JK Boundary

The Jurassic-Cretaceous boundary is one of the least studied and understood time period. Spanning between the Late Jurassic and the Early Cretaceous period, it saw an important pivotal change in biodiversity and paved the way for the life that would come after. This period saw a decline in many life-forms that were very common in the previous periods such as stegosaurs, sauropods, ammonites, itchyosaurs and various other life-forms. This "mini extinction" is not well understood for a number of reasons. 

1. The Jurassic-Cretaceous boundary’s rock record is not as globally uniform or well-preserved as other boundaries. This makes it harder to pinpoint precise events or changes associated with the transition.

2. A perceived lack of importance in the boundary compared to other boundaries such as the infamous Cretaceous-Paleocene boundary has caused less research into this area of study.

For these two reasons, the J-K boundary is often neglected despite its importance. There were great changes in the diversity of life, namely as many life-forms became less diverse and eventually went extinct. A good example of this is the stegosaurs. The stegosaurs were rather diverse during the Middle Jurassic and the Late Jurassic periods. However, in the Early Cretaceous, very few specimens remained. Mongolostegus, Paranthodon, Wuerhosaurus, and Regnosaurus are examples of stegosaurs that survived into the Early Cretaceous. On the other hand, more diverse species such as Stegosaurus, Kentrosaurus, Hesperosaurus, Chialingosaurus and many other stegosaurs roamed during the Jurassic period.

Many families of ancient creatures declined or went extinct during the Early Cretaceous period. In this post, we will go into some of them briefly.

• Non-neosauropods: Many of  the non-neosauropods appears to have declined heavily and were dominated by the titanosaurs, which were much more dominant and survived to the end of the Cretaceous period.  The non-neosauropods were very dominant during the Middle and Late Jurassic periods and declined soon afterwards. Examples of these are the well-known sauropods such as Diplodocus, Apatosaurus and Brachiosaurus. In the Early Cretaceous, titanosaurs flourished and yielded one of the biggest dinosaurs of all time: Argentinosaurus. One thing to consider is that non-neosauropods were not completely wiped out by the mini-extinction at the J-K boundary and there are a number of examples of non-neosauropods during the Early Cretaceous such as Amargasaurus and Cedarosaurus.                                                                       
• Stegosaurs: As previously explored, stegosaurs saw a decline towards the Cretaceous period and likely went extinct in the Early Cretaceous period. The stegosaurs were armored dinosaurs that seemed to have been replaced by other armored dinosaurs such as cerotopsians(horned dinosaurs) and ankylosaurs(heavily armored dinosaurs). The first known occurrence of cerotopsians is during the Late Jurassic and the first occurrence of the ankylosaurs is during the Middle Jurassic. Likely, the stegosaurs could not keep up with the changes and went extinct towards the Late Cretaceous or even earlier.                                                                                                                             
• Medium theropods: Medium-sized theropods underwent a decline during the Cretaceous and were replaced by larger theropods such as carcharodontosaurids, megalosaurids and spinosaurids. In the Late Cretaceous, other theropods would evolve, such as a special group known as the tyrannosaurs, which contains such famous theropods such as Tyrannosaurus and Albertosaurus. However, during this time, it can be seen that medium-sized theropods like allosaurids were gradually replaced by carcharodontosaurids while spinosaurids evolved as a separate special group. Needless to say, the Early Cretaceous saw a decline in medium-sized theropods.                                                                                                                                                                        
• Marine life: As a whole, marine life did not decline during the J-K boundary, but certain species did while others began to flourish. The itchyosaurs were already declining during the Jurassic and were heavily impacted by the Triassic-Jurassic boundary in which a meteor caused a mass extinction that paved the way for dinosaurs. Pleisiosaurs and ammonites also were declining during the J-K boundary. The decline was likely due to a number of reasons. First, it is important to note that other marine life, such as the mosasaurs were on the rise. During the J-K boundary, a common theme can be seen - a decline in shallow water creatures. Turtles, crocodilians and the itchyosaurs and pleisiosaurs declined heavily and in the case of itchyosaurs, went extinct during the middle of the Late Cretaceous. It is important to note that itchyosaurs and pleisiosaurs were more ocean-dwelling and therefore are not shallow water creatures.                                                 
• Certain pterosaurs: Certain groups such as rhamphorynchid pterosaurs went extinct completely while there was a shift in the diversity, Newer groups flourished and thrived until the Late Cretaceous. Pterosaurs also face competition as the only life-forms in the sky as birds evolved. These birds were now much better at flying compared to early forms such as the basal Archaeopteryx. However, birds were still evolving, and it wasn't until the Late Cretaceous that birds became direct competition to pterosaurs.

As the J-K boundary is not well studied, the exact cause of this "mini-extinction" is unknown. However, scientists speculate that it was caused by a mixture of volcanic activity, rising sea levels, tectonic activity and biotic change. 

• Volcanic activity: There is evidence for volcanic activity during the J-K boundary. Large igneous provinces (LIPs), which are large accumulates of volcanic rocks show there was likely intense volcanic activity during this period. This would release greenhouse gases, change ocean temperatures and kill anything that was too close to the volcano.                                                      
• Rising sea levels:  For example, there is evidence of a major eustatic sea-level fall in the Late Jurassic, which impacted marine ecosystems, such as marine life previously discussed. It is important to note that many marine species were already declining towards the end of the Triassic period, such as the itchyosaurs, which were growing less dominant and declining in diversity.        
• Changes in temperature: While changes in temperature are not well documented, it is known that the temperatures were gradually getting cooler towards the Cretaceous period which may have affected life-forms that were unable to adapt to the new temperatures.                                     
• Emergence of new life-forms: The Jurassic saw the beginning of a change in biodiversity as new life-forms such as birds and mammals begin to emerge and later become more successful in the Cretaceous periods. An example of this is the basal Archeaopteryx which is considered the ancestor of all birds.

In conclusion, the J-K boundary marked an important change in biodiversity that paved the way for life-forms that came after in the Cretaceous.