Trilobites are ancient marine arthropods that once roamed the world's oceans from 520 million years ago until their extinction about 250 million years ago, at the end of the Permian period. A recent study suggests that these ancient arthropods may have grown and aged similar to the marine crustaceans we know today.
Trilobites are one of the most vital early animals. Like many invertebrates, these animals belong to the phylum Arthropoda. Geologists deduced that they were marine animals due to the rocks found alongside their fossils.
Trilobites: Extinct Arthropods with Reminiscent Growth and Aging
In a paper published recently in the journal Paleobiology, titled "Reassessing growth and mortality estimates for the Ordovician trilobite Triarthrus eatoni," experts from the University of British Columbia and Uppsala University demonstrated that the trilobites, Ordovician trilobite Triarhrus eatoni, roughly 450 million years ago, reached lengths of up to 4 cm in about a decade, with a growth curve similar to small, slow-growing crustaceans of today.
Daniel Pauly, the lead author of the study and principal investigator of the Sea Around US Initiative at UBC, says that the trilobites thrived in low-oxygen environments, similar to extant crustaceans exposed to hypoxic conditions, exhibited very low rates of growth compared to the growth rates of species dwelling in more oxygenated conditions.
He adds that the low oxygen environment makes it difficult for water-breathers to grow and adds to breathing difficulties via gills, which, as 2D surfaces, cannot keep up with the growth of their 3D bodies. Hence, under such strenuous hypoxic conditions, the animals must remain small if they hope to maintain the rest of their bodily functions.
In the case of trilobites, the species' exopods - external branches on the upper parts of the animal's limbs- function like gills. Hence, these extinct animals had similar growth constraints to modern crustaceans.
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Analyzing the Growth Rate of Trilobites and Modern Marine Crustaceans
To reach this conclusion, researchers resorted to the analysis of length-frequency data, a technique developed within fisheries science for studying the growth of fish and invertebrates that lacked physical markings indicating their age.
The information to perform the technique was obtained via earlier publications with data on the length frequency distribution of 295 exceptionally preserved fossils of trilobites known as Beecher's Trilobite Bed in New York.
After estimating the growth model parameters, a widely used technique in fisheries science known as the von Bertalanffy growth function, the team compared their results with published data on the growth of extant crustaceans.
Researchers found that the growth parameters estimated for the extinct trilobites were within the range of modern-day slow-growing crustaceans.
James D Holms, a co-author of the study, explains that the findings provide the first reasonable estimates of the absolute growth in early animals using the technique known to characterize the growth in living species accurately. The data shows that half-a-billion years ago, the growth of marine arthropods, like the trilobites, was akin to modern examples of crustaceans living in the world's oceans today, reports EurekAlert.
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