The Ordovician is an era of extensive transgressions in continental areas in geological history, an era of volcanic activity and crustal movement, and an era of climate differentiation and glacier development, but the Seemingly not peaceful Ordovician is another era of biological development after the Cambrian life explosion. So, what exactly is the reason for the development of biology?
Producer: Popular Science China
Producer: Feng Weimin (Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences)
Producer: Computer Network Information Center, Chinese Academy of Sciences
The Ordovician is the second paleozoic era 485-443 million years ago, immediately after the Cambrian. The Ordovician is an era of extensive continental suffering in geological history, an era of intense volcanic activity and crustal movement, and an era of climate differentiation and glacier development.
In the Ordovician era, jawless species such as armored fish among vertebrates have appeared, but the real protagonists in the ocean are still marine invertebrates, and marine invertebrates have been greatly developed during that period and are the most important taxa of marine life. What is significant is that the biological radiation events that occur worldwide are happening again, the so-called "Ordovician biological radiation".
In terms of evolutionary level and classification level, although this large radiation is inferior to the "Cambrian explosion", its radiation scale is much larger than the latter, which is 3 times that of the Cambrian. Cambrian life explosions are mainly reflected in the large biological radiation at the phytosanitary level, while the Ordovician radiation is greatly enriched and developed in organisms at the order, family and genus levels.
The Ordovician fauna consists mainly of calcareous chickadeciform brachiopods, trilobites, hemizoans, echinoderms, bryozoans, and spiny animals. Moreover, the Ordovician period increased a large number of benthic wandering, planktonic ecological stratification and community complexity, and the food chain became more complex and perfect.
△ Figure 1 Proteozoic marine evolution fauna and Ordovician biological radiation (Source: According to Zhan Renbin's 2017 revision of JACK SEPKOSKI, 1978, 1981, etc.)
Various invertebrate "stars" competed to develop in the Ordovician
Ordovician marine invertebrates flourished unprecedentedly in shallow seas and dominated the marine ecosystem, especially cephalopods represented by the Nautilus gradually replaced the odd shrimp and became the new hegemons in the ocean, in the top position of the food chain.
The Nautilus is up to 4 meters long, the largest "body" animal since the birth of marine life, and is a veritable marine giant in the Ordovician. It has eyes on both sides of the head, the center of which is the mouth, and the mouth has jaws and teeth, like the mouth of a parrot, so it is called a nautilus; there are many long wrists around the mouth, and there are suction cups on the wrists, which can be preyed on or crawled. The nautilus began in the late Cambrian period and flourished in the Ordovician.
Figure 2 Ordovician Giant Nautilus (Source: Nanjing Paleontological Museum)
The penstone animals look delicate and cute. Its shape is very peculiar, some are tree-like, some are like geese flying with wings outstretched, and some are like open bows, but the most bizarre is the thin mesh penstone with a net pocket covered with various meshes on the surface and a cavity in the middle.
The pranulvar is very similar to the rod wallworms found in the ocean today, and may belong to the hemizoan. There are two ways of life, the primitive penstone animal is like a tree, fixed on the seabed, and then grow upwards; the more advanced penstone animals are mainly floating life with the waves, relying on the tentacle swing of the penstone insect body to filter the organic matter suspended in the seawater. The Ordovician and Silurian periods were the most prosperous epochs of penstone. Sometimes hundreds of individual petite animals can be seen in a single specimen, which shows how abundant the penstone animals were in the ocean at that time.
Figure 3 Fossils of penstone (Image source: provided by Zhang Yuandong)
Trilobites were already thriving during the Cambrian period, and new types evolved during the Ordovician period. Trilobites living on the bottom of the ocean have flat bodies and hard head structures that are easy to excavate; due to the emergence of a large number of carnivorous nautiluses, in order to defend, some trilobites have grown many needles in the chest and tail; and other trilobites have evolved very ingenious vertebral-like structures to resist the predation of fish, a natural enemy that has just appeared.
The Ordovician tooth-shaped animals were extremely diverse and reached the pinnacle of historical development. At that time, the ocean was a paradise for toothed animals, and there were a large number of tooth-shaped animals living in large numbers.
Ordovician brachiopods are also extremely abundant, with the total number of genera being 6 times greater than in the Cambrian period. Data from South China show that brachiopod radiation began in the fine clastic substrate area of shallow sea in the late Early Ordovician period, and later occupied the deeper seawater environment in the early Late Ordovician.
Figure 4 Brachiopod fossils (Image source: provided by Zhan Renbin)
Marine ecosystems became more diverse during this period
The Ordovician marine life is diverse and closely related to habitat diversity. In the terrace environment, there are deep burrowing bivalves, shallow burrowing polychaetes, some brachiopods and trilobites, low-fixed benthic brachiopods, bivalves, etc., high-solid benthic echinoderms, bryophytes, corals, tree-shaped penstones, recumbent soft-tongue snails, benthic wandering trilobites, bivalves, gastropods and mesenches, etc., forming ecological stratifications of different heights from below the seabed to above the seafloor.
In addition, in the land shelter waters, ocean waters and slope environments, there are swimming cephalopods, as well as floating (or floating) plitodactyls, suspected sources, chitinia and radiolaria. Therefore, in the Ordovician period, from the seabed upward to the surface water of the ocean, from the coast to the deep-water slope, the ecological space was effectively occupied, and each ecological space had a considerable abundance of different types of organisms.
Figure 5 Ordovician marine ecological relationship food network (Source: Zhang Yuandong et al., 2009)
Among the various ecological types of organisms in the Ordovician period, the benthic solid type predominates. Such as a large number of outbreaks of brachiopods, echinoderms, bryozoans, corals and sponges, etc., they move and feed in different water layers above the seabed. At the same time, endosophytes began to proliferate, with some animals (e.g., flappy gills) able to dig burrows or drill holes to a depth of more than 12 cm below the seafloor.
In the Ordovician, swimming and plankton began to rise, such as large swimming creatures - cephalopods and planktonic plitobactus, which evolved rapidly and occupied an important place in the ecological space of seawater at different depths.
Food chain relationships "reshuffle"
With the advent of the Ordovician era, the established food chain in the biological world changed a lot. The carnivores at the top of the food chain are no longer the strange shrimp that dominated the Cambrian era, and with the decline of the strange shrimp, they are replaced by larger, faster swimming and more adaptable carnivorous cephalopods, such as right-angled stone feet, whose descendants are the octopus, squid and nautilus in the modern ocean that everyone is very familiar with, and they also occupy a place in the ocean hegemony with their super swimming and predation ability.
In terms of feeding methods, the biggest feature of Ordovician marine life is the emergence of a large number of diverse filter-feeding organisms.
They feed on suspended solids or small plankton, such as some leaf gills, beaked shells and phosphorus shell brachiopods in the shallow layer of the seafloor, most lobed gills, veneers and gastropods move on the surface of the seafloor to filter feed, calcium crust brachiopods use cilia rings to feed low on the seabed, corals and mosses feed at different heights above the seabed with tentacles, sea lilies and marine foresters, etc. due to the presence of long stems, can feed on the seabed with their tentacles (25-100cm), Petreal animals, etc., filter food with cilia tentacles in open and distant waters, thus forming a feeding ecological stratification at different heights above the seafloor. The outbreak of a large number of filter-feeding organisms and the expansion of feeding ecological stratification are an important feature of the Ordovician biological radiation.
The Ordovicians added a number of carnivores, such as cephalopods, limbs and mouths, jawless fish, and starfish swimming on the seabed, some of which actively preyed directly on various predatory organs.
In addition, plant-eating organisms also increased significantly, in addition to the Sambrian veneers and mesozoans that had appeared, the Ordovicians added sea urchins, some gastropods, mesembranths and soft-onstilled arthropods (such as walnut shrimp) to feed on the microphytes on the seabed. At the same time, organisms that feed on the sedimentary soft mud on the seafloor continue to multiply, such as trilobites, some gastropods, and mesenchymals.
Overall, the emergence of many submarine and pelagic carnivores and the increase in plant-eating animal groups made the Ordovician marine life more complete in terms of food structure.
As a result, Ordovician marine life is likely to have formed a relatively stable and complex food web or food chain.
Based on a functional analysis of the mouthpiece of the catalpache, experts deduce that the catapult may have been the food of many other fossil phylums, among which the most likely predators include cephalopods and arthropods. In recent years, the specimens of penstone disabilities found in Zhejiang and Sweden in China, as well as the specimens with vesicular parasitic traces found in the Ordovician and Swedish Silurian strata in the United States, provide new evidence for the symbiotic relationship between the penstone and other organisms.
Among the feeding habits of trilobites, predation is a primitive way of being mainly limited to the early Cambrian type. The Ordovician type mainly consists of particulate matter and filter feeding, so although ordician trilobites could move freely on the seafloor, they were most likely not predators, but more likely predators.
Ordovician brachiopods were mostly fixed on the seafloor, living on filter feeding, and were more likely to be preyed upon, and the fossil brachiopods in the late Ordovician strata in the United States preserved the scars they left after being preyed on by cephalopods. Examples of horned stones parasitizing on the shells of living brachiopods in the late Ordovician brachiopod fauna of western Zhejiang and eastern Gandong have also been preserved.
In addition, evidence of cephalopod predation on brachiopods is also found in the Carboniferous stratigraphic record in the United States, and the relationship between predation and predation between the two may have been established as early as the Ordovician. In Sweden, Ordovician gastropods also have evidence of scarring caused by predation.
Changes in topography, geology and ocean currents were responsible for the great development of Ordovician organisms
The Ordovician period was another era of biological development after the Cambrian explosion of life, especially characterized by the competitive development of invertebrates. So, what exactly is the reason for the development of biology?
Recent studies by scientists have shown that as the temperature of the early Ordovician rose, the sea surface rose and the Yangtze Terrace gradually submerged. The purple-red, high oxygen-containing argillary limestone sediments covered the entire terrace, and while oxidation, the original mesa terrain was "transformed" into a relatively gentle slope, adding different ecological niches and providing space for related organisms to survive.
In addition, due to the regional geological formation movement and weathering, some rock fragments from the land entered the sea, increasing the nutrients on the seabed, thus providing suitable environmental conditions and rich sources of nutrients for the survival of different species of organisms, resulting in the first biological development of the Ordovician.
Later, as the Late Ordovician South China Plate drifted northward, there was a cold ocean current from the South Pole to the equator along the coast of the continent of Gondwana, also known as the "cold tongue". Affected by this, the cold water ocean current has brought rich nutrients to the South China Plate, promoted the differentiation and prosperity of marine biota, and thus developed a cool water type fauna, which is recorded as a set of cool water carbonate rocks. Therefore, the equatorial "cold tongue" is one of the background mechanisms of the Ordovician biological radiation in South China.
In short, the Ordovician was the era of marine invertebrates, which opened the curtain on the great development of Paleozoic biology.
bibliography:
1. Zhan Renbin, Jin Jisuo, Liu Jianbo. Ordovician biological radiation research: review and prospect. [J]. Chinese Science Bulletin, 58(33), 3357–3371.
2、 Rong Jiayu, Huang Bing. Thirty years of mass extinction research. SCIENCE CHINA Earth Sciences, 44, 1–28.
3. Zhang Yuandong,Zhan Renbin,Fan Junxuan,Cheng Junfeng,Liu Xiao. 2009. Key scientific issues in the study of ordovician biological radiation. Science in China Series D:Earth Sciences,39,129-143.]