Bairdops

Bairdops Temporal range:
Illustration of a fossil of B. elegans
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Malacostraca
Order:	Stomatopoda
Family:	†Perimecturidae
Genus:	†Bairdops Schram, 1979
Type species
†Perimecturus elegans Peach, 1908
Species
†B. beargulchensis Schram & Horner, 1978 †B. elegans Peach, 1908
Synonyms
synonyms of B. elegans Perimecturus elegans Peach, 1908 Perimecturus ensifer Peach, 1908

Bairdops is an extinct genus of mantis shrimp that lived during the Carboniferous period in what is now Scotland and the United States. It contains two named species. The type species, B. elegans, was described in 1908 by Ben Peach as a species of Perimecturus, and fossils of it are known from Scotland. A second species, B. beargulchensis, was named from specimens found in the Bear Gulch Limestone of Montana.

Discovery and naming

Fossils of Bairdops elegans as illustrated in 1908, originally labelled as Perimecturus ensifer

The fossilized remains of Bairdops have been discovered decades prior to the establishment of the genus. In 1908, Ben Peach became the first to study specimens of this animal, which were collected in Scotland by A. Macconochie. However, Peach believed they represented members of Perimecturus, another genus of early stomatopods. He erected the species Perimecturus elegans based on remains found near the River Esk in Glencartholm. In addition, he also erected the species Perimecturus ensifer from fossils found in Liddel Water, Newcastleton and Glencartholm.

In 1979, Frederick Schram found that P. elegans and P. ensifer were indistinguishable from each other, thus the latter was declared as a junior synonym of the former. He also noted that the tail fan, body shape and size of this species differed significantly from what is seen in other species of Perimecturus. Schram therefore erected the genus Bairdops with P. elegans as the type species, which was subsequently renamed as Bairdops elegans, and the specimen GSE 5879 was designated as the lectotype of this species. The generic name honors William Baird, the Assistant Curator of the fossil invertebrate collection at the Royal Scottish museum.

Additionally, Schram described a second species of Bairdops in 1978 which he named B. beargulchensis. The specific name refers to the Bear Gulch Limestone in Fergus County, Montana, where the holotype (UM 6217) was collected. Factor and Feldman (1985) declared this species to be a junior synonym of Tyrannophontes theridion. However, a later study by Jenner et al. (1997) found that B. beargulchensis is most similar to B. elegans, thus revalidating B. beargulchensis. They also found that some of the fossils in Schram's original sample represent a separate species they named Tyrannophontes acanthocercus (now moved to the genus Daidal), and that this is likely responsible for the earlier study finding B. beargulchensis similar to Tyrannophontes.

Description

Bairdops is a crustacean of moderate size, with B. elegans having a carapace length of 0.76–2.93 cm (0.30–1.15 in) and a total abdomen length of 1.50–5.03 cm (0.59–1.98 in). B. beargulchensis is slightly larger and more robust, with a carapace ranging from 1.7–2.6 cm (0.67–1.02 in) and a total abdomen length of 3.19–5.30 cm (1.26–2.09 in).

The rostrum is about a third the length of the carapace and is spatula-like in shape, with a tapering, pointed tip and a wide base. The antennae are not well preserved, but it is known the scaphocerite (exopod of the antenna) is very large. A pair of large and circular compound eyes are present on the head. The large, elongated carapace covers the entire thorax in B. elegans, whereas in B. beargulchensis it leaves only the top of the eighth thorax segment exposed. The carapace has a subrectangular shape and a furrow on its edges. This furrow is prominent in B. elegans but very slight in B. beargulchensis. The back margin of the carapace is concave at the top, with its sides extending slightly further and reaching the front of the first abdominal segment.

Pairs of raptorial appendages are attached to the thorax, though it is unclear whether there are four or five pairs of these. The coxae, ischiomeri and carpi of these appendages are short, whereas the propodi (penultimate segment) and dactyli (final segment) are longer. The longest segment of a raptorial appendage in Bairdops is the basis. Though Schram and Horner (1978) claimed the propodi of B. beargulchensis possessed two rows of spines, this observation was based on specimens now reassigned to Daidal acanthocercus. Propodus spines are currently believed to only be present in B. elegans.

All thoracic segments are shorter than the abdominal segments. The thoracic segments further front possess narrow sternites and are smaller than the sixth to eighth thoracic segments. The abdominal segments decrease in both height and width down the animal's length, and their side margins are rather straight. Moderate furrows are present on the lower margins of each abdominal segment. In B. beargulchensis, the fifth and sixth abdominal segments may possess one or two backward-pointing ridges extending past the back margin of their respective segments, a feature absent in B. elegans.

Telson of B. elegans

The somewhat delicate tail fan includes a telson which tapers into a long median spine, while a shorter spine is present on either side of this median spine. Three keels are seen on the telson, with one running down the middle and another on either side of it, which all converge near the end of the telson. The protopod (basal segment) of the uropod has a projection extending over the ramus, while the blade-like exopod is made up of a sclerotised outer part and more membranous inner part.

Classification

Fossil specimens of Bairdops were first assigned to the family Perimecturidae by Ben Peach in 1908, and to this day the genus is still placed within this family. Peach believed Perimecturidae to be intermediate forms between Lophogastridae and Anaspididae, and assigned the family to the currently defunct order Schizopoda. It was not until 1962 that the Perimecturidae family were recognized to be early forms of mantis shrimps, when H. K. Brooks reassigned the family to the order Palaeostomatopoda (currently delisted as a suborder and named Palaeostomatopodea). As cladistic analysis became more widely conducted, the palaeostomatopods were first recovered to be a paraphyletic grouping by Jenner et al. (1998), and this paraphyly was then confirmed by Schram (2007). The paraphyletic use of Palaeostomatopodea is used today for the convenience of referring to the evolutionary grade.

Though B. elegans and B. beargulchensis were initially placed in the same genus due to their overall similarity, the monophyly of the genus has been questioned. Jenner et al. (1998) was the first to propose that Bairdops may be polyphyletic, recovering B. beargulchensis to be closer to Perimecturus than B. elegans. Schram (2007) similarly suggested Bairdops to be polyphyletic, but instead found that B. elegans was the sister taxon to Perimecturus rapax. Haug et al. (2010) recovered B elegans as the second-earliest lineage of mantis shrimps to diverge (with Archaeocaris being the most basal), being a sister taxon to the clade including all other mantis shrimps except Archaeocaris. Smith et al. (2023) conducted a phylogenetic analysis which also supports the idea that Bairdops is polyphyletic, though the family Perimecturidae was found to be monophyletic. Their results are displayed in the cladogram below:

Stomatopoda †Archaeocaris Archaeocaris vermiformis Archaeocaris graffhami †Bairdops elegans †Perimecturus Perimecturus parki Perimecturus rapax †Bairdops beargulchensis †Daidal pattoni †Daidal schoellmanni †Daidal acanthocercus †Gorgonophontes Gorgonophontes fraiponti Gorgonophontes peleron †Chabardella spinosa †Tyrannophontes theridion †Tyrannophontes gigantion Unipeltata †Triassosculda ahyongi †Tyrannosculda laurae †Pseudosculda laevis †Archaeosculda phoenicia †Sculda pennata †Sculda syriaca †Ursquilla yehoachi †Lysiosquilla nkporoensis †Nodosculda fisherorum Squilla mantis

Archaeostomatopods Palaeostomatopods Pseudosculdids

Palaeobiology

Like all other mantis shrimp species , Bairdops would have been carnivorous, using its raptorial thoracic appendages to handle prey. Clarkson (1985) suggested that small fish and the crustacean Belotelson were among the prey of B. elegans. Unlike modern mantis shrimps of the suborder Unipeltata, the raptorial appendages of palaeostomatopods like Bairdops lack a click-joint mechanism formed by a specialized joint and its associated muscles. Such a mechanism is what allows modern mantis shrimps to quickly extend the second pair of thoracic appendages to capture prey. Lacking this, the appendages of Bairdops are more similar to the smaller, third to fifth pairs of thoracic appendages in unipeltatans used to manipulate prey after capture. Jenner et al. (1998) thus proposed that Bairdops was an opportunistic scavenger and specialized in dead food. In contrast, Haug and Haug (2021) suggested that since their appendage morphology would not allow them to capture prey from the seabed, early mantis shrimps would instead have been benthopelagic predators, possibly capturing prey from above while swimming just above the bottom. This is unlike modern mantis shrimp, which are bottom-dwelling predators as adults, though their larvae are benthopelagic and thus studying them could allow better knowledge of early stomatopods.