entire carapace.)
Using the same instrument, I also inspected the carpus of each cheliped. They're both preserved, but the middle sections of each of these legs feature weathering that causes the carpus to look fragmentary. However, they still both feature incredible detail including small mounds across the smoother sections of the leg as well as nice sectioning between where the carpus sections start and the merus sections end.
(Figure 6. Left carpus at 70X shows weathering causing the section to look weathered, but still featuring some nice details.)
Upon inspecting the ventral side of the specimen, in particular the abdominal flap, the same rounded craters found across the dorsal side of the carapace are also present here. However, instead of being in clusters and particular regions of the abdomen, they are seemingly covering this entire section. This puts doubts on my earlier thoughts of the craters possibly being evidence of bristles, as it's originally thought the bristles were to further enhance the species ability to camouflage (the fur-like structures giving the crab more of a algae/weed covered rock or coral.) However, the bristles could have also been used to sense movement or chemicals in the water or seafloor surrounding them. This means having the bristles present on both the dorsal and ventral sides of an individual would serve as a means to locate food or sense potential threats.
(Figure 7. Abdominal region of DBE12022 at 65X, showing the same rounded craters also present on the dorsal side of the carapace.)
Conclusion
After studying DBE12022 and researching both extant and extinct Dromiids I can conclude that both examples of decapod crustaceans provide ample wonder into the survival adaptations and fossil record of Brachyura. In particular, the question of whether or not D. belli shared similar adaptions of camouflage and appearance of modern, extant Dromiids is one that requires further probing. In terms of this particular specimen, I am rather pleased how well preserved the carapace and abdomen regions were despite the obvious signs of weathering and the presence of pyrite which usually causes specimens to fragment rather easily. I am extremely grateful of how it was able to help me in understanding the anatomy of Eocene crustaceans in comparison to modern ones and also how they gave me much more to look into in order to fully understand these amazing, grossly overlooked creatures.
Kieran Paul Farley - Studying Palaeontologist - Plymouth, United Kingdom - 18/03/2022
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