Ischnacanthiformes

{{Short description|Extinct order of cartilaginous fishes}}

{{Automatic taxobox

| fossil_range = {{fossilrange|418|409}}

| image = Nerepisacanthus.png

| image_caption = Nerepisacanthus

| taxon = Ischnacanthiformes

| authority =

| subdivision_ranks = Families

| subdivision = * Ischnacanthidae

• Poracanthidae
• Xylacanthus

}}

Ischnacanthiformes is a prehistoric order of "acanthodian" stem-chondrichthyans found in Canada, Ukraine and United Kingdom. Members of this order were nektonic carnivores,{{cite book|title=McGraw-Hill Dictionary of Scientific & Technical Terms |url=https://archive.org/details/mcgrawhilldictio0000unse_w4l3 |url-access=registration |edition=6 |date=2003 |publisher=McGraw-Hill Companies, Inc.}} eating animals that swim rather than plankton. They had slender builds, light armor, deeply inserted spines, shark-like teeth, and two dorsal fins. Some species were around 2 meters (6.56 feet) long.{{Cite web|url=http://palaeos.com/vertebrates/acanthodii/teleostomi.html#Ischnacanthiformes|title=Palaeos Vertebrates Acanthodii: Teleostomi|website=palaeos.com|access-date=2018-09-30}} It was described by Berg in 1940.{{cite web|title=†order Ischnacanthiformes Berg 1940 (spiny fish)|url=https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=149983|work=Fossilworks|access-date=11 January 2014|archive-date=12 October 2022|archive-url=https://web.archive.org/web/20221012115901/http://www.fossilworks.org/cgi-bin/bridge.pl?a=taxonInfo&taxon_no=149983|url-status=live}}{{cite book|last=Burrow|first=Carole|title=Acanthodians with dentigerous jaws: the Ischnacanthiformes and Acanthodopsis|publisher=Department of Zoology and Entomology, University of Queensland|url=https://www.academia.edu/428152/Acanthodians_with_dentigerous_jaws_the_Ischnacanthiformes_and_Acanthodopsis}}