Labechiida is an extinct order of stromatoporoid sponges. They lived from the Early Ordovician (Floian stage) to the Late Devonian (Famennian stage), though a few putative fossils have been reported from younger sediments. 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).
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). 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.
Taxonomy
Family Aulaceratidae Kühn, 1927 [Middle Ordovician (Darriwilian) – Upper Devonian (Famennian)] Family Labechiidae Nicholson, 1879 [Middle Ordovician (Darriwilian) – Upper Devonian (Famennian), Viséan?] Family Lophiostromatidae Nestor, 1966 [Middle Ordovician (Darriwilian) – Upper Devonian (Frasnian), Triassic?] Family Platiferostromatidae Khalfina & Yavorsky, 1973 [early Silurian (Llandovery) – Upper Devonian (Famennian)] Family Rosenellidae Khalfina & Yavorsky, 1973 [Lower Ordovician (Floian) – Upper Devonian (Famennian)] Family Stromatoceriidae Bogoyavlenskaya, 1969 [Upper Ordovician (Sandbian – Katian)] Family Stylostromatidae Webby, 1993 [Middle Ordovician (Darriwilian) – Upper Devonian (Famennian)]
Evolution
Ordovician
The earliest reported labechiid species is Cystostroma primordia, from the Floian stage (Early Ordovician) of South China. 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). 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. 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 or Cystostroma to be ancestral to other labechiids, while Russian paleontologists point to the Siberian Priscastroma. Labechiids as a whole may be descended from non-stromatoporoid sponges in the order Pulchrilaminida.
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. 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. 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. Only two labechiids (Aulacera and Pachystylostroma) retain any semblance of commonness during the extinction interval.
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. Even as stromatoporoids collectively reached their maximum Silurian diversity in the Wenlock Epoch (middle Silurian), labechiids were not among the beneficiaries. 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.