| Microconchida Temporal range: | |
|---|---|
 | |
| Palaeoconchus angulatus (Hall, 1861) on a brachiopod from the Middle Devonian (Givetian) of Michigan | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Class: | † Tentaculita |
| Order: | † Microconchida Weedon 1991 |
| Genera | |
The order Microconchida is a group of small, spirally-coiled, encrusting fossil "worm" tubes from the class Tentaculita found from the Upper Ordovician to the Middle Jurassic (Bathonian) around the world. [1] [2] [3] [4] [5] They have lamellar calcitic shells, usually with pseudopunctae or punctae and a bulb-like origin. Many were long misidentified as the polychaete annelid Spirorbis until studies of shell microstructure and formation showed significant differences. [6] All pre-Cretaceous "Spirorbis" fossils are now known to be microconchids. [6] Their classification at the phylum level is still debated. Most likely they are some form of lophophorate, a group which includes phoronids, bryozoans and brachiopods. Microconchids may be closely related to the other encrusting tentaculitoid tubeworms, such as Anticalyptraea , trypanoporids and cornulitids. [3] Their habitat is more controversial. While there is a consensus that they were present in the seas and in brackish water, there is a debate about their presence in freshwater. Some studies suggested that they colonised freshwater in the Early Devonian, [7] [8] whereas others suggest that microconchids never colonised that environment. [9] [10] A recent review of the associated fauna failed to find reliable occurrences of microconchids in the Middle Devonian to Early Permian time interval because microconchids seem to co-occur with other signs of marine influence, [11] such as xiphosurans and chondrichthyan egg capsules. [12]