Hybodontiformes

Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like cartilaginous fish which lived from the late Devonian to the Late Cretaceous. They share a close common ancestry with modern sharks and rays as part of the Euselachii group. Hybodonts can be recognized by their special fin spines and head spines found on male individuals.

These fish were very diverse and lived in many different places, such as oceans and freshwater rivers, during the late Paleozoic and early Mesozoic periods. By the end of the Jurassic period, they became less common in the open ocean, as modern sharks took over those areas. However, hybodonts continued to live in freshwater and near-shore ocean areas. They managed to survive until the end of the Cretaceous period before disappearing completely from Earth.

Etymology

The name "hybodont" comes from ancient Greek words. One word, ὕβος or ὑβός, means "hump" or "hump-backed," and the other, ὀδούς, ὀδοντ, means "tooth." Scientists used this name because these ancient fish had special conical, flat teeth.

Taxonomic history

Hybodonts were first found and described in the 1800s using old fossil teeth. Scientists later decided they were different from today's sharks. At first, some thought hybodonts were closely related to a modern shark group called Heterodontiformes, but this idea was proven wrong. Hybodonts are actually closely related to modern sharks and rays, grouped together as Euselachii. Scientists know hybodonts had many families, but they still don't fully understand how all the different types fit together, especially from the Mesozoic time period.

Description

The largest hybodonts reached lengths of 2–3 metres (6.6–9.8 ft), while some were as small as 25 centimetres (0.82 ft). Hybodonts had sturdy bodies, but because their skeletons were made of cartilage, they are mostly studied through their teeth and fin spines, which are more likely to be preserved.

Hybodonts had special teeth with a tall central point, and their teeth could be used for grinding, crushing, tearing, or cutting. They also had two dorsal fins, each with a spine in front. These spines were unique to each species and had hooked denticles along them. Male hybodonts had special spines on their heads. Like modern sharks, hybodonts had a tail fin where the top part was larger than the bottom part, and their skin was covered in tiny tooth-like scales. They also laid egg cases that looked like long, twisted tubes.

Ecology

Hybodont fossils have been found in many different places, from oceans to rivers. Many hybodonts could live in water with different salt levels. Some hybodonts lived only in freshwater, while others moved between saltwater and freshwater. One type of hybodont, called Onychoselache, might have been able to move on land like some sharks today, thanks to its strong fins. Scientists think that male hybodonts used special spines on their heads to hold onto females when having babies. Some hybodont eggs have been found stuck to plants, showing that they laid eggs in freshwater and places where saltwater and freshwater mix. This is something no modern fish does. Some hybodonts may have used special spots to raise their young, like areas near lakes where the babies could grow before moving deeper into the water.

Hybodonts were probably slow swimmers but could move quickly when needed. Some, like Hybodus, were active hunters that ate fast-moving animals, such as belemnites, an extinct type of squid-like creature. Hybodonts had many different kinds of teeth, which suggests they ate many types of food. Some with wide, flat teeth might have eaten hard-shelled animals, while others ate both hard and soft-bodied prey. Often, many hybodonts with different eating habits lived together in the same place.

Evolutionary history

Hybodonts, an ancient group of shark-like fish, first appeared about 360 million years ago during the Devonian period. They lived all over the world and were especially common during the Triassic and Early Jurassic periods. Over time, modern sharks began to appear and spread, making hybodonts less common in the open ocean. By the end of the Cretaceous period, around 66 million years ago, hybodonts had almost completely disappeared.

Families and genera

The groups and types of hybodonts are not fully understood, so the list below may not be completely accurate.

• Lonchidiidae Herman, 1977
  • Baharyodon
  • Diplolonchidion
  • Vectiselachos
  • Hylaeobatis
  • Isanodus
  • Parvodus
  • Lissodus?
  • Lonchidion
  • Lonchidionoides
  • Luopingselache
  • Jiaodontus
  • Pristrisodus
• Distobatidae
  • Distobatus
  • Reticulodus
  • Tribodus?
  • Aegyptobatus
• Acrodontidae
  • Acrodus
  • Strophodus?
• Hybodontidae
  • Dicrenodus
  • Egertonodus
  • Hybodus
  • Meristodonoides
  • Planohybodus
  • Priohybodus
  • Sphenonchus
  • Durnonovariaodus
  • Crassodus
• Incertae sedis
  • Tribodus
  • Strophodus
  • Asteracanthus
  • Roongodus
  • Polyacrodus
  • Palaeobates
  • Bdellodus
  • Thaiodus
  • Acrorhizodus
  • Khoratodus
  • Arctacanthus
  • Reesodus
  • Steinbachodus
  • Onychoselache
  • Omanoselache
  • Pororhiza
  • Mukdahanodus
  • Secarodus
  • Hamiltonichthys
  • Gansuselache
  • Dabasacanthus
  • Teresodus
  • Diablodontus
  • Gunnellodus?
  • Heteroptychodus
  • Lissodus
  • Columnaodus
  • Carinacanthus
• Form genera
  • Palaeoxyris (genus used for the egg capsules of hybodonts)