Tag: sexual dimorphism

Tachypleus syriacus

Tachypleus syriacus was a horseshoe crab from the late Cretaceous (~100-95 million years ago) of what is now Lebanon.

Closely related to modern tri-spine horseshoe crabs, it displayed a similar level of sexual dimorphism. Females grew to at least 25cm long (~10″), with rounded front edges to their carapaces and shorter rear spines, while males were around 30% smaller with a scalloped shape to the front of their carapaces.

One recently described female specimen also preserves distinctive nodules around the rim of its carapace, which may represent some sort of sensory structure.

This particular specimen is also unique for preserving a coprolite in the process of being expelled from the horseshoe crab’s body – that’s right, it died while pooping.

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Strange Symmetries #20: The 16 Million Year Fiddler Crab Rave

Many decapod crustaceans have slightly asymmetrical pincers, often with one claw being chunkier and specialized for “crushing” while the other is more slender and used for “cutting”.

But fiddler crabs take this sort of asymmetry to the extreme as part of their sexual dimorphism – males have one massively oversized claw, which is used for both visual display to potential mates and for physical fights against rivals.

Some of the earliest fiddler crabs are known from the Miocene of what is now northern Brazil. Although the fossils have been given several different taxonomic names since their discovery in the 1970s (including Uca maracoani antiqua, Uca antiqua, and Uca inaciobritoi) they’re currently considered to be indistinguishable from the modern Brazilian fiddler crab, Uca maracoani, meaning that these crabs have remained externally unchanged for the last 16 million years.

Up to about 4cm in carapace width (~1.6″), modern Uca maracoani are found in coastal mangrove swamps and tidal mudflats around the northern and eastern coasts of South America – and some of these environments have also undergone little change since the Miocene. Males of the species can develop their enlarged pincer on either side of their bodies, with lefties and righties seeming to occur in equal numbers.

Echinochimaera

Echinochimaera meltoni here was a cartilaginous fish found in the Bear Gulch Limestone deposits in Montana, USA, dating to the Early Carboniferous about 326-318 million years ago.

It was an early member of the chimaera lineage, but unlike its mostly-scaleless modern relatives its body was covered in small shark-like placoid scales.

It also showed a large degree of sexual dimorphism, with males and females almost looking like different species entirely. Males are identified by the presence of claspers and were up to 15cm long (6″), with four pairs of spiny “horns” on their heads, larger more pointed dorsal fins, and rows of spines along their tails. Females were less than half the size of males at just 7cm long (2.75″), with only one pair of smaller “horns” and none of the additional spines.

The rounded bodies and relatively small paddle-like tail fins of both sexes suggest they weren’t very strong swimmers, probably relying on their large dorsal fin spines to defend themselves – which may have been venomous much like those of modern chimaeras.

Squaloraja

Discovered in the late 1820s by pioneering paleontologist Mary Anning, the odd-looking fossil of the cartilaginous fish Squaloraja polyspondyla seemed to have characteristics of both sharks and rays.

It was initially thought to be a “missing link” transitional form between those two groups, but later it was identified as being something else entirely – it was actually part of the chimaera lineage, much closer related to modern ratfish, and its ray-like features were due to convergent evolution for a bottom-feeding lifestyle.

Living during the early Jurassic period, about 200-195 million years ago, Squaloraja fossils are now known from the south coast of England, southern Belguim, and northern Italy. Around 30cm long (1’), this weird fish had a massive wide flat snout that looked like an even more extreme version of the long noses seen in some of its modern relatives, and this enormous snoot would have been absolutely packed with sensory receptors to help it locate small aquatic prey hidden in the muddy seafloor.

Some specimens also have a distinctive long horn-like spine on their foreheads, and since these individuals also have claspers it seems like this was a sexually dimorphic feature. Much like the smaller head claspers on modern chimaeras, male Squaloraja probably used this “horn” to hang onto females’ pectoral fins during mating – and with it being such a large elaborate structure it may also have been used for visual display purposes, too.

Harpagofututor

Sometimes sexual dimorphism in the fossil record is hard to identify for certain – and sometimes it’s incredibly obvious.

Harpagofututor volsellorhinus here is a wonderful example of the second category. This 17cm long (~7″) cartilaginous fish was a distant relative of modern chimaeras, and lived during the Early Carboniferous about 326-318 million years ago in the shallow tropical sea that formed the Bear Gulch Limestone deposits in Montana, USA.

While all specimens show an elongated eel-like body, they come in two different forms: one with a fairly normal skull, and one with a pair of huge jointed cartilaginous appendages in front of its eyes that resemble antennae or antlers.

The presence of large claspers on the “antlered” forms indicated they were males, with the weird appendages probably being used either for display or as “grappling hooks” to hang onto females during mating.

(Modern male chimaeras also have clasping structures on their heads!)

Meanwhile a couple of non-antlered specimens preserved with unborn offspring still inside their bodies confirmed that these unadorned forms of Harpagofututor were indeed females. Some of their young were quite large and well-developed, suggesting live birth, and with multiple different fetal growth stages found within a single mother it’s also a rare example of fossilized superfetation.

Shringasaurus

Shringasaurus indicus, an archosauromorph reptile from the Middle Triassic of India (~247-242 mya). About 3-4m long (9′10″-13′1″), it was part of a group known as the allokotosaurs, specialized herbivores with lizard-like bodies, and was related to the strange long-beaked Teraterpeton.

Partial remains of about seven different Shringasaurus individuals of varying ages have been found, with several of them having large curving horns over their eyes convergently similar to those of the later ceratopsids. A couple of adult specimens lack horns entirely, suggesting the feature was sexually dimorphic with only the males developing ornamentation – a very rare arrangement among archosauromorphs, but similar to some horned mammals.