( ( ( This period ) ( of time ) ) ( ( is ( called ( the ( Cambrian period ) ) ) ) . ) )
(ROOT (S (NP (NP (DT This) (NN period)) (PP (IN of) (NP (NN time)))) (VP (VBZ is) (VP (VBN called) (NP (DT the) (NNP Cambrian) (NN period)))) (. .)))
This period of time is called the Cambrian period.
(ROOT (S (NP (NP (DT The) (JJ first) (NN period)) (PP (IN of) (NP (DT the) (JJ paleozoic) (NN era)))) (VP (VBD was) (VP (VBN called) (NP (NN cambrian)))) (. .)))