Labechiida is an extinct order of stromatoporoidsponges. They lived from the Early Ordovician (Floian stage)[1] to the Late Devonian (Famennian stage),[2] though a few putative fossils have been reported from younger sediments.[3][4] Labechiids were the first order of stromatoporoids to appear and were probably ancestral to all other orders in the main Paleozoic radiation (Stromatoporoidea sensu stricto).[2] They were most diverse and abundant during the Middle-Late Ordovician and the Famennian, when they were a major group of reef-building sponges. However, they were relatively uncommon through most of the Silurian and Devonian, in contrast to other stromatoporoids.[5]
Labechiids can be differentiated from other stromatoporoids by having an internal structure emphasizing cysts (pockets roofed by flat or convex cyst plates) and pillars (solid rods perpendicular to the surface).[2][6][7] The pillars may extend all the way to the surface and project out a short distance as small bumps known as papillae. Cyst plates may bear denticles (pointed thorns) or crenulations (pinched deflections) pointing upwards within the skeleton. Laminae (regular horizontal plates) are comparatively difficult to distinguish due to the more unorganized internal structure of labechiids compared to other stromatoporoids.
Mamelons are found in some species, while astrorhizae are indistinct or absent. Many labechiid fossils are poorly preserved, as their emphasis on cysts affords a lower skeletal density than other stromatoporoids. As a result, some traits used to distinguishing between taxa (such as pillar microstructure) may instead be taphonomic rather than biological in origin.[2]
The earliest reported labechiid species is Cystostroma primordia, from the Floian stage (Early Ordovician) of South China.[1] This species was rare and small, but massive reefs appeared simultaneously worldwide in the mid- to upper-Darriwilian stage, near the end of the Middle Ordovician (~460 Ma).[9] During the Darriwilian, 4 genera occur in the Chazy Group of eastern North America, 9 in the Machiakou Formation of North China, and several others are dispersed among Russia, Kazakhstan, Korea, and Malaysia.[5] Most specialists agree that the ancestral labechiid was a member of the family Rosenellidae, but disagreement persists over which genus serves as the ancestral form. Studies focusing on North America and China generally consider the widespread Pseudostylodictyon[9] or Cystostroma[1] to be ancestral to other labechiids, while Russian paleontologists point to the SiberianPriscastroma.[9][10] Labechiids as a whole may be descended from non-stromatoporoid sponges in the order Pulchrilaminida.[9][11]
In the Sandbian stage of the Late Ordovician, labechiids spread to regions equivalent to Scotland and Australia, and some former Chinese endemics (such as the tree-shaped aulaceratids) populated Laurentia (North America) for the first time.[5] During the Katian stage, labechiids reached their maximum diversity and were abundant in tropical reef ecosystems worldwide. They were significantly less common in temperate areas corresponding to Central Asia.[5] Most Late Ordovician labechiid genera were cosmopolitan, while endemics were few. Global cooling in the first pulse of the Late Ordovician mass extinction (~445 Ma) led to the extinction of the short-lived family Stromatoceriidae, and Hirnantian-age stromatoporoid fossils are limited to only a few localities in Canada and Northern Europe.[5] Only two labechiids (Aulacera and Pachystylostroma) retain any semblance of commonness during the extinction interval.[5]
Silurian
Labechiids which survived into the Llandovery Epoch (early Silurian) were inconspicuous relative to other diversifying stromatoporoid orders such as clathrodictyids and actinostromatids. South China and Siberia were notable exceptions where labechiids temporarily remained the most diverse group of stromatoporoids.[5] Even as stromatoporoids collectively reached their maximum Silurian diversity in the Wenlock Epoch (middle Silurian), labechiids were not among the beneficiaries.[5] Labechiids were still rare in the Ludlow and Pridoli epochs (late Silurian), though the typical Ordovician genus Lophiostroma made an unexpected recovery in Baltica (Eastern Europe) and Turkey.[5]
Devonian
No unambiguously new labechiid genera evolved through the entire Early and Middle Devonian, nor the Frasnian stage (the first part of the Late Devonian). Labechiid fossil occurrences during this time were spotty and consist entirely of hardy relicts which purportedly originated in the Ordovician.[5] Stromatoporoid specialists have mixed interpretations of this phenomenon. The Devonian relicts may truly be the same as their Ordovician predecessors, persisting as rare and reclusive ‘Lazarus taxa’. Alternatively, they could be misidentified fragments or new labechiid lineages acquiring a similar set of characteristics, so-called ‘Elvis taxa’.[12] In any case, Devonian labechiids never managed to repopulate southeast Laurussia (eastern North America) or high-latitude environments at any time during the period.[5]
Unlike other stromatoporoids, labechiids were not adversely affected by the Kellwasser event (Late Devonian mass extinction) at the end of the Frasnian (~372 Ma). Instead, they experienced a diversification in the Famennian stage, reacquiring a level of diversity and dominance in their niche not experienced since the Ordovician.[5] Most new genera arose from the families Stylostromatidae and Platiferostromatidae, which helped to form dense reef habitats alongside the older relict forms.[12] The labechiid resurgence was strongest in shallow water and the Eastern Hemisphere.[12][5]
Despite being reinvigorated after the Kellwasser event, labechiids and their reef ecosystems presumably did not survive the Hangenberg event (~359 Ma), a second mass extinction which marked the end of the Famennian stage and Devonian period.[11] This extinction is generally considered absolute, but several possible exceptions have been reported to imply post-Devonian survival: Labechia carbonaria (from the Viséan stage of Carboniferous England)[3] and Lophiostroma boletiformis (from Triassic rocks of the Pamir Mountains).[2] These discontinuous reports pose the same set of questions applied to Early and Middle Devonian occurrences.[12] Labechiid-like fossils are abundant in early Pennsylvanian (Bashkirian) reef deposits of the Akiyoshi Limestone Group in Japan, representing a Panthalassanseamount.[4]
_(Elkhorn_Formation,_Upper_Ordovician;_south_of_Dayton,_Ohio,_USA)_4.jpg)
