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The (Now Older) Age of Fishes: New bony fish from the Silurian of China

For all of the love and popularity that the “Age of Dinosaurs” receives, we wouldn’t be where we are without evolutionary innovations that occurred during the “Age of Fishes.” The Devonian Period (419.2–358.9 million years ago) witnessed a great increase in global abundance and diversity of jawed vertebrates. But a new study presents evidence that early jawed fishes may have evolved earlier, in the Silurian Period (443.7–419.2 million years ago), with China at its epicenter.

The study, published today in PLOS ONE by Brian Choo from Flinders University, Australia, with colleagues from the Institute of Vertebrate Paleontology and Paleoanthropology, China, describes a new fossil from the Kuanti Formation of Yunnan, southwestern China. The fish, dubbed Sparalepis tingi, is based on a beautifully-preserved and articulated partial specimen, and supports the idea that this area of Asia may represent an early boom of diversification of bony fishes, long before the Devonian “Age of Fishes.”

sparalepis fossil
The holotype of Sparalepis ting (V 17915), from Choo et al. 2017.

The Kuanti Formation (~423 Ma) is rich in marine fossils: corals, molluscs, trilobites, as well as several various groups of fishes, including placoderms, acanthodians, and osteichthyans. Only until recently, however, the knowledge of fishes from this formation was based on highly fragmentary material. For example, one bony fish from the Kuanti, Naxilepis gracilis, is based solely on isolated scales. Another osteichthyan, Megamastax amblyodus, is large but fragmentary.

Then in 2009, Min Zhu (second author in this study) and colleagues described Guiyu oneiros, the oldest articulated bony fish with a mosaic of gnathostome characteristics, including some characteristics that were once attributed solely to placoderms. In 2013, Zhu et al. then published Entelognathus, a maxillate placoderm. These taxonomic studies are based on fantastically-preserved specimens from the Kuanti Formation, and with the study published today (March 8, 2017), Sparalepis becomes the third articulated specimen from the Kuanti Formation, and only second known articulated Silurian osteichthyan.

sparalepis scales
Diagrammatic reconstruction of Sparalepis, and examples of scales preserved. From Choo et al. 2017

Sparalepis is named after the Sparabara infantry during the Persian Empire due to the similarity of the shape of the scales of the fish to the shape of the wicker sheilds carried by the Sparabara. The scales are particularly tall, thick, and narrow with distinct interlocking articulation mechanisms. Sparalepis is represented by a partial postcranium, body scales, and some fin elements. The pectoral girdles bear large fin spines, as well as large dorsal fin spines beautifully preserved in the holotype.

A lot of these characters observed in Sparalepis are shared with Guiyu, in particular the spine-bearing pectoral girdle and placoderm-like, dermal pelvic girdle. These articulated specimens from the Silurian helped the researchers solve another fishy mystery, by helping them definitively identify isolated elements from the Devonian Xitun Formation in Yunnan, attributing them to the enigmantic taxon Psarolepis. (Zhu et al. 2012)

So what sets Sparalepis aside from Guiyu and Psarolepis as a distinct species? Sparalepis possesses prominent linear ridges and pore openings on the dermal surfaces of all of the larger bones and median scutes are unique to Sparalepis and lacking in Guiyu. The scale ornament of Sparalepis and Guiyu are similar to each other, and different of that seen in Psarolepis.

What is important to note about these fishes is that they display characteristics that were once thought to be isolated to placoderms. The mosaic of characters seen in Sparalepis and Guiyu has caused questions regarding previous evolutionary hypotheses of relationships between placoderms, sarcopterygians, and actinopterygians.

The team sought to address the relationships of these enigmatic fishes, and provided a dataset to test their phylogenetic hypothesis. Their results suggest that Sparalepis is closely related to Guiyu, Achoania, and Psarolepis; this group is recovered at the base of Sarcopterygii, in a clade that this study is putatively calling “psarolepids.” This study also provides additional support to the hypothesis that the ancestral species to gnathostomes likely possessed a placoderm-like bodyplan, and that characteristics seen in stem sarcopterygians like Sparalepis are likely plesiomorphic conditions.

So this study is helping to narrow a wide morphological gap in the early history of jawed vertebrates. And with the evidence provided by this study, we now know that that history began earlier than we thought. Thanks to Sparalepis and Guiyu, we can see that the “Age of Fishes” arrived early during the SIlurian in China.

sparalepis recon
Life restoration of Sparalepis tingi (foreground) and other fauna from the Kuanti Formation. Also in the scene are numerous conodont animals, a pair of the maxillate placoderm Entelognathus (middle distance) and two examples of the osteichthyan Megamastax (background), the largest known Silurian vertebrate. Illustration by Brian Choo, released under Creative Commons Attribution Licence CC BY 4.0, 2016.

References:

Choo B, Zhu M, Qu Q, Yu X, Jia L, Zhao W (2017) A new osteichthyan from the late Silurian of Yunnan, China. PLoS ONE 12(3): e0170929. doi:10.1371/journal.pone.0170929 

Zhu M, Zhao WJ, Jia LT, Lu J, Qiao T, Qu QM (2009) The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature 458: 469–474. doi: 10.1038/nature07855

Zhu M, Yu XB, Ahberg PE, Choo B, Lu J, Qiao T, et al. (2013) A Silurian placoderm with osteichthyan- like marginal jaw bones. Nature 502: 188–193. doi: 10.1038/nature12617 PMID: 24067611 

Zhu M, Yu XB, Choo B, Qu QM, Jia LT, Zhao WJ, et al. (2012) Fossil fishes from China provide first evi- dence of dermal pelvic girdles in osteichthyans. PLoS ONE 7: e35103. doi: 10.1371/journal.pone. 0035103 PMID: 22509388 

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