Recently, distributed data storage has gained increasing popularity for efficient and robust data management in wireless sensor networks (WSNs). But the distributed architecture also makes it challenging to build a highly secure and dependable yet lightweight data storage system. On the one hand, sensor data are subject to not only Byzantine failures, but also dynamic pollution attacks, as along the time the adversary may modify/pollute the stored data by compromising individual sensors. On the other hand, the resource-constrained nature of WSNs precludes the applicability of heavyweight security designs. To address the challenges, we propose a novel dependable and secure data storage scheme with dynamic integrity assurance in this paper. Based on the principle of secret sharing and erasure coding, we first propose a hybrid share generation and distribution scheme to achieve reliable and fault-tolerant initial data storage by providing redundancy for original data components. To further dynamically ensure the integrity of the distributed data shares, we then propose an efficient data integrity verification scheme exploiting the technique of algebraic signatures. The proposed scheme enables individual sensors to verify in one protocol execution all the pertaining data shares simultaneously in the absence of the original data. Extensive security and performance analysis shows that the proposed schemes have strong resistance against various attacks and are practical for WSNs.