Leedsichthys problematicus

[theropod]

Leedsichthys problematicus is a species of giant pachycormid fish in the monotypic genus Leedsichthys, known from the Callovian, Oxfordian and Kimmeridgian (Middle to Upper Jurassic) of Europe and South America (Liston 2010). Pachycormidae include a number of large, pelagic fishes from the Middle Jurassic to the Upper Cretaceous, some of which filled the niche of modern large filter-feeding whales and sharks during the mesozoic (Friedmann et al. 2010)..

Systematic Palaeontology:
Vertebrata
Gnathostomata
Actinopterygii
Pachycormidae
Leedichthys Woodward 1889
L. problematicus Woodward 1889

Size comparison of various members of Pachycormidae, estimated maximum size of Leedsichthys in red

© Eotyrannu5 / Wikimedia CC BY-SA 4.0

Etymology:
The generic name references British fossil collector Alfred Nicholson Leeds, the species epithet refers to the fragmentary (hence, problematic) nature of the fossil material.

Description:

Large portions of the axial skeleton of Leedsichthys were cartilaginous, contributing to the fragmentary nature of its fossil records (Liston 2006). Among the ossified elements most commonly preserved are parts of the fins, skull and gill basket. The latter consists of 5 gill arches, bearing numerous gill-rakers that served as filtering implements (Liston 2006). The pectoral fins are long end elongated. The body was likely moderately deep (estimated 1.5-2.0 m in NHMUK PV P10000).

Martill (1986) originally estimated a length of 13.5-27.6 m for the largest specimen of L. problematicus, however the extreme variability between estimates based on different parts of the same specimen, and scaling from a much smaller relative, call the accuracy of these figures into question (Liston & Noè 2004).

Most Leedsichthys individuals were betwen 8 and 12 m long and would have massed between 5 and 19 t (Liston 2006). The largest individual, NHMUK PV P10156, estimated at a standard length of 16.5 m (Liston et al. 2013) would have massed roughly 44.9 t when based on regression equations for actinopterygian fish (Ferrón 2018), with very similar values (46.5 t) being suggested by a volumetric scale model (Liston 2006).

NHMUK PV P10000, a 2.74 m tall tail fin of L. problematicus, is estimated to have come from an individual with a total length approaching 9 m and body mass of c. 7 t (Liston & Noè 2004, Liston 2006).

Biology

Ferrón (2018) extrapolated swimming energetics and metabolic rates in actinopterygians, suggesting that a Leedsichthys the size of the largest individuals would have been energetically viable following the physiological scaling trends of other fish. Histological examination of gill rakers and  suggest that analyzed specimens were between 19 and 40 years old at time of death, and that Leedsichthys grew slowly and continuously throughout their lifetimes, in a manner similar to basking and whale sharks (Liston et al. 2013).

Leedsichthys and other, similar pachycormids are interpreted as occupants of the suspension-feeder niche today held by large sharks and mysticete whales (Friedmann et al. 2010). Furrow-like trace fossils on the Jurassic sea floor exposed in the Swiss alps have alternatively been interpreted as feeding traces of plesiosaurs or cases of Leedsichthys exploiting a benthic food source (Liston 2010).

Bite marks on fossil specimens of Leedsichthys suggest predation on or at least feeding on this taxon by large contemporaneous predators (Liston 2006). Specifically, two bite marks on fin rays are probably attributable to attack by large pliosaurs, possibly Liopleurodon. Teeth of metriorhynchid crocodiles and hybodontid sharks found embedded in Leedsichthys bones further suggest these taxa scavenged on the carcasses of Leedsichthys.

Ferrón, H.G., Holgado, B., Liston, J.J., Martínez‐Pérez, C. and Botella, H. 2018. Assessing metabolic constraints on the maximum body size of actinopterygians: locomotion energetics of Leedsichthys problematicus (Actinopterygii, Pachycormiformes). Palaeontology 61 (5): 775–783.
Friedman, M., Shimada, K., Martin, L.D., Everhart, M.J., Liston, J., Maltese, A. and Triebold, M. 2010. 100-million-year dynasty of giant planktivorous bony fishes in the Mesozoic seas. Science 327 (5968): 990–993.
Liston, J., Newbrey, M., Challands, T. and Adams, C. 2013. Growth, age and size of the Jurassic pachycormid Leedsichthys problematicus (Osteichthyes: Actinopterygii). .
Liston, J.J. 2006. A fish fit for Ozymandias?: the ecology, growth and osteology of Leedsichthys (Pachycormidae, Actinopterygii).PhD Thesis, University of Glasgow, 464pp.
Liston, J.J. 2010. The occurrence of the Middle Jurassic pachycormid fish Leedsichthys. Oryctos 9: 1–36.
Liston, J.J. and Noè, L.F. 2004. The tail of the Jurassic fish Leedsichthys problematicus (Osteichthyes: Actinopterygii) collected by Alfred Nicholson Leeds-an example of the importance of historical records in palaeontology. Archives of natural history 31 (2): 236–252.

Martill, D.M. 1986. The world’s largest fish. Geology today 2 (2): 61–63.

Woodward, A.S. 1889. Notes on some new and little-known British Jurassic fishes. Journal of Natural History 4 (23): 405–407.

[Infinity Blade]

Here's a full link to Martill (1986).

onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2451.1986.tb01022.x?casa_token=ppe9sx6BeAQAAAAA:hPCBBhFnjUsObIAzk6LCTDC0aagnWfnVEFXVztF2wH4GlnY6Q5XgEnPHvo97Qp7ufadP7c3-Hs916cY

Figure 3 shows you the actual metriorhynchid tooth embedded in the Leedsichthys hyomandibula.

This made me wonder: what would a Leedsichthys would do if confronted by a predator? I remember writing a profile for Bonnerichthys (a fellow pachycormid) where I cited one source for it having sharp edged pectoral fins for use as weapons. I also wrote a profile for yet another pachycormid, Protosphyraena, which was also described as having sharp edged, even serrated pectoral fins that have been described as weapons. Bracketing off of these two, I feel like Leedsichthys would have had the same structures.

EDIT: No it didn't. Fused leading edges are a derived trait in Cretaceous pachycormids. The first four rays of Leedsichthys don't bifurcate, but much of the leading edge was apparently still made of bifurcating rays.

Leedsichthys had a skull roof with bosses on them, though. If it did anything at all, it might have swung its head at predators like some baleen whales do today.