The Silurian period lasted from 443 to 417 million years ago. When the ice finally melted, ocean levels rose and the sea flooded vast areas of land, resulting in a milder climate. The Devonian Period (417 – 354 million years ago) was the time of the greatest cataclysms on the planet (Gensel et al., 2020). During this period, the lifeless land was covered with a carpet of vegetation creeping from the sea. Arachnids, primitive wingless insects, and amphibians appeared who made the first steps from water to land. This paper aims to discuss the organisms, plant fossils, and animals found in the Silurian and Devonian periods, and how the animals’ eyes evolved and adapted to the new conditions.
The most prosperous type of animal in the Silurian was the arthropod. The most extensive class of arthropods was trilobites. Crustacean was the second largest class of Silurian arthropods, which separated from the trilobites at the end of the Ordovician. The first representatives of a new class of arthropods, centipedes, appeared in the Silurian (Gensel et al., 2020). As for the eyes of arthropods, trilobites already possessed complex faceted eyes, which were planted on stalks in those animals buried in the mud. It is believed that the structure of the eyes of trilobites differed from the absolute majority of modern living organisms (Tinn et al., 2020). Instead of a crystalline lens, they had mineral lenses made of calcite. However, some scientists believe that the presence of calcite lenses in trilobites is a result of post-mortem mineralization.
Among modern organisms, mineral visual lenses are found only in the Ophiuroidea and Acanthopleura granulata. However, in terms of the complexity of their structure, their organs of vision cannot be compared with the eyes of trilobites. According to the location and number of prisms, trilobite eyes are divided into three groups. The first one is holochroic, consisting of a large number of up to 15 thousand prismatic lenses tightly pressed against each other, usually covered with a common transparent shell (Tinn et al., 2020). The second one is bathochromic, found in representatives of the Cambrian suborder Eodiscina and differing from schizophrenic in the smaller number of no more than 70 lenses and their size. The third group is schizophrenic, with a visual surface consisting of up to 700 rounded or polygonal lenses, each of which is covered with a shell and separated from the others.
As for the plants, at the end of the Silurian, some of them grew in shallow waters, so that the stems protruded above the water. They had to organize vessels inside themselves, supplying water to the upper parts of the plant and protecting them from drying out. This trick was done by miniature cooksonia and the first lycopsids (Gensel et al., 2020). Nematophytes have learned to protect themselves from drying out by covering their organisms with a membrane permeable to water only in one direction. In the short term, this decision proved to be the most effective and dermatophytes became the largest land plants in the Silurian.
In the Devonian Period (417 – 354 million years ago), new groups of fish, the so-called placoderms, were formed, for example, the huge predator of Dunkleosteus. Also at the same time, evolution gave rise to even more highly organized predators – sharks. Simultaneously with sharks, and even more promising group of fish began to spread in the seas – Osteichthyes or bony fish. Moreover, plants have colonized areas of land along the shores of swamps and lakes. The stems of plants such as rhinia, cuxonia, and zosterophyllum were smooth and leafless, while those of psilophyton and asteroxilon were covered with small scales (Sedorko et al., 2019). In the fossils of the Devonian Period, a variety of different rhinophytes have already been found. Since that time, two evolutionary lines have been distinguished among them. One of them will go from Zosterophyllum to lycopods, while another line leads to horsetails, ferns, and angiosperms.
As for the fauna of the Devonian Period, besides trilobites and other anthropods, chordates occupied an important place. Different classes independently acquired the characteristics of modern fish, such as jaws, fins, eyes, etc. One of the most widespread classes is anaspida, notable for perforated gills and long lateral fins that complement or replace the dorsal fins. It is important that anaspids were the first chordates that acquired a new type of complex eyes with a lens, pupil, and retina (Sedorko et al., 2019). Before that, the heads of chordates were equipped with simple eyes, like those of snails. The second class of jawless fish was osteostracans that acquired paired pectoral fins and a really powerful tail, much longer and more muscular than anaspids. The third class of jawless, somewhat less successful than anaspids and osteostracans, was thelodonti. For the first time, these fish acquired a complete set of fins, including a hydrodynamically perfect caudal fin.
In the Delurian, many different types of brachiopods and bryozoans appeared. In the stagnant swamp water, new animals have developed, such as amphibians. The earliest known amphibian is ichthyostega (Sedorko et al., 2019). Going out onto land, it most likely leaned on its forelimbs, in much the same way as sea lions lean on their front flippers. Among the first terrestrial invertebrates, one can distinguish scorpion-like creatures, such as a paleophone, possibly descended from aquatic eurypterids. Fish also continued to evolve rapidly, for example, acanthodes, representatives of the jawless carapace fish of the pteraspis and cephalaspis, as well as telodonts.
The Silurian and Devonian Periods follow the Ordovician, continuing its development. After some regression, the seas again transgress and their level rises, which is possibly associated with the melting of ice. Most of the previously formed groups of organisms, plants, and animals continue to evolve. On land, peculiar herbaceous plants, psilophytes, are widely developed. These periods are characterized by the appearance and widespread development of fish, as well as amphibians, insects, spiders, and centipedes. As for the development of eyes, creatures such as arthropods have evolved complex faceted eyes in the Silurian and Devonian.
Reference List
Gensel, P.G., Glasspool, I., Gastaldo, R.A., Libertin, M. and Kvaček, J. (2020) Back to the beginnings: The Silurian-Devonian as a time of major innovation in plants and their communities. In Nature through Time (pp. 367-398). Springer, Cham.
Sedorko, D., Netto, R.G. and Horodyski, R.S. (2019) Tracking Silurian-Devonian events and paleobathymetric curves by ichnologic and taphonomic analyzes in the southwestern Gondwana. Global and Planetary Change, 179, pp.43-56.
Tinn, O., Meidla, T. and Ainsaar, L. (2020) Diving with trilobites: Life in the Silurian–Devonian seas. In Nature through Time (pp. 345-366). Springer, Cham.