''Protichnites'' is an

ichnogenus An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxa'' comes from the Greek ίχνος, ''ichnos'' meaning ''track'' and ταξις, ''taxis'' meaning ...

trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...

consisting of the imprints made by the walking activity of certain arthropods. It consists of two rows of tracks and a medial furrow between the two rows. This furrow, which may be broken, set at an angle, and of varying width and depth, is thought to be the result of the tail region contacting the substrate.

The first footprints on land

Sir Richard Owen, the noted British paleontologist and anatomist who coined the term “ Dinosauria”, based ''Protichnites'' on trackways that were shipped to him from the Cambrian Potsdam Sandstone of

Quebec Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirtee ...

.Owen, 1851 He was correct in recognizing that these were the oldest known footprints on land, having been produced in intertidal and supratidal environments during what would now be called Cambrian times.Collette and Hagadorn, 2010

The ''Protichnites'' makers

Owen first thought that these trackways were made by tortoises, but new material convinced him that “articulates” (a group that included the arthropods) were responsible.Owen, 1852 He further suggested a kinship with ''

Limulus ''Limulus'' is a genus of horseshoe crab, with one extant species, the Atlantic horseshoe crab (''Limulus polyphemus''). One fossil species is currently assigned to the genus though several other species have been named, which have since been as ...

'', the modern horseshoe crab. Additional material was collected in Quebec,

Ontario Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central C ...

, New York,

Wisconsin Wisconsin () is a state in the upper Midwestern United States. Wisconsin is the 25th-largest state by total area and the 20th-most populous. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake M ...

, and

Missouri Missouri is a state in the Midwestern region of the United States. Ranking 21st in land area, it is bordered by eight states (tied for the most with Tennessee): Iowa to the north, Illinois, Kentucky and Tennessee to the east, Arkansas t ...

for the next 150 years without a single fossil of the maker of these traces. Finally, body fossils of potential makers were found in two of the same quarries that preserved ''Protichnites'' from the Elk Mound Group of Blackberry Hill, Wisconsin, and the Potsdam Group of Melocheville, Quebec. The animals, '' Mosineia macnaughtoni'' and ''Mictomerus melochevillensis'', were euthycarcinoids, extinct arthropods that may have given rise to the

mandibulate Mandibulata, termed "mandibulates", is a clade of arthropods that comprises the extant subphyla Myriapoda (millipedes and others), Crustacea and Hexapoda (insects and others). Mandibulata is currently believed to be the sister group of the clade ...

s. Fossils that clearly tie euthycarcinoids to ''Protichnites'' were then found at Blackberry Hill.Collette et al., 2012 It is possible that other extinct arthropods, such as members of the Aglaspidida, may also have produced some of these trackways.

Trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

s have been suggested as well; however, no trilobites have been found thus far in the strata that contain this ichnogenus. Similar trackways are present in post-Cambrian strata; however, those are seldom referred to as ''Protichnites''.

Possible mating behavior

It has been suggested that one form of ''Protichnites'', ''P. eremita'' Form 1, may have been produced by an ambulating pair of individuals, as in

amplexus Amplexus (Latin "embrace") is a type of mating behavior exhibited by some externally fertilizing species (chiefly amphibians and horseshoe crabs) in which a male grasps a female with his front legs as part of the mating process, and at the same ...

. In this form, the medial furrow is discontinuous and each section is set at an angle, such that the series of medial imprints have a shingled appearance, as might result if the female were displacing her tail to minimize its interference with external fertilization. A second set of medial imprints observed in Form 1 is postulated to have been produced by the tail region of the male of the pair. ''Protichnites eremita'' was originally interpreted as early evidence of

hermit crab Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an as ...

-like behavior.

Relationship with ''Diplichnites''

Differing only in the presence or absence of a medial furrow, ''Protichnites'' and '' Diplichnites'' trackways could both be produced by the same individual animal. In cases where the feet penetrate the sediment more deeply than does the tail, ''Diplichnites'' could be created on the underlying layer while ''Protichnites'' is being produced on the surface.Hoxie, 2005

References

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